The Astronomy Book (Big Ideas Simply Explained)

THE TELESCOPE REVOLUTION 49

See also: The geocentric model 20 ■ The Tychonic model 44–47 ■ Elliptical orbits 50–55 ■
Variable stars 86 ■ Measuring the universe 130–37

An artist’s impression shows
material flowing from Mira A (right)
onto the hot disk around its companion
white dwarf Mira B (left). The hot gas
in the system emits X-rays.

the sun rotated, providing further
proof of the variable nature of
heavenly bodies. However, the book
they published on the subject in
1611 was mostly overlooked, and the
credit for describing the movement
of sunspots went to Galileo, who
published his results in 1613.

To Fabricius’s amazement, a few Working with his son Johannes, Double-star system
days later, the brightness of this Fabricius also used a camera It is now known that Mira Ceti is a
star had increased by a factor of obscura to look at the sun. They double-star system 420 light-years
about three. After a few weeks, it studied sunspots, observing that away. Mira A is an unstable red
disappeared from view altogether, the spots moved across the sun’s giant star, about 6 billion years old
only to reappear some years later. disk from east to west at a constant and in a late phase of its evolution.
In 1609, Fabricius confirmed that speed. They then disappeared, It pulses in and out, changing not
Mira Ceti was a periodic variable only to reappear on the other side, only its size but also its temperature.
star, showing that, contrary to the having been out of sight for the During the cooler part of its cycle,
prevailing Greek philosophy that same time that it had taken them it emits much of its energy as
the cosmos was unchanging, stars to move across the sun’s disk. This infrared radiation rather than
were not constant. was the first concrete evidence that light, so its brightness diminishes
dramatically. Mira B is a white
dwarf star surrounded by a disk of
hot gas that is flowing from Mira A. ■

In short, this new star David Fabricius whom Fabricius pioneered
signifies peace … as well as the use of a camera obscura
change in the [Holy Roman] David Fabricius was born in to observe the sun.
1564 in Esens, Germany, and
Empire for the better. studied at the University of Little is known of Fabricius’s
David Fabricius Helmsted. He later became life beyond his letters and
a Lutheran pastor for a group publications. He died in 1617
in a letter to of churches in Frisia. after he was struck on the
Johannes Kepler head with a shovel by a local
Together with his son goose thief, whom he had
Johannes (1587–1615), he was denounced from the pulpit.
fascinated by astronomy and
an avid user of early telescopes, Key work
which his son had brought
back with him from a trip to 1611 Narration on Spots
the Netherlands. Fabricius Observed on the Sun and their
corresponded extensively Apparent Rotation with the Sun
with Johannes Kepler, with (with his son Johannes)

TTRHUEEMPOASTHT

ELLIPSEOF THE PLANET IS AN

ELLIPTICAL ORBITS



52 ELLIPTICAL ORBITS

IN CONTEXT B efore the 17th century, Kepler’s most productive years
all astronomers were also came in Prague under the patronage
KEY ASTRONOMER astrologers. For many, of Holy Roman Emperor Rudolf II
Johannes Kepler (1571–1630) including German astronomer (r.1576–1612). Rudolf was particularly
Johannes Kepler, casting horoscopes interested in astrology and alchemy.
BEFORE was the main source of their income
530–400 bce The works of and influence. Knowing where move around a small circle, the
Plato and Pythagoras convince the planets had been in the sky center of which moved around
Kepler that the cosmos can be was important, but of greater a larger circle. These circular
explained using mathematics. significance for constructing velocities were always assumed
1543 Copernicus’s sun- astrological charts was the ability to be constant.
centered cosmos helps to predict where the planets would
astronomers to visualize a be over the next few decades. Kepler supported the Copernican
physical solar system but still system, but the planetary tables it
gives no indication as to the To make predictions, astrologers produced could still easily be out by
true shape of a planetary orbit. assumed that the planets moved a day or two. The planets, the sun,
1600 Tycho Brahe convinces on specific paths around a central and the moon always appeared in
Kepler of the reliability of his object. Before Copernicus, in the a certain band of the sky, known
planetary observations. 16th century, this central body as the ecliptic, but actual paths of
was thought by most to be Earth. individual planets around the sun
AFTER Copernicus showed how the were still a mystery, as was the
1687 Isaac Newton realizes mathematics of planetary mechanism that made them move.
that an inverse square law of prediction could be simplified
gravitational force explains why by assuming that the central body Finding the paths
the planets obey Kepler’s laws. was the sun. However, Copernicus To improve the predictive tables,
1716 Edmond Halley uses assumed that orbits were circular, Danish astronomer Tycho Brahe
observations of the transit of and to provide any reasonable spent more than 20 years observing
Venus to convert Kepler’s ratios predictive accuracy, his system the planets. He next tried to
of planetary distance from still required the planets to ascertain a path of each planet
the sun into absolute values.

Kepler was never satisfied by a
moderate agreement between

theory and observation.
The theory had to fit exactly

otherwise some new
possibility had to be tried.

Fred Hoyle

THE TELESCOPE REVOLUTION 53

See also: The Copernican model 32–39 ■ The Tychonic model 44–47 ■
Galileo’s telescope 56–63 ■ Gravitational theory 66–73 ■ Halley’s comet 74–77

through space that would fit the distance from the other focus is Johannes Kepler
observational data. This is where always constant. Kepler found
the mathematical abilities of Kepler, that the sun was at one of these Born prematurely in 1571,
Brahe’s assistant, came into play. two foci. These two facts made Kepler spent his childhood
He considered specific models for up his first law of planetary motion: in Leonberg, Swabia, in his
the solar system and the paths the motion of the planets is an grandfather’s inn. Smallpox
of the individual planets in turn, ellipse with the sun as one of affected his coordination
including circular and ovoid the two foci. and vision. A scholarship
(egg-shaped) orbits. After many enabled him to attend the
calculations, Kepler determined Kepler also noticed that the Lutheran University of
whether or not the model led to speed of a planet on its ellipse was Tübingen in 1589, where
predictions of planetary positions always changing, and that this he was taught by Michael
that fit into Tycho’s precise change followed a fixed law (his Maestlin, Germany’s top
observations. If there was not exact second): a line between the planet astronomer at the time. In
agreement, he would discard the and the sun sweeps out equal areas 1600, Tycho Brahe invited
idea and start the process again. in equal times (p.54). These two Kepler to work with him at
laws were published in his 1609 Castle Benátky near Prague.
Abandoning circles book Astronomia Nova. On Tycho’s death in 1601,
In 1608, after 10 years of work, Kepler succeeded him as
Kepler found the solution, which Kepler had chosen to investigate Imperial Mathematician.
involved abandoning both circles Mars, which had strong astrological
and constant velocity. The planets significance, thought to influence In 1611, Kepler’s wife died,
made an ellipse—a kind of human desire and action. Mars and he became a teacher in
stretched-out circle for which the took variable retrograde loops— Linz. He remarried and had
amount of stretching is measured periods during which it would seven more children, five of
by a quantity called an eccentricity reverse its direction of movement— whom died young. His work
(p.54). Ellipses have two foci. and large variations in brightness. was then disrupted between
The distance of a point on an It also had an orbital period of 1615 and 1621 while he
ellipse from one focus plus the only 1.88 Earth years, meaning defended his mother from
that Mars went around the sun charges of witchcraft. The
about 11 times in Tycho’s data ❯❯ Catholic Counter-Reformation
in 1625 caused him further
Neither circular nor ovoid orbits fit problems, and prevented his
Tycho Brahe’s data on Mars. return to Tübingen. Kepler
died of a fever in 1630.
An ellipse fits the data, so the
path of Mars is an ellipse. Key works

The success of the The Three Laws of 1609 Astronomia Nova
predictions shows that Planetary Motion allow 1619 Harmonices Mundi
1627 Rudolphine Tables
the orbits of all the for new, improved
planets are ellipses. predictive tables.

54 ELLIPTICAL ORBITS

When just one body C searched for a divine purpose
goes around a larger body B within his scientific work. Since
undisturbed, the paths it can he saw six planets, he presumed
follow are known as Kepler A that the number six must have a
orbits. These are a group of profound significance. He produced
curves called conic sections, Large an ordered geometric model of the
which include ellipses, body solar system in which the sun-
parabolas, and hyperbolas. centered spheres that contained
The shape of the orbit is D each planetary orbit were inscribed
defined by a property called and circumscribed by a specific
eccentricity. An eccentricity regular “platonic” solid (the five
of 0 is a circle (A). Eccentricity possible solids whose faces and
between 0 and 1 is an ellipse internal angles are all equal). The
(B). Eccentricity equal to 1 sphere containing the orbit of
produces a parabola (C), and Mercury was placed inside an
greater than 1 a hyperbola (D). octahedron. The sphere that just
touched the points of this regular
set. Kepler was lucky to have chosen Today, astronomers might look at solid contained the orbit of Venus.
Mars, since its orbit has a high a list of planetary orbital sizes and This in its turn was placed inside
eccentricity, or stretch: 0.093 (where eccentricities and regard them as an icosahedron. Then followed the
0 is a circle and 1 is a parabola). the result of the planetary formation orbit of Earth, a dodecahedron,
This is 14 times the eccentricity process coupled with a few billion Mars, a tetrahedron, Jupiter, a cube,
of Venus. It took him another years of change. To Kepler, however, and finally Saturn. The system was
12 years to show that the other the numbers needed explanation. beautifully ordered, but inaccurate.
planets also had elliptical orbits. A deeply religious man, Kepler
F Planet near
Studying Brahe’s observations, A Focus 1 (the sun)
Kepler was also able to work out the aphelion
planets’ orbital periods. Earth goes B
around the sun in one year, Mars E
in 1.88 Earth years, Jupiter in 11.86,
and Saturn in 29.45. Kepler realized Focus 2
that the square of the orbital period (empty point
was proportional to the cube of
the planet’s average distance from in space)
the sun. This became his third
law and he published it in 1619
in his book Harmonices Mundi,
alongside lengthy tracts on
astrology, planetary music, and
platonic figures. The book had
taken him 20 years to produce.

Searching for meaning Planet near C Elliptical orbit
Kepler was fascinated by patterns perihelion
he found in the orbits of the planets.
He noted that, once you accepted D
the Copernican system for the
cosmos, the size of the orbits According to Kepler’s second law, the line joining a planet to the sun
of the six planets—Mercury, sweeps out equal areas in equal times. This is also known as the law of equal
Venus, Earth, Mars, Jupiter, areas. It is represented by the equal areas of the three shaded areas ABS,
and Saturn—appeared in the CDS, and EFS. It takes as long to travel from A to B as from C to D and from E
ratios 8 : 15 : 20 : 30 : 115 : 195. to F. A planet moves most rapidly when it is nearest the sun, at perihelion; a
planet’s slowest motion occurs when it is farthest from the sun, at aphelion.

THE TELESCOPE REVOLUTION 55

Kepler was convinced that
God created the world in
accordance with the principle
of perfect numbers, so that the
underlying mathematical
harmony … is the real and
discoverable cause of the

planetary motion.
William Dampier

Science historian

Kepler’s great breakthrough was his change, contradicting Aristotle’s In Harmonices Mundi, Kepler
calculation of the actual form of the idea of a “fixed cosmos.” A recent experimented with regular shapes
planetary orbits, but the physics planetary conjunction coupled to unlock the secrets of the cosmos.
behind his three laws did not with this new star led him to He linked these shapes with harmonics
seem to concern him. Rather, he speculate about the Biblical “Star to suggest a “music of the spheres.”
suggested that Mars was carried of Bethlehem.” Kepler’s fervent
on its orbit by an angel in a chariot, imagination also produced the the Rudolphine Tables (named
or swept along by some magnetic book Somnium, in which he after Emperor Rudolf, his patron
influence emanating from the sun. discusses space travel to the in Prague) were eventually
The idea that the movements were moon and the lunar geography published, and these tables of
due to a gravitational force only a visitor might expect on arrival. predicted planetary positions
arrived with the ideas of Isaac Many regard this as the first work helped him greatly with the well-
Newton some 70 years later. of science fiction. paid calendars that he published
between 1617 and 1624. The
Wider contributions Kepler’s most influential accuracy of his tables, proven
Kepler also made important publication, however, was a over a few decades, also did much
advances in the study of optics, textbook on astronomy called to encourage the acceptance of
and his 1604 book Astronomiae Epitome Astronomiae Copernicanae, both the Copernican sun-centered
Pars Optica is regarded as the and this became the most widely solar system and Kepler’s own
pioneer tome in the subject. used astronomical work between three laws. ■
Galileo’s telescope interested him 1630 and 1650. He ensured that
greatly and he even suggested
an improved design using convex
lenses for both the objective and
the magnifying eyepiece. He
wrote, too, about the supernova
that was first seen in October 1604,
today commonly called Kepler’s
supernova. Following Tycho, Kepler
realized that the heavens could

OUR OWN EYES

STHROWAUVS FEOULRISNTAGRS

AROUND JUPITER

GALILEO’S TELESCOPE



58 GALILEO’S TELESCOPE

IN CONTEXT G alileo Galilei’s effective The Milky Way is nothing
use of a telescope marked else but a mass of
KEY ASTRONOMER a watershed in the history
Galileo Galilei (1564–1642) of astronomy. There have been innumerable stars planted
other turning points—such as the together in clusters.
BEFORE introduction of photography, the Galileo Galilei
1543 Nicolaus Copernicus discovery of cosmic radio waves,
proposes a theory of a sun- and the invention of the electronic elements, analogous to individual
centered cosmos, but proof computer—but the invention of the pixels in a digital photograph (see
is needed because Earth does telescope was fundamental to the below). Dark lunar seas and lighter
not seem to move. advancement of the subject. lunar highland are discernible, but
individual mountains and their
1608 Dutch eyeglass-makers Limits of the naked eye shadows are beyond detection.
develop the first telescopes. Before Galileo, the naked eye was
all that was available to observe Looking up at the night sky
AFTER the sky. The naked eye is limited on a cloud-free, moonless night in
1656 Dutch scientist in two main ways: it is unable to Galileo’s Italian countryside, 2,500
Christiaan Huygens builds record detail, and it can only detect stars would be visible above the
ever-bigger telescopes that objects that are reasonably bright. horizon. The Milky Way—the disk
are capable of detecting more of the solar system seen side-on—
detail and fainter objects. When looking at a full moon, the looks like a river of milk to the
lunar diameter subtends (spans) an naked eye. Only a telescope shows
1668 Isaac Newton produces angle of 1⁄2º at Earth’s surface. This that the Milky Way seems to be
the first reflecting telescope, means that two lines extending made up of individual stars; the
an instrument that is far less from opposite sides of the moon bigger the telescope, the more stars
affected by the distortion of meet at the eye to make an angle of
chromatic aberration (p.60). 1⁄2º. However, the naked eye can only
detect separate objects that are more
1733 The first flint glass/crown than about 1⁄60º apart. This is the
glass achromatic lens is made. eye’s resolution, and determines
This greatly improves the the level of detail it can detect.
potential image quality of Looking at the full moon with
refracting telescopes. the naked eye, the lunar diameter
is resolved into only 30 picture

The resolution of the naked eye is about 1⁄60°. Moon
The moon subtends an angle of 1⁄2° seen from Earth,
meaning that the lunar diameter can be resolved
into 30 picture elements.

Eye 1 picture element

½°
1⁄60°

THE TELESCOPE REVOLUTION 59

See also: The Copernican model 32–39 ■ The Tychonic model 44–47 ■ Elliptical orbits 50–55 ■
Barnard (Directory) 337

To the naked eye,
Jupiter looks like

a bright star.

The telescope shows
a finer resolution than

the naked eye.

This reveals that
Jupiter is a disk with
four stars around it.

Galileo demonstrates his telescope telescope.) After hearing about The four stars
to Leonardo Donato, the Doge of this new instrument, Galileo had can be seen to be
Venice. Like other astronomers of his resolved to make one for himself. orbiting Jupiter.
time, Galileo relied on patronage to
fund and legitimize his work. A telescope does two important Jupiter has at least
things. Its resolution (the detail a four moons.
are visible. By turning his new telescope can detect) is proportional
telescope to the night sky, Galileo to the diameter of the objective of four, and objects of similar light
would become one of the very first lens—the large lens at the front output can be detected if they are
people to appreciate the true nature that collects the light. The larger twice as far away. Objective lenses
of this band of stars across the sky. the objective lens, the better the of 1, 2, and 4 cm enable the eye to
resolution. An eye that has fully discern 20,000, 160,000, and
Building a telescope adapted to the dark has a pupil 1,280,000 stars respectively.
Galileo did not invent the telescope that is about ¼ in (0.5 cm) across,
himself. The idea of combining two and a resolution of around 1⁄60º. Put Galileo was not satisfied with
lenses—a large one at the front of the eye at the back of a telescope his first instrument, which only
a tube to collect the light, and a with an objective lens of 1, 2, or 4 magnified three times. He realized
small one at the back to magnify cm diameter, and the resolution that a telescope’s magnification ❯❯
the image—had come from the improves to 1⁄120º, 1⁄240º, and 1⁄480º
Dutchmen Hans Lipperhey, Jacob respectively. Details then spring into
Metius, and Sacharias Janssen view. Jupiter, for example, looks like
in around September 1608. (It had a disk and not just a point.
taken over 300 years to progress
from the invention of reading A telescope also acts as a “light
glasses to the invention of a bucket.” Every time the diameter
of the objective lens is doubled, the
light gathered increases by a factor

60 GALILEO’S TELESCOPE

My dear Kepler, what developed a new telescope with Galileo had the
would you say of the learned 33 times magnification, and it was experience of beholding
with this improved instrument
who … have steadfastly that he discovered the Jovian the heavens as they
refused to cast a glance (“of Jupiter”) moons. actually are for perhaps
through the telescope?
“Three little stars” the first time.
Galileo Galilei Galileo discovered the moons I Bernard Cohen
of the planet Jupiter on the night of
was directly related to the ratio of January 7, 1610. At first, he thought they appeared to be arranged
the focal length of the objective lens he was looking at distant stars, exactly along a straight line and
to the focal length of the eyepiece. but he quickly realized that the new parallel to the ecliptic ….”
A longer-focus convex lens for bodies were moving around Jupiter,
the objective, or a shorter-focus At the time, Galileo was a 45-year- Repeated observations
concave lens for the eyepiece old professor of mathematics at the Galileo’s unexpected discovery
was required. Since these were not University of Padua near Venice. fascinated him. As he observed
available, Galileo taught himself to When he published his pioneering Jupiter night after night, it soon
grind and polish lenses and made telescopic observations, he wrote: became clear that the new stars
them for himself. Living in northern “Through a spyglass, Jupiter were not beyond Jupiter, in the
Italy, the glassmaking center presented himself. And since distant heavens. They not only
of the world at the time, helped I had prepared for myself a accompanied the planet as it
him considerably. He eventually superlative instrument, I saw moved along its path across
(which earlier had not happened the sky, but also moved around
because of the weakness of other the planet.
instruments) that three little stars
were positioned near him—small Just as the moon orbits Earth
but yet very bright. Although every month, Galileo realized that
I believed them to be among there were four moons in orbit
the number of fixed stars, they around Jupiter, staying with it as it
nevertheless intrigued me because orbited the sun. The more distant
moons took longer to complete
Single Green light is their orbits than the closer ones.
lens in between red The time to complete one orbit
from the inner to the outer moon
Red light refracts and blue. is 1.77, 3.55, 7.15, and 16.69 days,
the least. respectively. The Jovian moon
system looked like a small model
Different of the sun’s planetary system.
foci It was proof that not everything in
the cosmos orbited Earth, as had
Blue light refracts been thought in pre-Copernican
the most. days. The observation of these four
moons was a boost to the theory
Refracting telescopes suffer from a problem known of the sun-centered cosmos.
as chromatic aberration. The different wavelengths of
light are brought to slightly different foci, so the final
image is surrounded by a halo of color.

Galileo’s telescope had a concave lens as an eyepiece. THE TELESCOPE REVOLUTION 61
When viewing a celestial object a great distance away, the
distance between the two lenses would equal the focal length Focal point of
of the objective lens minus the focal length of the eyepiece. objective lens
Parallel
Eye
light
rays
from
star

Objective Concave Refracting telescopes
lens eyepiece lens
There were two kinds of
Objective focal length early refracting telescope: the
Galilean, and the Keplerian,
Kepler’s telescope, developed soon after, had a convex lens developed in 1611 by Johannes
as an eyepiece. The length of the telescope was equal to the Kepler (see left). They both had
objective focal length plus the focal length of the eyepiece. a long-focus, large diameter
lens at the front, called the
Objective lens Focal point of objective objective. This collected the
and eyepiece lenses light and brought it to a focus.
The image at the focus was
Eye magnified using the smaller,
short-focus eyepiece lens.
Objective focal length Convex
eyepiece The magnification of the
lens instrument is equal to the
focal length of the objective
Eyepiece focal length lens divided by the focal
length of the eyepiece. A
Galileo quickly published his At first, many were sceptical, flatter convex objective lens
discovery in his book Siderius suggesting that the moons were no reduced chromatic aberration
Nuncius (The Starry Messenger), more than defects in the telescope (see opposite), gave a longer
published on March 10, 1610. In lens. However, other pioneering focal length, and, for a fixed
the hope of advancement, Galileo telescopic astronomers such as eyepiece, greater magnification.
dedicated the book to a former Thomas Harriot, Joseph Gaultier For this reason, telescopes
pupil of his who later became the de la Vatelle, and Nicolas-Claude became longer in the 17th
Grand Duke of Tuscany, Cosimo II Fabri de Peiresc confirmed their century. The minimum focal
de’ Medici. He named the moons existence when Jupiter returned length of eyepieces at the
the Medicean Stars in honor of to the night sky later in 1610, after time of Galileo and Kepler was
the four royal Medici brothers. This passing behind the sun. about 1–1½ in (2–4 cm). This
political thoughtfulness won him meant that, for a magnification
the position of Chief Mathematician Disputed priority of x30, an objective lens with
and Philosopher to the Medici In 1614, German astronomer Simon a focal length of 24–48 in
at the University of Pisa. However, Marius published Mundus Iovialis, (60–120 cm) was needed. Built
the name did not catch on. in which he described Jupiter’s ❯❯ in 1888, the huge James Lick
Telescope on Mount Hamilton,
California (above), has a 36-in
(90-cm) lens and a focal length
of 57 ft (17.37 m).

62 GALILEO’S TELESCOPE

moons and claimed to have four times farther away from Earth When Jonathan Swift published
discovered them before Galileo. than the sun, the Jovian system Gulliver’s Travels in 1726, he
Galileo would later accuse looks the same from anywhere predicted, in the chapter on
Marius of plagiarism, but it is on Earth, so a “Jovian clock” Laputa, that Mars would have
now generally accepted that he would work from anywhere. The two moons simply because Earth
made his discovery independently longitude problem was finally had one and Jupiter four. In 1877,
at around the same time. Marius solved with the introduction of this prediction was fortuitously
named the moons Io, Europa, accurate chronometers by the proved to be correct when Asaph
Ganymede, and Callisto after the English clockmaker John Harrison Hall discovered Mars’s two small
Roman god Jupiter’s love conquests, around 1740. This was well before moons, Phobos and Deimos, using
and these names are still used. the orbits of Jupiter’s moons had a new 26-in (66-cm) refracting
They are now known collectively been worked out in detail. telescope at the US Naval
as the Galilean moons. Observatory in Washington.
Galileo’s discovery of four
A Jovian clock satellites around Jupiter had Support for Copernicus
Galileo carefully studied the another interesting consequence. In Galileo’s time, there was still a
changing positions of the Jovian heated debate between believers
moons from day to day. He The Bible shows the way of the old biblical theory that Earth
concluded that, like the planets, to go to heaven, not the was stationary at the center of the
their positions could be calculated cosmos and Copernicus’s new
in advance. Galileo saw that, if way the heavens go. idea that the Earth was in orbit
this could be done accurately, the Galileo Galilei around the sun. The geocentric
system would act as a universal (Earth-centered) idea stressed the
clock and could solve the problem uniqueness of the planet, while the
of measuring longitude at sea. heliocentric (sun-centered) proposal
To establish longitude requires made Earth just one of a family
the ability to tell the time, but of planets. The assumption that
in Galileo’s day, there were no Earth does not occupy a privileged
timepieces that would work on place in the cosmos is now known
a boat. Because Jupiter is at least as the Copernican principle.

THE TELESCOPE REVOLUTION 63

The challenge now was to find Starting with the closest to Jupiter, beyond reasonable doubt. His
observations to prove that one the Galilean moons are, from left discoveries also included the
theory was correct and the other to right, Io, Europa, Ganymede, and phases of Venus, which are best
false. The discovery of moons Callisto. Ganymede is larger than explained if the planet is in orbit
around Jupiter was great support for the planet Mercury. around the sun, and the fact that
a sun-centered system. It was now the sun is spinning, shown by the
clear that everything did not orbit every year, it had to have an orbital movement of sunspots. By 1619,
around a central Earth, but there speed of 20 miles/sec (30 km/sec). Galileo’s pugnacious defense of
were still unanswered questions. In Galileo’s time, the exact distance Copernicus had drawn him into
If the sun-centered system was from Earth to the sun was not conflict with the Church, which had
correct, Earth must be moving. If known, but it was clearly far enough declared in 1616 that heliocentricism
Earth had to travel around the sun that Earth would need to be moving was heretical. In 1633, he appeared
quickly, and humans cannot before the Inquisition. His books
apprehend this movement. Also, were banned, and he spent the last
this orbital motion should make the 10 years of his life under house arrest.
stars appear to swing from side to
side every year in a phenomenon New moons
called stellar parallax (p.102). This Jupiter only had four known moons
again was not observed at the time. for 283 years. A fifth satellite,
Galileo and his contemporaries Amalthea, was discovered by the
did not suspect that the typical American astronomer E. E. Barnard
distance between stars in the in 1892, using the 36-in (91-cm)
Milky Way was about 500,000 refractor at the Lick Observatory
times larger than the distance in California. It was the last solar
between Earth and the sun, which system satellite to be discovered
makes stellar parallax so small that by direct observation. Subsequently,
it is difficult to measure. It was not satellites have been found by
until the mid-19th century that the meticulous examination of
vastly improved instruments made photographs. The number of known
it possible to detect this swing. Jupiter satellites had crept up to
12 by the mid-1950s, and has now
Despite these questions, reached 67. Many smaller moons
Galileo considered that his findings may be found in the future. ■
had proved Copernicus correct

Galileo Galilei Galileo Galilei was born in Pisa, that the planet Jupiter had
Italy on 15 February 1564. He four moons, Venus underwent
was appointed to the Chair of phase changes, the moon was
Mathematics at the University mountainous, and the sun was
of Pisa in 1589, moving to the spinning round once in about
University of Padua in 1590. a month. He was a prolific
Galileo was an astronomer, writer and made his findings
physicist, mathematician, accessible to a wide audience.
philosopher, and engineer,
who played a pivotal role in the Key works
process of intellectual advances
in Europe now known as the 1610 The Starry Messenger
Scientific Revolution. 1632 Dialogue Concerning the
Two Chief World Systems
He was the first person to 1638 The Discourses and
effectively turn the refractor Mathematical Demonstrations
telescope on the heavens. Relating to Two New Sciences
During 1609–10, he discovered

64

  CAIPRECRUFLEACRTLSYPOT
 C  ENTERED ON THE SUN

THE TRANSIT OF VENUS

IN CONTEXT I n 1639, a 20-year-old English The most recent transit of Venus
astronomer named Jeremiah in 2012 (the tiny dot in the top right
KEY ASTRONOMER Horrocks predicted a transit of the sun’s disk) was captured by
Jeremiah Horrocks of Venus across the face of the sun NASA’s Solar Dynamics Observatory.
(1618–1641) after finding errors in tables made by
Johannes Kepler. Because the transit Mercury subtended the same angle
BEFORE was only four weeks away, Horrocks as Venus. He guessed that all the
c.150 ce Ptolemy estimates the wrote to his collaborator, William planets subtend the same angle
Earth–sun distance at 1,210 Crabtree, urging him to observe at the sun, and calculated the
times Earth’s radius—around it. On December 4, 1639, Horrocks distance from Earth to the sun to
5 million miles (8 million km). and Crabtree independently set up be 59 million miles (95 million km).
helioscopes that focused an image
1619 Kepler’s third law of the sun from a telescope onto a Horrocks’s guess is now known
gives the ratio of the sizes plane. They became the first people to be wrong: Earth subtends 17.8
of planetary orbits but the to witness a transit of Venus. arcseconds at the sun, which is 93
absolute values are not known. million miles (150 million km) away.
As it crossed the sun’s disk, Nevertheless, he was the first to
1631 French astronomer Pierre Horrocks tried to calculate Venus’s have a reasonably accurate idea
Gassendi observes a transit of size and distance. He noted that it of the size of the solar system. ■
Mercury across the solar disk, subtended, or spanned, an angle
the first planetary transit to be of 76 arcseconds (76⁄3600°) at Earth
recorded in history. (p.58), which was smaller than the
value guessed by Kepler. Using the
AFTER ratios of planetary distances known
1716 Edmond Halley from Kepler’s third law, Horrocks
suggests that an accurate calculated that the disk of Venus
timing of the transit of Venus subtended an angle of about 28
could lead to an accurate arcseconds as seen from the sun.
Earth–sun distance.
Using data from a transit of
2012 The most recent transit Mercury that had taken place in
of Venus takes place. The next 1631, Horrocks calculated that
two will be in 2117 and 2125.
See also: Elliptical orbits 50–55 ■ Halley’s comet 74–77

THE TELESCOPE REVOLUTION 65

 NAREWOUMNDOOSNASTURN

OBSERVING SATURN’S RINGS

IN CONTEXT W orking at the Panzano With these observations, Cassini
Observatory near single-handedly nearly doubled
KEY ASTRONOMER Bologna, Italian the number of known satellites
Giovanni Domenico Cassini astronomer Giovanni Cassini was in the solar system. This number
(1625–1712) provided in 1664 with a state-of- has since increased dramatically.
the-art refracting telescope made by
BEFORE Guiseppe Campini of Rome. With it, Jupiter and Saturn have more
1610 Galileo announces he discovered the bands and spots than 60 known satellites each.
the discovery of four moons on Jupiter, measured the planet’s The gas giants in the outer solar
around Jupiter. spin period and polar flattening, system have two types of moon—
and made observations of the orbits large ones that were formed at the
1655 Christiaan Huygens of Jupiter’s four known moons. same time as the planet and smaller
discovers Titan, a moon orbiting ones captured from the asteroid belt.
Saturn that is 50 percent larger Observing Saturn In the inner solar system, Mars has
than Earth’s moon. Cassini’s reputation as a brilliant two small captured asteroidal moons,
observer led to an invitation to while Mercury and Venus have no
AFTER oversee the completion of the new moons. Earth has one huge moon,
1801 The first asteroid Paris observatory. There, he turned 1⁄81 its mass, and astronomers are
is discovered in an orbit his telescope on Saturn, the largest still not certain how it formed. ■
between Mars and Jupiter. moon of which, Titan, had been
discovered in 1655 by Christiaan The largest gap in Saturn’s rings,
1859 Scottish physicist Huygens. Cassini discovered two called the Cassini Division, separates
James Clerk Maxwell proves more satellites: Iapetus in 1671 and the outer A-ring from the inner B-ring.
that Saturn’s rings cannot be Rhea in 1672. In 1675, he noticed It is 3,000 miles (4,800 km) wide.
solid, since they would break a large gap in the Saturnian rings
apart under the force of gravity. and concluded, correctly, that the
rings were not solid but made up
1960s onward In recent of a multitude of small orbiting
decades, spacecraft have gone bodies. In 1684, he discovered two
into orbit around Jupiter and fainter satellites, Tethys and Dione.
Saturn, and Voyager 2 flew by
Uranus and Neptune. Large See also: Galileo’s telescope 56–63 ■ The origin of the moon 186–87 ■
numbers of moons have Huygens (Directory) 335
been discovered.

GRAVITY

MEXPOLTAIINOSNTHSE

OF THE PLANETS

GRAVITATIONAL THEORY



68 GRAVITATIONAL THEORY

IN CONTEXT G ravity is the name To myself I am only a child
given to the force of playing on the beach, while
KEY ASTRONOMER attraction between any
Isaac Newton (1642–1726) two masses. It is the force that vast oceans of truth lie
attracts all objects to Earth, giving undiscovered before me.
BEFORE them weight. It draws objects
1609 Johannes Kepler downward, toward the center Isaac Newton
shows that Mars has an of Earth. If the object were on
elliptical orbit. the moon, a much smaller mass described the relation between
than Earth, the force would be six the time taken to complete one
AFTER times less and its weight would orbit and the distance from the
1798 Henry Cavendish be one sixth of its weight on Earth. sun: the time taken for one orbit,
measures the gravitational English physicist, astronomer, and squared, was equal to the cube of
constant for the first time. mathematician Isaac Newton was the average distance between the
the first person to realize that planet and the sun. For instance,
1846 French mathematician gravity is a universal force, acting Earth goes around the sun in one
Urbain Le Verrier uses on all objects, and that it explains year, while Jupiter is 5.2 times
Newton’s laws to calculate the movement of planets. farther away from the sun than
the planet Neptune’s position. Earth. 5.2 cubed equals 140, and
the square root of 140 gives the
1915 Albert Einstein Describing orbits correct figure for one Jupiter year:
introduces general relativity The shapes of the orbits of the 11.86 Earth years.
and explains the gravitational planets were already well-known
force as a function of the in Newton’s time, based on the However, although Kepler
curvature of spacetime. three laws of planetary motion had correctly discovered the
introduced by Johannes Kepler. shapes and speeds of planetary
2014 The gravitational constant Kepler’s first law stated that these
is measured by studying the orbits were ellipses, with the sun
behavior of atoms. The latest at one focus of each ellipse. The
figure is given as 6.6719 × 10−11 second law described the way that
m3 kg−1 s−2. This is about planets moved along their orbits
1 percent less than the value more quickly when they were
Henry Cavendish calculated. close to the sun than when they
were farther away. The third law

Isaac Newton Isaac Newton was born on a farm why the equinox moved, and
in Woolsthorpe, Lincolnshire, on formalized the physics of the
December 25, 1642. After school speed of sound. He spent much
in Grantham, he attended Trinity time on biblical chronology and
College Cambridge, where he alchemy. Newton was at various
became a Fellow and taught times President of the Royal
physics and astronomy. His book Society, Warden and Master
Principia set out the principle of of the Royal Mint, and member
gravity and celestial mechanics. of parliament for Cambridge
University. He died in 1727.
Newton invented the reflecting
telescope; wrote theses on optics, Key works
the prism, and the spectrum
of white light; was one of the 1671 Method of Fluxions
founders of calculus; and studied 1687 Philosphiae Naturalis
the cooling of bodies. He also Principia Mathematica
explained why Earth was oblate 1704 Optics
(a squashed sphere) in shape and

THE TELESCOPE REVOLUTION 69

See also: Elliptical orbits 50–55 ■ Halley’s comet 74–77 ■ The discovery of Neptune 106–07 ■
The theory of relativity 146–53 ■ Lagrange (Directory) 336

The Great Comet appeared in 1680,
then again in 1681. John Flamsteed
proposed that it was the same comet.
Newton disagreed, but changed his
mind after examining Flamsteed’s data.

orbits, he did not know why the
planets moved as they did. In his
1609 book Astronomia Nova, he
suggested that Mars was being
carried around its orbit by an
angel in a chariot. A year later,
he had changed his mind,
suggesting that the planets
were magnets and were being
driven around by magnetic “arms”
extending from the spinning sun.

Newton’s insight F 1/r 2. It means that doubling comets at the time, its orbit was
Before Newton, several scientists, the distance between the objects a mystery, and the two sightings
including Englishman Robert Hooke reduces the strength of the attractive were at first not widely recognized
and Italian Giovanni Alfonso Borelli, force to a quarter of the original force. as the same object. Astronomer
suggested that there was a force John Flamsteed suggested that the
of attraction between the sun and The Great Comet two sightings might be of the same
the individual planets. They also Newton was a shy, reclusive comet, which had come from the
stated that the force decreased man, and reluctant to publish his outer edge of the solar system,
with distance. breakthrough. Two things forced swung around the sun (where it
his hand. The first was the Great was too close to the sun to be
On December 9, 1679, Hooke Comet of 1680, and the second was seen), and moved out again.
wrote to Newton saying that he the astronomer Edmond Halley.
thought the force might decrease Halley was fascinated by the
as the inverse square of distance. The Great Comet of 1680 was mysterious form of cometary orbits,
However, Hooke did not publish the brightest comet of the 17th and traveled to Cambridge to
the idea and did not possess century—so bright that for a short discuss the problem with his friend
the mathematical skills to fully time it was visible in the daytime. Newton. Using his law that related
demonstrate his proposition. By Two comets were seen: one that force to acceleration and his
contrast, Newton was able to prove was approaching the sun in insistence that the strength of the
rigorously that an inverse square November and December 1680; and force varied as the inverse square
law of attractive force would result another that was moving away from of distance, Newton calculated the
in an elliptical planetary orbit. the sun between late December parameters of the comet’s orbit as
1680 and March 1681. As with all it passed through the inner solar ❯❯
Newton used mathematics to
demonstrate that, if the force of
attraction (F) between the sun
and the planets varied precisely as
an inverse square of the distance
(r) between them, this fully explained
the planetary orbits and why they
follow Kepler’s three laws. This
is written mathematically as

70 GRAVITATIONAL THEORY

system. This breakthrough The planets’ elliptical orbits are explained
intrigued Halley so much that he by an attractive force that reduces at a rate of
went on to calculate the orbits of the square of the distance between objects.
24 othercomets, and to prove that
one comet (Halley’s comet) returned Gravity explains This force is
to the sun around every 76 years. the motions of the universal and applies
Perhaps more importantly, Halley planets, but does not
was so impressed by Newton’s explain what sets to all bodies with
work that he strongly encouraged mass at all distances.
him to publish his findings. This them in motion.
resulted in the book Philosophiae
Naturalis Principia Mathematica, In his book, Newton stressed that center. One final value is needed
published in Latin on July 5, 1687, his law was universal—gravity to calculate the force—the constant
in which Newton describes his affects everything in the universe, of proportionality, a number that
laws of motion, his gravitational regardless of distance. It explained gives the strength of the force:
theory, the proof of Kepler’s three how an apple fell on his head in the the gravitational constant (G).
laws, and the method he used to orchard of Woolsthorpe where his
calculate a comet’s orbit. mother lived, the tides in the seas, Measuring G
the moon orbiting Earth, Jupiter Gravity is a weak force, and this
The masses of orbiting the sun, and even the means that the gravitational
the two bodies elliptical orbit of a comet. The constant is rather difficult to
physical law that made the apple measure accurately. The first
(m1 and m2) fall in his yard was exactly the laboratory test of Newton’s theory
same as the one that shaped the was made by the English aristocrat
The gravitational solar system, and would later be scientist Henry Cavendish in 1798,
constant (G) discovered at work between stars 71 years after Newton’s death. He
and distant galaxies. Evidence copied an experimental system
F = Gm1m2 was all around that Newton’s law proposed by the geophysicist John
r2 of gravitation worked. It not only Michell and successfully measured
explained where planets had been, the gravitational force between
The force but also made it possible to predict two lead balls, of diameters 2 and
of attraction where they would go in the future.
between the
bodies (F) Constant of proportionality
Newton’s law of gravitation states
The distance between
the bodies (r) that the size of the gravitational

Newton’s law of universal force is proportional to the masses Nature and Nature’s
gravitation shows how the force laws lay hid in night:
produced depends on the mass of of the two btoogdeiethse(rma1nadnddivmid2)ed God said, “Let Newton
the two objects and the square multiplied be!” and all was light.
of the distance between them. Alexander Pope
by the square of the distance, r,

between them (see left). It always

draws masses together and acts

along a straight line between them. If

the object in question is spherically

symmetrical, like Earth, then its

gravitational pull can be treated as

if it were coming from a point at its

THE TELESCOPE REVOLUTION 71

Large ball Wire twists Small ball

M m
F F
m M

Henry Cavendish measured the gravitational constant using a torsion balance. Two
large balls (M) were fixed in place, while two smaller balls (m) were attached at either end
of a wooden arm suspended from a wire. The gravitational attraction (F) of the small balls
to the large ones caused the balance to rotate slightly, twisting the wire. The rotation
stopped when the gravitational force equaled the torque (twisting force) of the wire.
Knowing the torque for a given angle made it possible to measure the gravitational force.

12 in (5.1 and 30 cm) (see above). proportional to the velocity of the other and have fallen into resonant
Many have tried to refine and planet along its orbit. The slower intervals, similar to the way musical
repeat the experiment since. This a planet moved, the lower the notes resonate. Looking at three of
has led to a slow improvement in note that it emitted. The difference the moons of Jupiter, for every once
the accuracy of G. Some scientists between the notes produced by that Ganymede orbits the planet,
suggested that G changed with adjacent planets turned out to be Europa goes around twice and Io
time. However, recent analysis of well-known musical intervals such four times. Over time, they have
type 1a supernovae has shown that, as major thirds. been gravitationally locked into
over the last nine billion years, G this resonance.
has changed by less than one part There is some scientific merit
in 10 billion, if at all. The light from behind Kepler’s idea. The solar The three-body problem
distant supernovae was emitted system is about 4.6 billion years The solar system as a whole
nine billion years ago, allowing old. During its lifetime, the planets has fallen into similar resonant
scientists to study the laws of physics and their satellites have exerted proportions to Jupiter’s moons.
as they were in the distant past. gravitational influences on each On average, each planet has an
orbit that is about 73 percent larger
Seeking meaning than the planet immediately closer
Like many of the scientists of his to the sun. Here, however, there
time, Newton was deeply pious appears a difficult mathematical
and sought a religious meaning problem, and one that Newton had
behind his observations and laws. grappled with. The movement of a
The solar system was not regarded low-mass body under the gravitational
as a random collection of planets, influence of a large-mass body can
and the sizes of the specific orbits be understood, and predicted. But
were thought to have some specific when three bodies are involved,
meaning. For example, Kepler had the mathematical problem becomes
sought meaning with his notion exceedingly difficult. ❯❯
of “the music of the spheres.”
Building on ideas first put forward Distant supernovae are seen today
by Pythagoras and Ptolemy, Kepler as they were billions of years ago.
suggested that each planet was Analysis of their structure shows that
responsible for an inaudible the law of gravity operated with the
musical note that had a frequency same value of G then as today.

72 GRAVITATIONAL THEORY

An example of a three-body system 76.1, 76.3, 76.9, 77.4, 76.1, 76.5, 77.1, Gravitation is also responsible for
is the moon-Earth-sun. Newton 77.8, and 79.1 years respectively the size of the deviations from a
thought about this system but the due to the combined gravitational sphere that can occur on a planet.
mathematical difficulties were influence of the sun, Jupiter, Saturn, There are no mountains on Earth
insurmountable, and human and other planets on the comet. higher than the 5.5 miles (8.8 km)
knowledge of where the moon will of Mount Everest because the
be in the distant future is still very Shaping the planets gravitational weight of a taller
limited. Variations in the orbit of While Newton searched for religious mountain would exceed the
Halley’s comet are another indicator meaning in his scientific work, he strength of the underlying mantle
of the influence of the gravitational could find none behind his theory rock, and sink. On planets with
fields of the planets operating in of gravity. He did not discover the lower mass, the weight of objects
addition to the gravitation of the hand of God setting the planets in is less, and so mountains can be
sun. Recent orbits have taken 76.0, motion, but he had found a formula bigger. The highest mountain on
that shaped the universe. Mars, for instance, Olympus Mons,
I have not been able is nearly three times as high as
to discover the cause of The action of gravity is key to Everest. The mass of Mars is about
these properties of gravity understanding why the universe one-tenth that of Earth, and its
from phenomena, and I looks as it does. For instance, diameter is about half Earth’s.
frame no hypotheses. gravity is responsible for the Putting these numbers into
spherical shapes of the planets. Newton’s formula for gravitation,
Isaac Newton If a body has sufficient mass, the this gives a weight on the surface
gravitational force that it exerts of Mars of just over one-third that
exceeds the strength of the material on Earth, which explains the size
of the body and it is pulled into of Olympus Mons.
a spherical shape. Astronomical
rocky bodies, such as the asteroids In his great work Principia, Newton
between the orbits of Mars and plotted the parabolic path of the Great
Jupiter, are irregular in shape if Comet by taking accurate observations
they have a diameter of less than and correcting them to allow for the
about 240 miles (380 km) (the motion of Earth.
Hughes-Cole limit).

The motions of the comets THE TELESCOPE REVOLUTION 73
are exceedingly regular,
and they observe the Newton illustrated escape velocity with a thought experiment of a
same laws as the cannon firing horizontally from a high mountain. At velocities less than
motions of the planets. orbital velocity at that altitude, the cannonball will fall to earth (A and
Isaac Newton B). At exactly orbital velocity, it will enter a circular orbit (C). At greater
than orbital velocity but less than escape velocity, it will enter an
elliptical orbit (D). Only at escape velocity will it fly off into space (E).

A E
B

Gravity thus also shapes life on C
Earth by limiting the size of animals.
The largest land animals ever were D
dinosaurs weighing up to 40 tons.
The largest animals of all, whales, are or comet can have when it hits is an excellent approximation in
found in the oceans, where the water Earth’s surface, and this affects the vast majority of cases. General
supports their weight. Gravity is also the size of the resulting crater. relativity only needs to be invoked
responsible for the tides, which are in cases requiring extreme
produced because water bulges Today, gravity is held to be precision or where the gravitational
toward the sun and moon on the most accurately described by field is very strong, such as close
side of Earth nearer to them, and the general theory of relativity  to the sun or in the vicinity of a
also bulges away from them on the proposed by Albert Einstein massive black hole. Massive bodies
other side where their gravitational in 1915. This does not describe that are accelerating can produce
pull is weaker. When the sun and gravity as a force, but instead as waves in spacetime, and these
moon are aligned, there is a high a consequence of the curvature of propagate out at the speed of light.
spring tide; when they are at right the continuum of spacetime due The first detection of one of these
angles, there is a low neap tide. to the uneven distribution of mass gravitational waves was announced
inside it. This said, Newton’s in February 2016 (pp.326–29). ■
Escape velocity concept of a gravitational force
Gravity profoundly affects human
mobility. The height a person
can jump is determined by the
gravitational field at ground level.
Newton realized that the strength
of gravity would affect the ease of
travel beyond the atmosphere. To
break free from Earth’s gravitational
pull, it is necessary to travel at
25,020 mph (40,270 km/h). It is much
easier to get away from less massive
bodies such as the moon and Mars.
Turning the problem around, this
escape velocity is also the minimum
velocity that an incoming asteroid

74 IN CONTEXT

IIRTTNDHOETTAEFUHRCOREEORNYVMEEETAENAEGTRTLAWLUI1NR7TILE5HL8AT KEY ASTRONOMER
Edmond Halley (1656–1742)
 HALLEY’S COMET
BEFORE
c.350 bce Aristotle declares
that comets are weather
phenomena in Earth’s
upper atmosphere.

1577 Tycho Brahe calculates
that a comet he has observed
must exist far outside
Earth’s atmosphere.

AFTER
1758 The comet that Halley
predicted duly reappears, 76
years after its last sighting.

1819 German astronomer
Johann Encke discovers a
second periodic comet, which
reappears every 3.3 years.

1950 Dutch astronomer Jan
Oort proposes that the solar
system is surrounded by a
huge cloud of comets, and that
stars may perturb their orbits.

I n the 16th century and for
much of the 17th, advances
were made in understanding
the motions of planets, but the
nature of comets remained a
mystery. Up until at least 1500,
comets had been feared as
harbingers of doom in Europe.
Astronomers were familiar with
these bright blotches of light and
their long, beautiful tails moving
slowly across the sky over periods
of a few weeks or months, but had
no idea where they came from,
nor where they disappeared to.

Things changed, however,
in 1577, when an exceptionally
bright comet lit up the night sky
for several months. By studying

THE TELESCOPE REVOLUTION 75

See also: The Tychonic model 44–47 ■ Elliptical orbits 50–55 ■
Gravitational theory 66–73

observational data from different Halley’s comet appeared in 1066 Edmond Halley
parts of Europe, the Danish and is shown in the Bayeaux Tapestry,
astronomer Tycho Brahe calculated with Anglo-Saxons pointing fearfully Edmond Halley was born
that the comet must be at least four at the sky. Its appearance was taken in 1656 in London. In 1676,
times farther away than the moon, by some to foretell the fall of England. he sailed to the island of
and this allowed him to fit comets St. Helena in the South
into his model of the universe. theory of gravitation. Using his Atlantic where he charted
He saw them as objects that new theory, Newton analyzed and the stars of the southern
could move freely through the predicted the future path that the hemisphere, publishing a
same regions of space as planets. 1680 comet would take. He came catalog and star charts after
But what was not agreed on in to the conclusion that comets his return. In 1687, he helped
Brahe’s time, nor for many decades (like planets) had orbits in the persuade Isaac Newton to
afterward, was the shape of the shapes of ellipses, with the sun publish Principia, which
paths that comets carved through at one focus of the ellipse. These included details on how to
space. Brahe’s one-time student ellipses were so stretched out, calculate cometary orbits.
Johannes Kepler believed that they however, that they could be
traveled in straight lines. Polish approximated to an open-ended Halley was appointed
astronomer Johannes Hevelius, curve called a parabola. If Newton Astronomer Royal in 1720,
however, suggested that a comet was right, then once a comet had and he resided at the Royal
of 1664 had traveled in a curved visited the inner solar system and Observatory, Greenwich,
orbit around the sun. curved around the sun, it would until his death in 1742.
either never return (if its orbit was Although remembered mainly
Newton tackles comets parabolic) or would not return for as an astronomer, Halley
From about 1680, stimulated by thousands of years (if its orbit was did important work in many
the appearance of a particularly an extremely stretched-out ellipse, fields. He published studies
bright comet that year, the great but not a parabola). on variations in Earth’s
English scientist Isaac Newton magnetic field; invented and
began studying cometary orbits In 1684, Newton received a tested a diving bell; devised
while developing his universal visit from a young acquaintance methods for calculating life
named Edmond Halley, who was ❯❯ insurance premiums; and
produced oceanic charts of
unprecedented accuracy.

Key works

1679 Catalogus Stellarum
Australium
1705 Astronomiae cometicae
synopsis
1716 An Account of Several
Nebulae

76 HALLEY’S COMET

interested in discussing what force great book on gravity and the laws Even in an age renowned for
might account for the motions of of motion, Philosophiae Naturalis unusual savants, Halley stands
planets and other celestial bodies Principia Mathematica. out as a man of extraordinary
such as comets. Newton told his
astonished visitor that he had been Historical records breadth and depth.
studying the matter himself and Halley suggested to Newton that J. Donald Fernie
had already solved the problem (the he might apply his new theory to
answer was gravity), but that he studying the orbits of more comets. Professor Emeritus of Astronomy
had not yet published his findings. However, Newton’s mind had at the University of Toronto
This meeting eventually led to turned to other matters so, from the
Halley editing and financing the early 1690s, Halley conducted his had a few characteristics that
publication in 1687 of Newton’s own detailed study. In all, over a clearly distinguished it from the
period of more than 10 years, he orbits of other comets, such as its
Three comets of 1531, studied the orbits of 24 comets— orientation in relation to the stars.
1607, and 1682 had very some that he had observed himself, However, three of the comets he
and others for which he had had studied—one he had seen
similar orbits. obtained data from historical himself in 1682, and others
records. He suspected that, while observed by Kepler in 1607, and
The small differences some comets followed paths that Petrus Apianus in 1531—seemed
in their orbits can be are parabolas (open-ended curves) to have remarkably similar orbits.
as Newton had proposed, others He suspected that these were
accounted for in terms of followed elliptical orbits, meaning
the gravitational pull that they might pass through the
of Jupiter and Saturn. inner solar system, and thus
become visible from Earth, more
than once in a person’s lifetime.

During his studies, Halley
had noticed something strange.
In general, the orbit of each comet

Hyperbola

Parabola (ellipse stretched to infinity)

AB

The three comets D Ellipse C
are therefore the same Sun
comet, which reappears Ellipse (moderately stretched)
Earth
every 75–76 years.

The comet Some comets follow a parabolic (A),
will reappear or hyperbolic (B) path, meaning that they
around 1758. will never return. Others follow elliptical
curves of varying extent (C). Halley
suggested that if a comet followed a
moderately stretched elliptical curve (D)
it could return every 50 to 100 years.

THE TELESCOPE REVOLUTION 77

On its last appearance in 1986,
Halley’s comet passed to within
0.42 astronomical units (AU) of Earth.
It has passed much closer. In 1066,
for instance, it came within 0.1 AU.

successive reappearances, which Lepaute—spent several arduous predicted. By then, Halley had
occurred about once every 75 to months recalculating when it been dead for 17 years, but the
76 years, of the same comet, which might reappear, and where comet’s reappearance brought
was traveling on a closed, elliptical it might first be seen in the night him posthumous fame. The comet
orbit. In 1705, Halley outlined his sky. Amateur and professional was named Halley’s comet in his
ideas in a paper called Astronomiae astronomers alike began watching honor by the French astronomer
cometicae synopsis (A synopsis of for the comet’s return as early as Nicolas-Louis de Lacaille.
the astronomy of comets). He wrote: 1757. On December 25, 1758, it
“Many considerations incline me to was finally spotted by Johann Halley’s comet was the first
believe the Comet of 1531 observed Palitzsch, a farmer and amateur object other than a planet that
by Apianus to have been the same astronomer from Germany. had been proven to orbit the sun.
as that described by Kepler and It also provided one of the earliest
Longomontanus in 1607 and which The comet passed closest to the proofs of Newton’s theory of gravity,
I again observed when it returned sun in March 1759, only a couple demonstrating that the theory
in 1682. All the elements agree. of months later than Halley had could be applied to all celestial
Whence I would venture confidently bodies. Comets themselves, once
to predict its return, namely in Aristotle’s opinion feared as unpredictable omens of
the year 1758.” that comets were nothing ill fortune, were now understood.
else than sublunary vapors
One uncertainty still worried prevailed so far that this Subsequent research found
Halley. The time intervals between sublimest part of astronomy that the comet had made regular
the three appearances were not lay altogether neglected. appearances going back to at
precisely the same—they differed least 240 bce, including some
by about a year. Remembering Edmond Halley particularly bright apparitions in
research he had done some years 87 bce, 12 bce, 837 ce, 1066, 1301,
earlier on Jupiter and Saturn, and 1456. In 1986, the comet was
Halley suspected that the closely approached by spacecraft,
gravitational pull from these two which provided data on the
giant planets might slightly throw structure of its nucleus (solid part)
the comet off its course and delay and its tail. It is the only known
its timing. Halley asked Newton short-period comet (comet with
to reflect on this problem, and an orbit of less than 200 years) that
Newton came up with gravitational may be seen with a naked eye and
calculations by which Halley was appear twice in a human lifetime. ■
able to refine his forecast. His
revised prediction was that the
comet would reappear either in
late 1758 or in early 1759.

Halley is proved right
Interest in Halley’s prediction
spread throughout Europe. As
the year of the comet’s predicted
return approached, three French
mathematicians—Alexis Clairaut,
Joseph Lalande, and Nicole-Reine

78

   BOTARHRFIEETLSHTLEHIEADECNIMESTNCOATOSNUVTDREYURSIEESFUL

STELLAR ABERRATION

IN CONTEXT I n the 1720s, while seeking observer (in this case, Earth as it
proof that Earth was moving moves through space). Aberration
KEY ASTRONOMER by tracking changes in the angles are tiny: no more than the
James Bradley (1693–1762) apparent positions of stars, Oxford speed of Earth perpendicular to the
astronomer James Bradley found star’s direction divided by the speed
BEFORE another phenomenon that also of light, which is 20 arcseconds
17th century The general provided proof—stellar aberration. at most. Earth moves at about 20
acceptance of a sun-centered The aberration of light causes miles/s (30 km/s), but both its speed
cosmos leads astronomers to objects to appear to be angled and direction of travel change as it
search for stellar parallax—the toward the direction of a moving orbits the sun. As a result, a star’s
apparent movement of stars observed position follows a small
caused by Earth’s movement. Observed position ellipse around its real position.
Bradley observed this in the case of
1676 Danish astronomer Ole Real position the star Gamma Draconis—the first
Rømer estimates the speed of irrefutable proof that Earth moves.
light using observations of the Stellar aberration
Jovian satellites. is caused by He also discovered another
Earth’s movement. periodic variation in star positions,
1748 Swiss mathematician Changes in called nutation. Like aberration,
Leonhard Euler outlines the Earth’s velocity the effect is small. Earth’s spin axis
physical cause of nutation. can be detected gradually changes its orientation
through changes in space. The greatest change is
AFTER in the position precession, and a full cycle takes
1820 German optician Joseph of the stars. 26,000 years to complete. Nutation
von Fraunhofer builds a new is a small wobble in precession
type of heliometer (a device for Earth with an 18.6-year cycle. Precession
measuring the sun’s diameter) and nutation are both caused by
for the study of stellar parallax. Movement of Earth gravitational interactions between
the moon, Earth, and sun. Bradley
1838 Friedrich Bessel made his discovery public in 1748,
measures the parallax of the after 20 years of observations. ■
star 61 Cygni. He finds that it
is 600,000 times farther away See also: Shifting stars 22 ■ Stellar parallax 102 ■ Rømer (Directory) 335
from Earth than the sun.

THE TELESCOPE REVOLUTION 79

  OSAOFCUTATHTHAEELRONG SKY

MAPPING SOUTHERN STARS

IN CONTEXT F rench astronomer and Lacaille laid the
mathematician Nicolas- foundation of exact
KEY ASTRONOMER Louis de Lacaille had the sidereal astronomy in the
Nicolas-Louis de Lacaille idea to use trigonometry to measure southern hemisphere.
(1713–1762) the distance to the planets after
observing them from different Sir David Gill
BEFORE places. To provide the longest
150 ce Ptolemy lists the 48 possible baseline for his calculations, that are still recognized and used
constellations that can be seen Lacaille needed simultaneous today, and he defined the boundaries
from Mediterranean latitudes. observations in Paris and at the of existing southern constellations.
Cape of Good Hope. To this end, Before leaving South Africa, he also
1597 Petrus Plancius, a he traveled to South Africa in carried out a major surveying project
founder of the Dutch East 1750 and set up an observatory with the aim of better understanding
India Company, uses the at Cape Town. There, he not only the shape of the Earth.
findings of explorers Keyser observed the planets, but also
and de Houtman to introduce measured the positions of 10,000 Lacaille was a zealous and
12 new southern constellations southern stars. His results were highly skilled observer who
on his celestial globes. published posthumously in 1763 appreciated the value of accurate
in Coelum Australe Stelliferum. measurements. He demonstrated
c.1690 Prodomus Astronomiae, They proved to be his greatest an exceptional ability and energy
by Polish astronomer Johannes legacy to astronomy. to pioneer a thorough survey of
Hevelius, names seven new the southernmost sky. ■
constellations still in use. Southern stars
Parts of the sky surveyed by Lacaille
AFTER are too far south to be visible from
1801 Johann Bode’s Europe and many of the stars he
Uranographia, a collection of observed had not been allocated to
20 star maps, is the first near constellations. To give designations
complete guide to stars that to the stars in his catalog, Lacaille
are visible to the naked eye. introduced 14 new constellations

1910 School master Arthur See also: Consolidating knowledge 24–25 ■ The southern hemisphere 100–01
Norton produces his star atlas,
which is popular for a century.

NUREPATNUUN

1750–1850

UUNSETO

82 INTRODUCTION

French astronomer English clergyman Ernst Chladni studies
Charles Messier John Michell first reports of rock falls and
compiles a list of proposes the concept concludes that chunks
103 known nebulae. of black holes, which
he calls “dark stars.” of rock and metal
fall from space.

1771 1783 1794
1781 1786 1801

William Herschel Pierre-Simon Laplace Giuseppe Piazzi
discovers Uranus, puts forward the theory discovers Ceres, the
believing at first that he biggest asteroid in
has found a new comet. that the solar system
formed from a rotating the asteroid belt.

mass of gas.

I n the space of 75 years across than any of his contemporaries spectrum of the sun in 1800.
the 18th and 19th centuries, and was an obsessive observer Better telescopes led to far more
two new planets were of apparently boundless stamina detailed surveys of the sky. William’s
discovered, bringing the number and enthusiasm. In addition, he son, John Herschel, inherited his
of known major planets to eight persuaded members of his family father’s aptitude for astronomy and
(including Earth). However, the to help his enterprises, notably spent five years in South Africa
circumstances under which his sister Caroline, who gained completing his father’s surveys.
Neptune was found in 1846 recognition as an astronomer
were very different from those in her own right. All the effects of Nature
that resulted in the accidental are only the mathematical
identification of Uranus in 1781. William was not looking for a consequences of a small
In between these two discoveries, planet when he noticed Uranus, but number of immutable laws.
many other bodies were found in his discovery was a consequence Pierre-Simon Laplace
the solar system, showing that of his skill at telescope-making and
it contains a far greater number systematic approach to observing,
and variety of objects than had which enabled him to spot the
previously been imagined. movement of the planet over time.
Herschel also studied double and
Powers of observation multiple stars, cataloged nebulae
Briton William Herschel is and star clusters, and attempted to
considered by many to have been map the structure of the Milky Way.
the greatest visual astronomer of Always alert to the unexpected,
all time. He built better telescopes he discovered infrared radiation
by accident when studying the

French astronomer German Friedrich Bessel URANUS TO NEPTUNE 83
Jean Baptiste successfully measures the
stellar parallax of the star Neptune is discovered
Joseph Delambre very close to the
produces a good 61 Cygni to give a good position predicted
estimate of the approximation of its
speed of light. distance from Earth. through mathematics
by Urbain Le Verrier.

1809 1838 1846
1833 1845 1849

John Herschel begins a Lord Rosse makes a US astronomer Benjamin
comprehensive survey of the drawing of nebula M51, Apthorp Gould boosts
southern sky to complement now called the Whirlpool US astronomy by founding
The Astronomical Journal.
his father’s surveys of the galaxy, showing its
northern sky. spiral structure.

William Parsons, 3rd Earl of Rosse, the first asteroid, Ceres, in 1801, interplay of gravitational forces
took the next big step in the as he was observing for a new star between the larger bodies of the
investigation of nebulae. In the catalog. Three more were located solar system. The calculations
1840s, he set himself the ambitious in the following six years. The next of German mathematician Carl
task of constructing the largest was not found until 1845, after which Friedrich Gauss in 1801 allowed
telescope in the world. With it, the pace of discovery increased. Ceres to be relocated, while
he discovered that some nebulae between 1799 and 1825, Frenchman
(which we now know to be Meanwhile, German Ernst Pierre-Simon Laplace produced
galaxies) have a spiral structure. Chladni had correctly concluded a monumental definitive work
that meteorites reaching Earth are on celestial mechanics.
More planets chunks of rock and metal from space.
Herschel’s discovery of Uranus Clearly, the solar system contained It soon became evident that
aroused fresh interest in the wide a great variety of bodies. Uranus was not following its
gap in the solar system between predicted course. The pull of an
the orbits of Mars and Jupiter. The might of mathematics unknown planet was suspected.
The regular spacing of the other By contrast to the fortuitous Building on Laplace’s work,
planets suggested that there was discovery of Uranus, the discovery compatriot Urbain Le Verrier
an unknown planet in the gap. of Neptune was a demonstration tackled the problem of predicting
It turned out to be occupied not of the power of mathematics. While the undiscovered planet’s possible
by a single major planet but by astronomers were working with position. Neptune was duly found
numerous minor planets, which better telescopes, mathematicians close to where Le Verrier thought
William Herschel dubbed “asteroids.” grappled with the practical it would be. For the first time,
Italian Giuseppe Piazzi discovered difficulties of applying Newton’s astronomers now had an idea of the
theory of gravitation to the complex true extent of the solar system. ■

84

ICCFHOOMAUNENGTDEFDTOHIRTASTITPITHLAAISSCEA

OBSERVING URANUS

IN CONTEXT Uranus has been observed but
not recognized as a planet.
KEY ASTRONOMER
William Herschel (1738–1822) Observations a few days apart show that it has moved,
meaning that it might be a comet.
BEFORE
1660s Mirror-based reflecting Calculations show that its orbit is almost
telescopes are developed by circular, so it must be a planet.
Isaac Newton and others.
Irregularities in its orbit indicate that there
1690 John Flamsteed may be an eighth planet in the solar system.
observes Uranus, but
believes it to be a star. U ranus, the seventh planet a star. It was also observed by the
from the sun, is visible French astronomer Pierre Lemonier
1774 French astronomer to the naked eye, and several times between 1750 and
Charles Messier publishes it is believed that the ancient Greek 1769. However, none of the observers
his astronomical survey, which Hipparchus observed it in 128 bce. figured out that it was a planet.
inspires Herschel to begin The development of telescopes
work on a survey of his own. in the 17th century led to further William Herschel observed
sightings, such as one by English Uranus on March 13, 1781, while
AFTER astronomer John Flamsteed in 1690 looking for multiple star systems.
1846 Unexplained changes when it was recorded as 34 Tauri, He spotted it again four nights
to the orbit of Uranus lead later, and on this second occasion
French mathematician Urbain
Le Verrier to predict the
existence and position of
an eighth planet—Neptune.

1930 US astronomer Clyde
Tombaugh discovers Pluto, a
ninth planet. It has since been
reclassified as a dwarf planet,
the brightest member of the
Kuiper belt of small, icy worlds.

URANUS TO NEPTUNE 85

See also: Shifting stars 22 ■ Gravitational theory 66–73 ■ The discovery of Neptune 106–07

I compared it to H. Geminorum at Herschel’s discovery and decided predicted according to Newton’s
and the small star in the that the new object was as likely laws—irregularities that could only
to be a planet as a comet. Swedish- be explained by the gravitational
quartile between Auriga and Russian Anders Johan Lexell influence of an eighth, even more
Gemini, finding it so much and German Johann Elert Bode distant planet. This led to the
larger than either of them. independently computed the discovery of Neptune by Urbain
orbit of Herschel’s object and Le Verrier in 1846. ■
William Herschel concluded that this was a planet
in a near-circular orbit, roughly Herschel observed Uranus using a
he noticed that its position had twice as far away as Saturn. 7-ft (2.1-m) reflector telescope. He would
changed in relation to the stars later construct a 40-ft (12-m) telescope,
around it. He also noted that if Naming the planet which was the largest telescope in the
he increased the power of the Herschel’s discovery was praised world for half a century.
telescope he was using the new by King George III, who appointed
object increased in size more Herschel “The King’s Astronomer.”
than the fixed stars. These two Maskelyne asked Herschel to name
observations indicated that it was the new planet, and he chose
not a star, and when he presented Georgium Sidus (George’s Star) in
his discovery to the Royal Society honor of his patron. Other names,
he announced that he had found including Neptune, were proposed,
a new comet. The Astronomer and Bode suggested Uranus. His
Royal, Nevil Maskelyne, looked suggestion became universal in
1850, when the UK’s Greenwich
Observatory finally abandoned
the name Georgium Sidus.

The detailed study of the
orbit of Uranus by subsequent
astronomers showed that there
were discrepancies between
its observed orbit and the orbit

William Herschel Born in Hanover, Germany, Herschel discovered a new
Frederick William Herschel form of radiation, now known
emigrated to Britain at the age as infrared radiation.
of 19 to make a career in music.
His studies of harmonics and Herschel’s sister Caroline
mathematics led to an interest in (1750–1848) acted as his
optics and astronomy, and he set assistant, polishing mirrors
out to make his own telescopes. and recording and organizing
his observations. She began
Following his discovery of to make observations of her
Uranus, Herschel detected two own in 1782, and went on to
new moons of Saturn and the discover a number of comets.
largest two moons of Uranus.
He also showed that the solar Key works
system is in motion relative
to the rest of the galaxy and 1781 Account of a Comet
identified numerous nebulae. 1786 Catalogue of 1,000 New
While studying the sun in 1800, Nebulae and Clusters of Stars

86

  OWTHFAETSHBAERLITSGETHRATERNDESS

VARIABLE STARS

IN CONTEXT A ncient Greek astronomers Bright star
were the first to classify
KEY ASTRONOMER stars by their apparent A
John Goodricke (1764–1786) brightness—that is, their brightness
as observed from Earth. In the Dim star
BEFORE 18th century, British amateur
130 bce Hipparchus defines astronomer John Goodricke grew B
a magnitude scale for the interested in changes in apparent
apparent brightness of stars, brightness after his neighbor, In an eclipsing binary system,
which is popularized by astronomer Edward Pigott, maximum brightness occurs when
Ptolemy in the Almagest. provided him with a list of stars both stars are visible (A); minimum
known to vary. In the course of his brightness occurs when the dim star
1596 David Fabricius observations, he discovered more. is eclipsing the bright star (B).
discovers that the star Mira
Ceti varies in brightness with In 1782, Goodricke observed Goodricke also discovered that the
periodic regularity. the variation in brightness of Algol, star Delta Cephei in the constellation
a bright star in the constellation Cepheus varies in brightness with
AFTER Perseus. He was the first person a regular period. It is now known
1912 Henrietta Swan Leavitt to propose a reason for this change that Delta Cephei is one of a class
discovers that the period of in brightness, suggesting that of stars whose apparent brightness
some variable stars is related to Algol was in fact a pair of stars varies because the star itself
their absolute (true) brightness. orbiting one another, with one changes. Stars such as this are
brighter than the other. When called Cepheid variables, and they
1913 Ejnar Hertzsprung the dimmer of the two stars are key to calculating the distance
calibrates this variation in passed in front of the brighter to other galaxies.
brightness, allowing Cepheid one, the eclipse would reduce the
variables to be used as brightness detected by observers. Goodricke presented his
“standard candles” to calculate Today this is referred to as an findings to the Royal Society in
the distance to galaxies. eclipsing binary system (it is 1783. He died shortly after from
now known that Algol is actually pneumonia, at just 21 years old. ■
1929 Edwin Hubble identifies a three-star system).
the link between the velocity
of a galaxy and its distance. See also: A new kind of star 48–49 ■ Measuring the universe 130–37 ■
Beyond the Milky Way 172–77

URANUS TO NEPTUNE 87

IOTASHRUTEERHNTMEHEIDBLEWKUCYLIETALWIELEAISNYG

  MESSIER OBJECTS

IN CONTEXT B y the 18th century, large listed 80 objects. These nebulae
telescopes that could are now known as Messier objects.
KEY ASTRONOMER magnify images by Other astronomers added further
Charles Messier (1730–1817) several hundred times were being nebulae that were observed by
produced. This allowed astronomers Messier but not recorded by him
BEFORE to identify various fuzzy patches in his catalog, bringing the
150 ce Ptolemy records five of light, which were called nebulae, total to 110.
stars that appear nebulous and after the Latin word for “cloud.”
one nebula not linked to a star. With more powerful telescopes,
French astronomer Charles it has been possible to determine
964 Persian astronomer Messier was primarily interested in the nature of the Messier objects.
Abd al-Rahman al-Sufi notes finding comets, which often look just Some are galaxies beyond the
several nebulae in his Book like nebulae. A fuzzy object could Milky Way, some are clouds of
of Fixed Stars. only be identified as a comet if it gas where stars are forming, and
changed position against the stars others are the remains of supernova
1714 Edmond Halley publishes over a period of weeks or months. explosions or the gas thrown off by
a list of six nebulae. Messier therefore compiled a list of dying stars the size of our sun. ■
known nebulae to eliminate them
1715 Nicolas Louis de Lacaille as potential comets. His initial list Messier 31 is also known as the
identifies 42 nebulae. was published in 1774 and identified Andromeda Galaxy. It is the nearest
45 nebulae. The final 1784 version major galaxy to the Milky Way.
AFTER
1845 Lord Rosse observes See also: Halley’s comet 74–77 ■ Mapping southern stars 79 ■ Examining
that some nebulae have a nebulae 104–05 ■ Properties of nebulae 114–15 ■ Spiral galaxies 156–61
spiral structure.

1864 William Huggins
examines the spectra of 70
nebulae, finding that a third of
them are clouds of gas, while
the rest are masses of stars.

1917 Vesto Slipher
identifies spiral nebulae
as distant galaxies.

88

OOCNFONTTSHHTEERHUECATVIEONNS

THE MILKY WAY

IN CONTEXT O ne of the most spectacular From Earth, the Milky Way appears
features in the sky visible as a band of light whose individual
KEY ASTRONOMER to the naked eye is the stars cannot be seen with the naked
William Herschel (1738–1822) dense band of light called the Milky eye. The band is the galaxy’s disk-
Way. This light from billions of stars shaped structure viewed from within.
BEFORE is not seen by many people today
1725 English astronomer because of light pollution, but was a randomly scattered but formed
John Flamsteed’s catalog of common sight before street lighting. a vast ring around Earth, held
3,000 stars is issued, followed together by gravity.
by his star atlas in 1729. In the 1780s, British astronomer
William Herschel attempted to The Milky Way appeared to circle
1750 Thomas Wright suggests determine the shape of the Milky Earth and so Herschel concluded
that the solar system is part Way and the sun’s position within that the galaxy was disklike. He
of a disk of stars. it by observing the stars. In this observed the numbers of stars of
endeavor, Herschel built upon different magnitudes (brightness)
1784 Charles Messier the work of his compatriot Thomas and discovered that these were
produces his final catalog Wright, who, in 1750, had argued equally distributed within the band
of nebulae. that the stars appeared as a band of the Milky Way in all directions.
of light because they were not This led him to assume that the
AFTER
1833 John Herschel continues
his father’s work, publishing a
systematic mapping of the sky
including observations made
from the southern hemisphere.

1845 Lord Rosse observes
that some nebulae have a
spiral structure.

1864 William Huggins uses
emission spectra to determine
that some nebulae are masses
of stars.

URANUS TO NEPTUNE 89

See also: Messier objects 87 ■ The southern hemisphere 100–01 ■ Properties of nebulae 114–15 ■
Spiral galaxies 156–61 ■ The shape of the Milky Way 164–65

Far 3kpc Carina–Sagittarius

Norma I have observed stars of
Scutum– which the light, it can be
Centaurus proved, must take two million
years to reach the Earth.

William Herschel

Perseus

Near 3kpc

New Outer Sun Orion–Cygnus

The Milky Way comprises stellar arms spiraling out from the nature and size to the Milky Way,
bulging “bar” at the center. The arms are labeled here. The sun is decades before it was confirmed
located in the Orion–Cygnus arm, 26,000 light-years from the center. that nebulae were in fact galaxies
in their own right.
brightness of a star indicated its new objects in 1786, with further
distance from Earth, with dimmer catalogs appearing in 1789 and The current model of the Milky
stars being more distant. The 1802. Herschel classified the objects Way is a barred spiral galaxy.
even distribution, he believed, must he listed into eight categories, Around two-thirds of spiral
mean that the solar system was depending on their brightness, galaxies have central bars like
close to the center of the galaxy. size, or whether they appeared the Milky Way’s. The early idea
Herschel’s model was refined by to consist of dense or scattered of a disk of stars is broadly correct,
other astronomers, but was not clusters of stars. He also conjectured but the stars within the disk are
replaced until the early 20th century. that most nebulae were similar in arranged in a series of spiral arms,
with the sun in a sparse area of
the Orion–Cygnus arm. ■

New nebulae There is a dense The solar system
Herschel did not limit himself to the band of stars across is positioned within
study of stars in his investigation
into a galaxy’s shape. He also the night sky. a disk of stars.
observed nebulae, the fuzzy patches This suggests that Stars of different
of light that dotted the sky. Herschel the solar system magnitudes are
was a skilled telescope-maker as is in the center distributed evenly
well as an astronomer, and used within this band.
two large, powerful telescopes of the disk.
with 49½-in (126-cm) and 18½-in
(47-cm) apertures. From 1782, he
used these instruments to conduct
systematic observations of the
“deep sky,” searching for objects
that were not stars. He listed these
as nebulae or as clusters of stars,
and published details of 1,000

90

RFROOCMKSSFPAALCLE

ASTEROIDS AND METEORITES

IN CONTEXT I n the 18th century, the real and flung into the air. Chladni
source and nature of what are then examined an object found in
KEY ASTRONOMER now called meteorites was not 1772 that had a mass of more than
Ernst Chladni (1756–1827) known. Interplanetary space was 1,500 lb (700 kg). It had a rough
thought to be empty, and the fiery surface, was filled with cavities,
BEFORE lumps of rock and iron that fell from and was totally unlike the rock of
1718 Isaac Newton proposes the sky were believed to originate the landscape where it was found.
that nothing can exist either in volcanoes on Earth that It had also very clearly been melted.
between the planets. had thrown them up, or from dust
in the atmosphere, perhaps by the Falling from space
1771 A spectacular fireball is action of lightning. Chladni realized that neither
recorded passing over Sussex lightning nor a forest fire could
in southern England and This idea can be traced back have produced enough heat to
continuing to be seen over to Isaac Newton, who wrote that melt bedrock (the solid rock that
northern France. it was “necessary to empty the underlies loose deposits). Yet the
Heavens of all Matter” in order for rock he examined had become a
AFTER the planets and comets to move mass of metallic iron. This “iron,”
1798 British chemist unimpeded in their regular orbits. he concluded, could only have come
Edward Howard and French from space. It had melted on its
mineralogist Jacques-Louis In the early 1790s, a German passage through the atmosphere.
de Bournon analyze stones physicist named Ernst Chladni
and irons from falls in Italy, attempted to solve the mystery
England, and India. They find of these “fallen stones” by
similar proportions of nickel examining historical records.
in the stones, indicating a link One that he studied had landed in
between them. 1768 in France, where it had been
subjected to chemical analysis.
1801 Giuseppe Piazzi The results showed that it had
discovers Ceres, the largest formed from a lump of sandstone
object in the asteroid belt, that had been struck by lightning
now classed as a dwarf planet.
This iron-nickel meteorite was found
on an Arctic ice sheet. The odd shape
of the meteorite is due to spinning and
tumbling at a high temperature on
entry into the atmosphere.

URANUS TO NEPTUNE 91

See also: Gravitational theory 66–73 ■ The discovery of Ceres 94–99 ■
Investigating craters 212

Reports of rock-falls from the sky are all very similar.

These are The rocks do not
reliable reports. resemble local rocks.

The rocks melted as The rocks show the effects Ernst Chladni
they fell through the of extreme heating.
Ernst Chladni was born in
atmosphere. Saxony to a family of prominent
academics. Chladni’s father
Rocks fall from space. disapproved of his son’s
interest in science and
Chladni published his findings in Jean-Baptiste Biot investigated insisted he study law and
a book in 1794, which set out his this fall. He concluded that philosophy. He obtained a
main conclusions: that masses of they could not have originated degree in these subjects from
iron or stone fall from the sky; and anywhere nearby. the University of Leipzig in
that friction in the atmosphere 1782. However, when his
causes them to heat up, creating Solar system fragments father died that year, he
visible fireballs (“shooting stars”); Thanks to Chladni’s work, scientists turned to physics.
that the masses do not originate in know that shooting stars are lumps
Earth’s atmosphere but far beyond of rock or metal from space heated Initially, Chladni applied his
it; and that they are fragments of to glowing point as they pass physics knowledge to work in
bodies that never joined together through the atmosphere. The object acoustics, which brought him
to make planets. that causes the glowing trail is renown. He identified the way
called a meteor. If any of it survives rigid surfaces vibrate, and his
Chladni’s conclusions were to reach the ground, it is termed a observations were applied
correct, but at the time he was meteorite. Meteorites can originate to the design of violins. His
ridiculed—until some chance in the asteroid belt between Jupiter later work on meteorites drew
rock-falls helped to change opinion. and Mars, or they can be rocks less favorable attention from
The first of these took place within thrown up from Mars or the moon. the scientists of the day, and
two months of the publication Many meteorites contain small might have vanished into
of Chladni’s book, when a large particles called chondrules, which obscurity had it not been for
fall of stones came down on the are thought to be material from the the popular writing of Jean-
outskirts of Siena, Italy. Analysis asteroid belt that never formed into Baptist Biot, whose findings
of them showed they were very larger bodies. These are some of supported Chladni’s ideas.
different from anything found on the oldest materials in the solar
Earth. Then, in 1803, nearly 3,000 system, and can tell scientists Key works
stones fell in fields around L’Aigle much about its early composition. ■
in Normandy. French physicist 1794 On the Origin of the
Iron Masses Found by 
Pallas and Others Similar
to it, and on Some Associated
Natural Phenomena
1819 Igneous Meteors and the
Substances that have Fallen
from them

92

OTHFETHMEECHHEAAVNEISNMS

GRAVITATIONAL DISTURBANCES

IN CONTEXT There are Without divine
disturbances in intervention these
KEY ASTRONOMER the mechanism disturbances look like they
Pierre-Simon Laplace of the heavens. should make the orbits of
(1749–1827) the planets unstable.

BEFORE But the disturbances continually self-correct over time.
1609 Johannes Kepler
determines that the planets The self-correction is made by the force of
move in elliptical orbits. gravity that caused the disturbance itself.

1687 Isaac Newton publishes B y the end of the 18th planets’ orbits. By this he meant
Principia Mathematica, which century, the structure a disturbance to the orbits caused
includes his law of universal of the solar system was by an additional force, which
gravitation and a mathematical well-known. The planets moved would make the orbits unstable if
derivation of Kepler’s laws of in elliptical orbits around the not corrected. As a result, Newton
planetary motion. sun, held in place by gravity. decided that the hand of God was
Isaac Newton’s laws allowed occasionally required to maintain
1734 Swedish philosopher a mathematical basis for this the solar system in a stable state.
Emanuel Swedenborg outlines model of the solar system to be
the nebular theory of the developed, but there were still Orbital resonance
formation of the solar system. problems. Newton himself tested French mathematician Pierre-
his ideas against observations, Simon Laplace rejected the notion
AFTER but noted “perturbations” to the of divine intervention, however. In
1831 Mary Somerville translates
Laplace’s Méchanique céleste
to English.

1889 French mathematician
Henri Poincaré shows that
it is not possible to prove that
the solar system is stable,
laying the foundations for
chaos theory.

URANUS TO NEPTUNE 93

See also: Elliptical orbits 50–55 ■ Galileo’s telescope 56–63 ■ Gravitational theory 66–73 ■
The theory of relativity 146–53 ■ Delambre (Directory) 336

1784, he turned his attention to Orbital resonance occurs Saturn
a long-standing question known when the gravity of orbiting
as the “great Jupiter-Saturn bodies produces a stable Sun
inequality.” Laplace showed that and self-correcting Jupiter
perturbations in the orbits of these system. An example
two planets were due to the orbital is the orbits of the
resonance of their motions. This neighboring giant
refers to the situation in which the planets Jupiter
orbits of two bodies relate to each and Saturn, whose
other in a ratio of whole numbers. orbital periods are
In the case of Jupiter and Saturn, in a ratio of 5:2.
Jupiter orbits the sun almost
exactly five times for every two Two orbits
orbits of Saturn. This means that
their gravitational fields have a Five orbits
greater effect on each other than
they would for orbits that are not cooled and contracted, breaking Mary Somerville, and this led to
in resonance. off rings from its outer edge. The a wide dissemination of his ideas.
core material formed the sun and Using Laplace’s new theorems,
The nebular hypothesis the matter in the rings cooled to his fellow countryman Jean
Laplace published his work on form the planets. Baptiste Joseph Delambre was
the solar system in two influential able to produce far more accurate
books—a popular account called Shortly after Laplace’s death, tables predicting the motions of
Exposition du système du monde his work was translated into English Jupiter and Saturn. ■
and the mathematical Méchanique by the Scottish mathematician
celeste. In his Exposition, Laplace
explored the idea that the solar
system developed from a primeval
nebula. Laplace described a
rotating mass of hot gases that

Pierre-Simon Laplace Pierre-Simon Laplace was born When Napoleon seized power
in Normandy, France, to a minor in 1799, Laplace became a
landowner. His father intended member of the senate and
him for the Church, and he studied served on many scientific
theology at the University of Caen, commissions. He continued
but it was there that he developed to work on the mathematics
his interest in mathematics. He of astronomy until his death,
gave up any intention of entering publishing five volumes on
the priesthood and moved to Paris, celestial mechanics.
where he obtained a teaching
position in the École Militaire. Key works
Here, he taught a young Napoleon
Bonaparte. The post allowed him 1784 Théorie du movement et de
time to devote himself to research, la figure elliptique des planètes
and through the 1780s, he 1786 Exposition du système
produced a string of influential du monde
mathematical papers. 1799–1825 Méchanique céleste

I SURMISE THAT

BETTER IT COULD BE SOMETHING

 THAN A COMET

THE DISCOVERY OF CERES



96 THE DISCOVERY OF CERES

IN CONTEXT The orbits of the F or centuries, the number
planets appear to follow a of known “wandering
KEY ASTRONOMER mathematical formula. stars,” or planets, that
Giuseppe Piazzi (1746–1826) The formula predicts that trailed through the night sky was
the gap between Mars five. Together with the sun and
BEFORE moon, that brought the total of
1596 Johannes Kepler suggests and Jupiter should major celestial bodies visible from
there are unobserved planets contain an orbiting body. Earth to seven—a number imbued
in the solar system. with mystical significance. Then,
Ceres, a body seen in in 1781, William Herschel spotted
1766 Johann Titius predicts the gap, is too small to be Uranus out beyond the orbit of
the gap between Mars and a planet, but it does not Saturn, which forced astronomers
Jupiter contains a planet. to rethink this number. However,
have the orbit of when the new planet’s orbit was
1781 William Herschel’s a comet. placed in an updated plan of the
discovery of Uranus confirms solar system, it revealed another
the pattern of orbits proposed Ceres is a minor planet, number conundrum.
by Johann Bode. or asteroid—one of
thousands in that Finding a gap
1794 Ernst Chladni suggests region of space. In 1766, a German astronomer
meteorites are rocks that were named Johann Titius discovered
once in orbit. a mathematical link between the
orbital distances of the planets.
AFTER He divided the orbital distance
1906 Trojan asteroids are of Saturn by 100 to create a unit
found in the orbit of Jupiter. to measure all the other orbits.
Mercury’s orbit was 4 units from
1920 Hidalgo, the first “centaur” the sun, and every other planet’s
asteroid (an asteroid with position from there was linked
an unstable orbit), is found to a doubling of 3, or the number
between Jupiter and Neptune. sequence 0, 3, 6, 12, 24, 48, and 96.
So Mercury was located at 4 + 0
2006 Ceres is classified units from the sun, Venus at 4 + 3,
as a dwarf planet.

Guiseppe Piazzi As was common for younger time. Piazzi was famed for
sons in wealthy Italian families, his diligence, and would take
Guiseppe Piazzi’s career began measurements on at least four
in the Catholic Church. By his consecutive nights to average
mid-20s, it was obvious that out errors. In 1806, he recorded
his abilities lay in academia. In the large proper motion of the
1781, he was appointed math star 61 Cygni. This prompted
professor at a newly founded several astronomers to use the
academy in Palermo, Sicily, but parallax of that star to measure
soon switched to astronomy. the distance between stars.
His first task in this role was to
build a new observatory, which Key works
he equipped with the Palermo
Circle, a telescope built in London 1803 Præcipuarum stellarum
with a 5-ft (1.5-m) wide altitude inerrantium (Star catalog)
scale. It was the most accurate 1806 Royal Observatory of
telescope in the world at the Palermo (Book 6)

URANUS TO NEPTUNE 97

See also: Elliptical orbits 50–55 ■ Observing Uranus 84–85 ■ Asteroids and meteorites 90–91

From Mars there follows
a space of 4 + 24 = 28 such
parts, but so far no planet
was sighted there. But should
the Lord Architect have left
that space empty? Not at all.

Johann Titius

Earth at 4 + 6, and Mars was at Schröter decided to launch a search Photographed by NASA’s Dawn
4 + 12. Jupiter was at 4 + 48 and of the gap. Their plan was to divide spacecraft in 2015, Ceres is the largest
Saturn was at 4 + 96. There was no up the zodiac—the strip of sky in object in the asteroid belt, and the
known planet in the sequence at which all the planets move—and only object large enough to have been
4 + 24 = 28, so there appeared to be ask Europe’s top 24 astronomers made spherical by its own gravity.
a gap in the solar system between to patrol one zone each, searching
Mars and Jupiter. Titius proposed for planetlike motion. The team as the Palermo Circle. Although
that the gap must be occupied by they put together was dubbed the it was not the most powerful
an unknown body. However, his Celestial Police. But in the end telescope of its day, its altazimuth
findings seemed too good to be it was straightforward luck, not mounting could move both vertically
true—and the results for Mars and efficiency, that filled the gap. and horizontally, enabling it to
Saturn were slightly out, so few make very accurate measurements
astronomers paid them much heed. Surveying telescope of stellar positions, a feature that
One of the astronomers among would pay rich dividends.
A few years later, in 1772, a the Celestial Police was Giuseppe
fellow German named Johann Piazzi, who was based in Palermo, On the evening of New Year’s
Bode published a slightly modified Sicily. Like most astronomers at the Day 1801, the instructions from
version of Titius’s work, which met time, Piazzi was mainly concerned the Celestial Police were still en
with greater acclaim. As a result, with creating accurate star maps. route to Piazzi so he spent the
the theory is best remembered To that end, he had acquired a evening surveying stars and
as Bode’s law. When Uranus was surveying telescope now known recorded a new, faint object (with
discovered, Bode’s law predicted a magnitude of eight) in the ❯❯
that it would be 196 units from
the sun. It was finally shown to
be nearer to 192 units, but that
seemed close enough. Surely, it
meant the 28-unit gap must also
contain a planet.

In 1800, a group of German-based
astronomers led by Franz Xaver von
Zach, Heinrich Olbers, and Johann

98 THE DISCOVERY OF CERES

constellation of Taurus. The approached the sun, Piazzi’s do so.) Other astronomers
following night, Piazzi checked his discovery took a more stable, preferred the name Hera, but
measurements and found that the circular path. In his letter to Bode, Piazzi, still the only person who
object had moved slightly. This Piazzi made his suspicions clear: had actually seen the object, had
meant it was definitely not a star. this could be the missing planet opted for Ceres, after the Roman
everyone was looking for. goddess of agriculture.
Piazzi watched the object for
24 days before informing Bode. On hearing the news in late By June, Ceres’s orbit had taken
He thought at first that it was March, Bode wasted no time it into the glare of the sun. Piazzi
a comet—a relatively common in announcing the discovery of had been sick in the interim and
discovery—but his observations a new planet, which he named so had not had the chance to
soon suggested otherwise. He Juno. (He had recently chosen map anything but the simplest
could see no fuzzy coma, or tail, the name for Uranus, and clearly orbital arc. He calculated that his
and while comets sped up as they felt confident in his right to discovery would be visible again
in the fall. But, try as they might,
neither Piazzi nor anyone else
could find Ceres.

Mathematical hunch
Von Zach decided to follow a
hunch, and sent the details of
Ceres’s orbit to the mathematician
Carl Friedrich Gauss. In less than
six weeks, Gauss calculated all
the places Ceres was likely to be. It
took von Zach most of December to
search through Gauss’s predictions,
but on the evening of New Year’s
Eve 1801, almost exactly a year
to the day after it was first seen,
he found Ceres once again.

The orbital distance of Ceres
was 27.7 Bode units, a remarkably
close fit to the predicted location.
However, the orbital data showed
that this new member of the solar
system was far smaller than the
known planets. William Herschel’s
early estimate put Ceres at just
160 miles (260 km) across. A few
years later, Schröter proposed a
diameter of 1,624 miles (2,613 km).
The actual figure is 588 miles (946
km), which means it would be
a comfortable fit over the Iberian
Peninsula or Texas.

Piazzi’s telescope, the Palermo
Circle, was built by Jesse Ramsden.
Its precision mounting allowed it to
measure stellar positions with an
accuracy of a few seconds of an arc.