The Astronomy Book (Big Ideas Simply Explained)

COSMIC EXPANSION THE UNMOVING THE SEARCH FOR
IS ACCELERATING STARS GO
UNIFORMLY
GRAVITY EXPLAINS WESTWARD
THE MOTIONS OF
THE PLANETS THE EXTRATERRESTRIAL
UNIVERSE IS INTELLIGENCE IS
A SEARCH FOR
OURSELVES
EXPANDING
IN ALL
DIRECTIONS RIPPLES THROUGH SPACETIME

THE THE
ASTRONOMYI FOUND WAY TO
BOOKCOMET, FOR THE
THAT IT IS A
STARS IS
IT HAS BIG IDEAS SIMPLY EXPLAINED OPEN
CHANGED
ITS PLACE
THE MOST TRUE PATH FINALLY WE
OF THE PLANET IS SHALL PLACE
THE SUN HIMSELF
AN ELLIPSE AT THE CENTER

OF THE UNIVERSE

A SLOW AN EXACT STARS ARE
PROCESS OF SOLUTION TO FACTORIES FOR
ANNIHILATION RELATIVITY THE CHEMICAL
OF MATTER
PREDICTS ELEMENTS
BLACK HOLES









DK LONDON PRE-PRODUCTION PRODUCER First American Edition, 2017
Jacqueline Street-Elkayam Published in the United States by
SENIOR EDITOR DK Publishing, 345 Hudson Street,
Victoria Heyworth-Dunne SENIOR PRODUCER
Mandy Inness New York, New York 10014
US EDITOR
Margaret Parrish DK DELHI Copyright © 2017
Dorling Kindersley Limited
SENIOR ART EDITORS JACKET DESIGNER DK, a Division of Penguin
Gillian Andrews, Nicola Rodway Suhita Dharamjit
Random House LLC
MANAGING EDITOR EDITORIAL COORDINATOR 17 18 19 20 21 10 9 8 7 6 5 4 3 2 1
Gareth Jones Priyanka Sharma
001—283974—Sep/2017
SENIOR MANAGING ART EDITOR SENIOR DTP DESIGNER
Lee Griffiths Harish Aggarwal All rights reserved.
Without limiting the rights under the
ART DIRECTOR MANAGING JACKETS EDITOR copyright reserved above, no part of this
Karen Self Saloni Singh publication may be reproduced, stored in
or introduced into a retrieval system, or
ASSOCIATE PUBLISHING produced for DK by transmitted, in any form, or by any means
DIRECTOR (electronic, mechanical, photocopying,
Liz Wheeler TALL TREE LTD. recording, or otherwise), without the prior
written permission of the copyright owner.
PUBLISHING DIRECTOR EDITORS
Jonathan Metcalf Rob Colson, David John Published in Great Britain by
Dorling Kindersley Limited.
SENIOR JACKET DESIGNER DESIGN
Mark Cavanagh Ben Ruocco A catalog record for this book is available
from the Library of Congress.
JACKET EDITOR ILLUSTRATIONS
Claire Gell James Graham ISBN: 978-1-4654-6418-7

JACKETS DESIGN original styling by DK books are available at special discounts
DEVELOPMENT MANAGER when purchased in bulk for sales
STUDIO 8
Sophia MTT promotions, premiums, fund-raising,
or educational use. For details, contact:
DK Publishing Special Markets, 345 Hudson

Street, New York, New York 10014
[email protected]

Printed in China

A WORLD OF IDEAS:
SEE ALL THERE IS TO KNOW

www.dk.com

CONTRIBUTORS ROBERT DINWIDDIE

JACQUELINE MITTON, CONSULTANT EDITOR Robert Dinwiddie is a science writer specializing in
educational illustrated books on astronomy, cosmology,
Jacqueline Mitton is the author of more than 20 books earth science, and the history of science. He has written
on astronomy, including books for children. She has been or contributed to more than 50 books, including the DK
a contributor, editor, and consultant for many other books. titles Universe, Space, The Stars, Science, Ocean, Earth,
Becoming an astronomer was Jacqueline’s childhood and Violent Earth. He lives in southwest London and
ambition. She studied physics at Oxford University and enjoys travel, sailing, and stargazing.
then earned her Ph.D. at Cambridge, where she still lives. 
PENNY JOHNSON
DAVID W. HUGHES
Penny Johnson started out as an aeronautical engineer,
David W. Hughes is Emeritus Professor of Astronomy working on military aircraft for 10 years, before becoming
at the University of Sheffield, UK. He is an international a science teacher, and then a publisher producing science
authority on comets, asteroids, and the history of astronomy. courses for schools. Penny has been a full-time educational
He has spent more than 40 years explaining the joys of writer for the last 15 years.
astronomy and physics to his students, and has published
well over 200 research papers, as well as books on the moon, TOM JACKSON
the solar system, the universe, and the Star of Bethlehem.
He was a co-investigator on the European Space Agency’s Tom Jackson is a science writer based in Bristol, UK.
GIOTTO space mission to Halley's Comet and also on ESA’s He has written about 150 books and contributed to many
Smart 1 mission to the moon. David has served on a host others, covering all kinds of subjects from fish to religion.
of space and astronomy committees, and has been a vice Tom writes for adults and children, mostly about science
president of both the Royal Astronomical Society and the and technology, with a focus on the histories of the
British Astronomical Association. sciences. He has worked on several astronomy books,
including collaborations with Brian May and Patrick Moore.

6

CONTENTS

10 INTRODUCTION 26 The unmoving stars go
uniformly westward
FROM MYTH Earth’s rotation
TO SCIENCE
27 A little cloud in
600 BCE–1550 CE the night sky
Mapping the galaxies
20 It is clear that Earth
does not move 28 A new calendar for China
The geocentric model The solar year
21 Earth revolves around the
sun on the circumference 30 We have re-observed
of a circle all of the stars in
Early heliocentric model Ptolemy’s catalog
22 The equinoxes Improved instruments
move over time
Shifting stars 32 Finally we shall place the 64 A perfectly circular spot
23 The moon’s brightness sun himself at the center centered on the sun
is produced by the of the universe The transit of Venus
radiance of the sun The Copernican model 65 New moons around Saturn
Theories about the moon Observing Saturn’s rings
24 All matters useful to the THE TELESCOPE 66 Gravity explains the
theory of heavenly things REVOLUTION motions of the planets
Consolidating knowledge Gravitational theory
1550–1750 74 I dare venture to foretell
that the comet will return
44 I noticed a new again in the year 1758
and unusual star Halley’s comet
The Tychonic model
48 Mira Ceti is a variable star
A new kind of star

50 The most true path of 78 These discoveries are the
the planet is an ellipse most brilliant and useful
Elliptical orbits of the century
56 Our own eyes show us Stellar aberration
four stars traveling 79 A catalog of the
around Jupiter southern sky
Galileo’s telescope Mapping southern stars

7

URANUS 100 A survey of the whole
TO NEPTUNE surface of the heavens
The southern hemisphere

1750–1850 102 An apparent movement
of the stars
Stellar parallax
84 I found that it is a comet,
for it has changed its place 103 Sunspots appear in cycles
Observing Uranus The surface of the sun

86 The brightness of the 104 A spiral form of 118 Photographing the stars
star was altered arrangement was detected Astrophotography
Variable stars Examining nebulae 120 A precise measurement
87 Our Milky Way is the 106 The planet whose position of the stars
dwelling, the nebulae you have pointed out The star catalog
are the cities actually exists
Messier objects The discovery of Neptune
88 On the construction
of the heavens THE RISE OF 122 Classifying the stars
The Milky Way ASTROPHYSICS according to their
90 Rocks fall from space spectra reveals their
Asteroids and meteorites 1850–1915 age and size
The characteristics of stars

92 The mechanism 112 Sodium is to be found 128 There are two kinds
of the heavens in the solar atmosphere of red star
Gravitational disturbances The sun’s spectrum Analyzing absorption lines
94 I surmise that it could 113 Stars can be grouped 129 Sunspots are magnetic
be something better by their spectra The properties of sunspots
than a comet Analyzing starlight 130 The key to a distance
The discovery of Ceres 114 Enormous masses scale of the universe
of luminous gas Measuring the universe
Properties of nebulae 138 Stars are giants
116 The sun’s yellow or dwarfs
prominence differs from Refining star classification
any terrestrial flame 140 Penetrating radiation
The sun’s emissions is coming from space
117 Mars is traversed by Cosmic rays
a dense network 141 A white hot star
of channels that is too faint
Mapping Mars’s surface Discovering white dwarfs

8

178 White dwarfs have 196 It took less than an hour
a maximum mass to make the atomic nuclei
The life cycles of stars The primeval atom

ATOMS, STARS, 179 The radio universe 198 Stars are factories for
AND GALAXIES Radio astronomy the chemical elements
180 An explosive transition Nucleosynthesis
1915–1950 to a neutron star 200 Sites of star formation
Supernovae Dense molecular clouds
146 Time and space and
gravitation have no 182 The source of energy in NEW WINDOWS
separate existence stars is nuclear fusion ON THE UNIVERSE
from matter Energy generation
The theory of relativity 184 A reservoir of comets 1950–1975 
exists beyond the planets
The Kuiper belt

185 Some galaxies have active 206 A vast cloud surrounds
regions at their centers the solar system
Nuclei and radiation The Oort cloud

154 An exact solution 186 The match of lunar and 207 Comets are dirty snowballs
to relativity predicts Earth material is too perfect The composition of comets
black holes The origin of the moon
Curves in spacetime 208 The way to the stars is open
156 The spiral nebulae 188 Important new discoveries The launch of Sputnik
are stellar systems will be made with flying
Spiral galaxies telescopes 210 The search for interstellar
162 Stars are dominated by Space telescopes communications
hydrogen and helium Radio telescopes
Stellar composition
164 Our galaxy is rotating 212 Meteorites can vaporize
The shape of the Milky Way on impact
166 A slow process of Investigating craters
annihilation of matter
Nuclear fusion within stars 213 The sun rings like a bell
The sun’s vibrations
168 A day without yesterday
The birth of the universe 214 The data can best be
explained as X-rays
from sources outside
the solar system
Cosmic radiation

172 The universe is expanding 218 Brighter than a galaxy,
in all directions but it looks like a star
Beyond the Milky Way Quasars and black holes

9

222 An ocean of whispers 298 Cosmic expansion
left over from our is accelerating
eruptive creations Dark energy
Searching for the Big Bang 304 Peering back over
13.5 billion years
228 The search for Studying distant stars
extraterrestrial 306 Our mission is to
intelligence is a search land on a comet
for ourselves Understanding comets
Life on other planets 312 The violent birth of
the solar system
236 It has to be some 268 Most of the universe The Nice model
new kind of star is missing 314 A close-up view of an
Quasars and pulsars Dark matter oddball of the solar system
240 Galaxies change over time 272 Negative pressures Studying Pluto
Understanding stellar produce repulsive gravity 318 A laboratory on Mars
evolution Cosmic inflation Exploring Mars
242 We choose to go 274 Galaxies appear to 326 The biggest eye on the sky
to the moon be on the surfaces of Looking farther into space
The Space Race bubblelike structures 328 Ripples through spacetime
250 The planets formed from Redshift surveys Gravitational waves
a disk of gas and dust
The nebular hypothesis 332 DIRECTORY
340 GLOSSARY
252 Solar neutrinos can 276 Stars form from 344 INDEX
only be seen with the inside out 352 ACKNOWLEDGMENTS
a very large detector Inside giant molecular clouds
The Homestake experiment 280 Wrinkles in time
254 A star that we couldn’t see Observing the CMB
Discovering black holes 286 The Kuiper belt is real
255 Black holes emit radiation Exploring beyond Neptune
Hawking radiation 288 Most stars are orbited
by planets
THE TRIUMPH Exoplanets
OF TECHNOLOGY 296 The most ambitious map
of the universe ever
1975–PRESENT  A digital view of the skies

260 A grand tour of the 297 Our galaxy harbors
giant planets a massive central
Exploring the solar system black hole
The heart of the Milky Way

INTRODU

CTION

12 INTRODUCTION

T hroughout history, the aim through telescopes and detectors predictability of repetitive cycles
of astronomy has been to of various kinds, one of our biggest had vital practical applications
make sense of the universe. discoveries is what we do not in marking the passage of time.
In the ancient world, astronomers understand at all: more than 95
puzzled over how and why the percent of the substance of the Archaeology provides abundant
planets moved against the backdrop universe is in the form of “dark evidence that, even in prehistoric
of the starry sky, the meaning of matter” and “dark energy.” times, astronomical phenomena
the mysterious apparition of comets, were a cultural resource for societies
and the seeming remoteness of The origins of astronomy around the world. Where there is
the sun and stars. Today, the In many of the world’s most no written record, we can only
emphasis has changed to new populated areas today, many of us speculate as to the knowledge
questions concerning how the are barely aware of the night sky. and beliefs early societies held.
universe began, what it is made We cannot see it because the blaze The oldest astronomical records
of, and how it has changed. The of artificial lighting overwhelms to survive in written form come
way in which its constituents, such the faint and delicate light of the from Mesopotamia, the region
as galaxies, stars, and planets, fit stars. Light pollution on this scale that was between and around the
into the larger picture and whether has exploded since the mid-20th valleys of the Tigris and Euphrates
there is life beyond Earth are some century. In past times, the starry rivers, in present-day Iraq and
of the questions humans still patterns of the sky, the phases of neighboring countries. Clay tablets
endeavor to answer. the moon, and the meanderings inscribed with astronomical
of the planets were a familiar information date back to about
Understanding astronomy part of daily experience and a
The baffling cosmic questions of perpetual source of wonder. Philosophy is written in
the day have always inspired big this grand book, the universe,
ideas to answer them. They have Few people fail to be moved the
stimulated curious and creative first time they experience a clear which stands continually
minds for millennia, resulting in sky on a truly dark night, in which open to our gaze.
pioneering advances in philosophy, the magnificent sweep of the Milky Galileo Galilei
mathematics, technology, and Way arches across the sky. Our
observation techniques. Just ancestors were driven by a mixture
when one fresh breakthrough of curiosity and awe in their search
seems to explain gravitational for order and meaning in the great
waves, another discovery throws vault of the sky above their heads.
up a new conundrum. For all we The mystery and grandeur of the
have learned about the universe’s heavens were explained by the
familiar constituents, as seen spiritual and divine. At the same
time, however, the orderliness and

INTRODUCTION 13

1600 bce. Some of the constellations century onward, however, moon’s phases. The combined
(groupings of stars) we know today astronomy as a scientific activity motion in space of Earth, the sun,
have come from Mesopotamian diverged from traditional astrology. and the moon also determines
mythology going back even earlier, Today, astronomers reject astrology, the timing and magnitudes of the
to before 2000 bce. because it is unfounded in scientific oceanic tides, which are of crucial
evidence, but they have good reason importance to coastal communities
Astronomy and astrology to be grateful to the astrologers of and seafarers.
The Babylonians of Mesopotamia the past for leaving an invaluable
were greatly concerned with historical record. Astronomy played an equally
divination. To them, planets were important role in navigation, the
manifestations of the gods. The Time and tide stars acting as a framework of
mysterious comings and goings of The systematic astronomical reference points visible from
the planets and unusual happenings observations once used for astrology anywhere at sea (cloud permitting).
in the sky were omens from the started to become increasingly In 1675, British King Charles II
gods. The Babylonians interpreted important as a means of both commissioned an observatory, the
them by relating them to past timekeeping and navigation. Royal Observatory at Greenwich,
experience. To their way of Countries had highly practical near London. The instruction to
thinking, detailed records over long reasons—civil, as well as military — its director, the first Astronomer
periods were essential to establish to establish national observatories, Royal, John Flamsteed, was to
connections between the celestial as the world industrialized and apply himself diligently to making
and the terrestrial, and the practice international trade grew. For many the observations needed “for the
of interpreting horoscopes began in centuries, only astronomers had perfecting of the art of navigation.” ❯❯
the 6th century bce. Charts showed the skills and equipment to preside
where the sun, moon, and planets over the world’s timekeeping. You have to have the
appeared against the backdrop of This remained the case until the imagination to recognize a
the zodiac at some critical time, development of atomic clocks in discovery when you make one.
such as a person’s birth. the mid-20th century.
Clyde Tombaugh
For some 2,000 years, there was Human society regulates itself
little distinction between astrology, around three natural astronomical
which used the relative positions of clocks: Earth’s rotation, detectable
celestial bodies to track the course by the apparent daily march of the
of human lives and history, and the stars around the celestial sphere to
astronomy on which it relied. The give us the day; the time our planet
needs of astrology, rather than pure takes to make a circuit around the
curiosity, justified observation of sun, otherwise known as a year;
the heavens. From the mid-17th and the monthly cycle of the

14 INTRODUCTION

Astronomy was largely discarded as surface inhabitants, the protection Arthur Eddington took advantage
the foundation of navigation in the afforded by the atmosphere and of a total solar eclipse to observe
1970s, and replaced by artificial Earth’s magnetic field may make how the paths of starlight deviated
satellites, which created a global us feel secure, but in reality we from a straight line when the light
positioning system. are at the mercy of a harsh space passed through the gravitational
environment, blasted by energetic field of the sun, just as relativity
The purpose of astronomy particles and radiation, and at risk predicted. Then, in 1979, the first
The practical reasons for pursuing of colliding with rocks. The more example of a gravitational lens
astronomy and space science may we know about that environment, was identified, when the image
have changed, but they still exist. the better equipped we are to deal of a quasar was seen to be double
For example, astronomy is needed with the potential threats it presents. due to the presence of a galaxy
to assess the risks our planet faces along the line of sight, again as
from space. Nothing illustrated A universal laboratory relativity had predicted. The most
Earth’s apparent fragility more There is another very important recent triumphant justification of
powerfully than the iconic images, reason for doing astronomy. The Einstein’s theory came in 2015 with
such as “Earthrise” and “Blue universe is a vast laboratory in the first detection of gravitational
Marble,” taken from space by Apollo which to explore the fundamental waves, which are ripples in the
astronauts in the 1960s. These nature of matter, and of time and fabric of spacetime, generated by
images reminded us that Earth is space. The unimaginably grand the merging of two black holes.
a small planet adrift in space. As scales of time, size, and distance,
and the extremes of density, When to observe
What a wonderful and pressure, and temperature go far One of the main methods scientists
amazing scheme have we beyond the conditions we can use to test ideas and search for new
readily simulate on Earth. It would phenomena is to design experiments
here of the magnificent be impossible to test the predicted and carry them out in controlled
vastness of the universe. properties of a black hole or watch laboratory conditions. For the most
Christiaan Huygens what happens when a star explodes part, however, with the exception
in an Earth-bound experiment. of the solar system—which is
close enough for experiments to be
Astronomical observations carried out by robots—astronomers
have spectacularly confirmed the have to settle for a role as passive
predictions of Albert Einstein’s collectors of the radiation and
general theory of relativity. As elementary particles that happen
Einstein himself pointed out, his to arrive on Earth. The key skill
theory explained apparent anomalies astronomers have mastered is that
in Mercury’s orbit, where Newton’s of making informed choices about
theory of gravity failed. In 1919,

INTRODUCTION 15

what, how, and when to observe. which is the branch of astronomy science.” This is the combination
For instance, it was through the concerned with the movement of technology and practical
gathering and analysis of telescopic of bodies, especially in the solar applications that blossomed with
data that the rotation of galaxies system. The term “planetary the establishment of the “space
could be measured. This, in turn, science” encompasses every age” in the mid-20th century.
quite unexpectedly led to the aspect of the study of planets,
discovery that invisible “dark matter” including Earth. Solar physics Collaboration of science
must exist. In this way, astronomy’s is another important discipline. Every space telescope and mission
contribution to fundamental to explore the worlds of the solar
physics has been immense. Technology and innovation system makes use of space
With the spawning of so many science, so sometimes it is hard
Astronomy’s scope branches of enquiry connected to separate it from astronomy.
Up to the 19th century, astronomers with everything in space, including This is just one example of how
could only chart the positions and Earth as a planet, the meaning of developments in other fields,
movements of heavenly bodies. the word “astronomy” has evolved especially technology and
This led the French philosopher once again to become the collective mathematics, have been crucial
Auguste Comte to state in 1842 name encompassing the whole of in propelling astronomy forward.
that it would never be possible the study of the universe. However, Astronomers were quick to take
to determine the compositions one closely related subject does advantage of the invention of
of planets or stars. Then, some not come under astronomy: “space telescopes, photography, novel
two decades later, new techniques ways of detecting radiation,
for the spectrum analysis of light If astronomy teaches and digital computing and data
began to open up the possibility anything, it teaches that handling, to mention but a few
of investigating the physical man is but a detail in the technological advances. Astronomy
nature of stars and planets. A new evolution of the universe. is the epitome of “big science”—a
word was invented to distinguish large-scale scientific collaboration.
this new field from traditional Percival Lowell
astronomy: astrophysics. Understanding our place in
the universe goes to the heart of
Astrophysics became just one our understanding of ourselves:
of many specialisms in the study the formation of Earth as a life-
of the universe in the 20th century. supporting planet; the creation of
Astrochemistry and astrobiology the chemical building blocks from
are more recent branches. They join which the solar system formed; and
cosmology—the study of the origin the origin of the universe as a
and evolution of the universe as a whole. Astronomy is the means by
whole—and celestial mechanics, which we tackle these big ideas. ■

FTROOSMCIME

600 BCE–1550 CE

NYCTHE

18 INTRODUCTION

Anaximander of In his On the Heavens, Aristotle In Alexandria, Eratosthenes
Miletus produces one outlines an Earth-centered measures the circumference
of the earliest attempts model of the universe. Many
of his ideas will dominate of Earth and estimates the
at a scientific thinking for 2,000 years. distance to the sun.
explanation of

the universe.

C.550 BCE 350 BCE C.200 BCE

C.530 BCE C.220 BCE C.150 CE

Pythagoras establishes a Aristarchus of Ptolemy writes the
school in Croton, where he Samos proposes a Almagest, which sets
sun-centered model out an Earth-centered
promotes the idea of a of the universe, but his model of the universe
cosmos in which bodies idea does not gain
move in perfect circles. wide acceptance. that becomes
widely accepted.

T he traditions on which put forward two centuries later inconsistencies, his philosophy
modern astronomy is by Aristotle (384–322 bce) were to was adopted as the most acceptable
built began in ancient underpin the whole of astronomy overall framework of ideas for science
Greece and its colonies. In nearby until the 16th century. and was later incorporated into
Mesopotamia, although the Christian theology.
Babylonians had become highly Aristotle’s beliefs
proficient at celestial forecasting Aristotle was a pupil of Plato, Geometrical order
using complicated arithmetic, and both were influenced by the Mathematically, much of Greek
their astronomy was rooted in thinking of Pythagoras and his astronomy was based on geometry,
mythology, and their preoccupation followers, who believed that the particularly motion in circles,
was with divining the future. To natural world was a “cosmos” as which were considered to be the
them, the heavens were the realm opposed to “chaos.” This meant most perfect shapes. Elaborate
of the gods, outside the scope of that it is ordered in a rational way geometrical schemes were created
rational investigation by humans. rather than incomprehensible. for predicting the positions of
the planets, in which circular
By contrast, the Greeks tried Aristotle stated that the motions were combined. In 150 ce,
to explain what they observed heavenly realms are unchanging the Graeco–Egyptian astronomer
happening in the sky. Thales and perfect, unlike the world of Ptolemy, working in Alexandria, put
of Miletus (c.624–c.546 bce) is human experience, but he promoted together the ultimate compendium
regarded as the first in a line of ideas that were consistent with of Greek astronomy. However, by
philosophers who thought that “common sense.” Among other 500 ce, the Greek approach to
immutable principles in nature things, this meant Earth was astronomy had lost momentum.
could be revealed by logical stationary and at the center of the In effect, after Ptolemy, there were
reasoning. The theoretical ideas universe. Although it contained

In the Aryabhatiya, Italian scholar Gerard of FROM MYTH TO SCIENCE 19
Indian astronomer Cremona makes Arabic texts,
Aryabhata suggests including Ptolemy’s Almagest, Mongol ruler Ulugh Beg
that the stars move corrects many of the
across the sky because accessible in Europe by postions of stars
Earth is rotating. translating them into Latin.
found in the Almagest.

499 CE C.1180 1437
1543
1025 1279

Arab scholar Ibn Chinese astronomer Nicolaus Copernicus’s
al-Haytham produces Guo Shoujing produces an book De revolutionibus
a work that criticizes accurate measurement of the orbium coelestium is
the Ptolomaic model published, outlining a
length of the solar year. sun-centered cosmos.
of the universe for
its complexity.

no significant new ideas in astronomy progress. Chinese, Arab, and The invention of the printing
in this tradition for nearly 1,400 Japanese astronomers recorded press in the mid-15th century
years. Independently, great cultures the 1054 supernova in the widened access to books. Nicolaus
in China, India, and the Islamic constellation Taurus, which Copernicus, who was born in 1473,
world developed their own traditions made the famous Crab nebula. collected books throughout his life,
through the centuries when Although it was much brighter including the works of Ptolemy. To
astronomy in Europe made little than Venus, there is no record of its Copernicus, Ptolemy’s geometrical
appearance being noted in Europe. constructions failed to do what
It is the duty of an astronomer the original Greek philosophers
to compose the history of the The spread of learning saw as their objective: describe
Ultimately, Greek science returned nature by finding simple underlying
celestial motions through to Europe via a roundabout route. principles. Copernicus intuitively
careful and expert study. From 740 ce, Baghdad became a understood that a sun-centered
Nicolaus Copernicus great center of learning for the method could produce a much
Islamic world. Ptolemy’s great simpler system, but in the end
compendium was translated into his reluctance to abandon circular
Arabic, and became known as the motion meant that real success
Almagest, from its Arabic title. eluded him. Nevertheless, his
In the 12th century, many texts in message that physical reality
Arabic were translated into Latin, so should underpin astronomical
the legacy of the Greek philosophers, thinking arrived at a pivotal
as well as the writings of the Islamic moment to set the scene for the
scholars, reached Western Europe. telescopic revolution. ■

20

 IDTTHOIEASTSCENLAOERTATRMHOVE

THE GEOCENTRIC MODEL

IN CONTEXT O ne of the most influential stayed the same, and spun daily
of all Western philosophers, around Earth. The moon, sun, and
KEY ASTRONOMER Aristotle, from Macedonia planets, too, appeared to move in
Aristotle (384–322 bce) in northern Greece, believed that unchanging orbits around Earth.
the universe was governed by Their motion, he believed, was
BEFORE physical laws. He attempted to circular and their speed constant.
465 bce Greek philosopher explain these through deduction,
Empedocles thinks that there philosophy, and logic. His observations of the shadow
are four elements: earth, water, cast by Earth on the moon’s
air, and fire. Aristotle contends Aristotle observed that the surface during a lunar eclipse
that the stars and planets are positions of the stars appeared to convinced him that Earth was
made of a fifth element, aether. be fixed in relation to each other, a sphere. His conclusion was
and that their brightness never that a spherical Earth remained
387 bce Plato’s student changed. The constellations always stationary in space, never spinning
Eudoxus suggests that the or changing its position, while the
planets are set in transparent Earth casts a circular shadow cosmos spun eternally around it.
rotating spheres. on the moon during a lunar eclipse. Earth was an unmoving object
This convinced Aristotle that at the center of the universe.
AFTER Earth was a sphere.
355 bce Greek thinker Aristotle believed that Earth’s
Heraclides claims that the sky Earth’s shadow atmosphere, too, was stationary.
is stationary and Earth spins. At the top of the atmosphere,
moon friction occurred between the
12th century Italian Catholic sun’s atmospheric gases and the rotating
priest Thomas Aquinas begins rays sky above. Episodic emanations
teaching Aristotle’s theories. of gases from volcanoes rose to
Earth the top of the atmosphere. Ignited
1577 Tycho Brahe shows that by friction, these gases produced
the Great Comet is farther comets, and, if ignited quickly,
from Earth than the moon. they produced shooting stars.
His reasoning remained widely
1687 Isaac Newton explains accepted until the 16th century. ■
force in his Philosophiae
Naturalis Principia Mathematica. See also: Consolidating knowledge 24–25 ■ The Copernican model 32–39 ■
The Tychonic model 44–47 ■ Gravitational theory 66–73

FROM MYTH TO SCIENCE 21

EOTAAHRFREOATUCCHNIIRDRRCCETUVLHMOEELFSVEUERSNENOCNE

  EARLY HELIOCENTRIC MODEL

IN CONTEXT A n astronomer and Aristarchus was the
mathematician from the real originator of the
KEY ASTRONOMER Greek island of Samos, Copernican hypothesis.
Aristarchus (310–230 bce) Aristarchus is the first person Sir Thomas Heath
known to have proposed that the
BEFORE sun, not Earth, is at the center Mathematician and classical scholar
430 bce Philolalus of Craton of the universe, and that Earth
proposes that there is a revolves around the sun. the case until the 15th century, when
huge fire at the center of the the heliocentric viewpoint was
universe, around which the Aristarchus’s thoughts on this revived by Nicolaus Copernicus.
sun, moon, Earth, five planets, matter are mentioned in a book
and stars revolve. by another Greek mathematician, Aristarchus also believed that
Archimedes, who states in The the stars were much farther away
350 bce Aristotle states that Sand Reckoner that Aristarchus than had previously been imagined.
Earth is at the center of the had formulated a hypothesis that He made estimates of the distances
universe and everything else “the fixed stars and sun remain to the sun and moon, and their
moves around it. unmoved” and “Earth revolves sizes relative to Earth. His estimates
about the sun.” regarding the moon were reasonably
AFTER accurate, but he underestimated
150 ce Ptolemy publishes Unfashionable idea the distance to the sun, mainly
his Almagest, describing an Aristarchus persuaded at least because of an inaccuracy in one
Earth-centered (geocentric) one later astronomer—Seleucus of of his measurements. ■
model of the universe. Seleucia, who lived in the second
century bce—of the truth of his
1453 Nicolaus Copernicus heliocentric (sun-centered) view
proposes a heliocentric of the universe, but otherwise it
(sun-centered) universe. seems his ideas did not gain wide
acceptance. By the time of Ptolemy,
1838 German astronomer in about 150 ce, the prevailing view
Friedrich Bessel is the was still a geocentric (Earth-
first to obtain an accurate centered) one, and this remained
measurement of the
distance to a star, using a See also: The geocentric model 20 ■ Consolidating knowledge 24–25 ■
method known as parallax. The Copernican model 32–39 ■ Stellar parallax 102

22

  MTHOEVEEOOUVIENROXTEIMSE

SHIFTING STARS

IN CONTEXT I n about 130 bce, the Greek exactly defined points and curves
astronomer and mathematician on the surface of this sphere as
KEY ASTRONOMER Hipparchus of Nicaea noticed references for describing the
Hipparchus (190–120 bce) that a star named Spica had moved positions of stars and other celestial
2o east of a point on the celestial objects. The sphere has north and
BEFORE sphere, called the fall equinox south poles, and a celestial equator,
280 bce Greek astronomer point, compared to its position which is a circle lying above Earth’s
Timocharis records that the recorded 150 years earlier. Further equator. The ecliptic is another
star Spica is 8° west of the fall research showed him that the important circle on the sphere,
equinox. positions of all stars had shifted. which traces the apparent path
This shift became known as of the sun against the background
AFTER “precession of the equinoxes.” of stars over the course of the year.
4th century ce Chinese The ecliptic intersects the celestial
astronomer Yu Xi notices The celestial sphere is an equator at two points: the spring
and measures precession. imaginary sphere surrounding and fall equinox points. These mark
Earth, in which stars are found at the positions on the celestial sphere
1543 Nicolaus Copernicus specific points. Astronomers use that the sun reaches on the
explains precession as a equinoxes in March and September.
motion of Earth’s axis. Industrious, and a The precession of the equinoxes
great lover of the truth. refers to the gradual drift of these
1687 Isaac Newton two points relative to star positions.
demonstrates precession to Ptolemy
be a consequence of gravity. Hipparchus put this precession
describing Hipparchus down to a “wobble” in the movement
1718 Edmond Halley discovers of the celestial sphere, which he
that, except for the relative believed to be real and to rotate
motion between stars and around Earth. It is now known
reference points on the that the wobble is actually in
celestial sphere, stars have a the orientation of Earth’s spin
gradual motion relative to each axis, caused by the gravitational
other. This is because they are influence of the sun and the moon. ■
moving in different directions
and at different speeds. See also: Gravitational theory 66–73 ■ Halley’s comet 74–77

FROM MYTH TO SCIENCE 23

RBPTRHRAODEIGDIMAHUNOTCCNOEEENDS’OSBSFYITSTHHEESUN

 THEORIES ABOUT THE MOON

IN CONTEXT T he Chief Astrologer at the The sun is like fire
court of Chinese emperor and the moon like water.
KEY ASTRONOMER An-ti, Zhang Heng was The fire gives out light
Zhang Heng (78–139 ce) a skilled mathematician and a and the water reflects it.
careful observer. He cataloged
BEFORE 2,500 “brightly shining” stars and Zhang Heng
140 bce Hipparchus discovers estimated that there were a further
how to predict eclipses. 11,520 “very small” ones. sun is fully lit, and the side that
is away from it is dark.” He also
1st century bce Jing Fang Also a distinguished poet, described a lunar eclipse, during
advances the “radiating Zhang expressed his astronomical which the sun’s light cannot reach
influence” theory, stating that ideas through simile and metaphor. the moon because Earth is in the
the light of the moon is the In his treatise Ling Xian, or The way. He recognized that the planets
reflected light of the sun. Spiritual Constitution of the were similarly subject to eclipses.
Universe, he placed Earth at the
AFTER center of the cosmos, stating that Zhang’s work was developed
150 ce Ptolemy produces “the sky is like a hen’s egg, and is further in the 11th century by
tables for calculating the as round as a crossbow pellet, and another Chinese astronomer,
positions of celestial bodies. Earth is the yolk of the egg, lying Shen Kuo. Shen demonstrated
alone at the center.” that the waxing and waning of
11th century Shen Kuo’s the moon proved that the moon
Dream Pool Essays explains Shape but no light and sun were spherical. ■
that heavenly bodies are round Zhang concluded that the moon
like balls rather than flat. had no light of its own, but rather
reflected the sun “like water.” In
1543 Nicolaus Copernicus’s this, he embraced the theories of
On the Revolutions of the his compatriot Jing Fang who, a
Celestial Spheres describes century earlier, had declared that
a heliocentric system. “the moon and the planets are Yin;
they have shape but no light.” Zhang
1609 Johannes Kepler saw that “the side that faces the
explains the movements of
the planets as free-floating See also: The Copernican model 32–39 ■ Elliptical orbits 50–55
bodies, describing ellipses.

24

HTAOLELATVMHEEANTLTTYHEETROHSRINYUGSOSEFFUL

CONSOLIDATING KNOWLEDGE

IN CONTEXT I n his greatest known work, the The constellations devised by
Almagest, the Graeco-Egyptian Ptolemy are used in this 17th-century
KEY ASTRONOMER astronomer Ptolemy produced star map. The number of stars per
Ptolemy (85–165 ce) a summary of all the astronomical constellation ranges from two (Canis
knowledge of his time. Rather than Minor) to 42 (Aquarius).
BEFORE producing radical new ideas of his
12th century bce The own, Ptolemy mostly consolidated Ptolemy’s model of the solar system
Babylonians organize the and built upon previous knowledge, had a stationary Earth at its center,
stars into constellations. particularly the works of the Greek with the heavens spinning daily
astronomer Hipparchus, whose around it. His model required
350 bce Aristotle asserts star catalog formed the basis complicated additions to make it
that the stars are fixed in of most of the calculations in the match the data and allow it to be
place and Earth is stationary. Almagest. Ptolemy also detailed used to calculate the positions of the
the mathematics required to planets; nonetheless, it was largely
135 bce Hipparchus produces calculate the future positions of unchallenged until Copernicus
a catalog of over 850 star the planets. His system would be placed the sun at the center of
positions and brightnesses. used by generations of astrologers. the cosmos in the 16th century.

AFTER
964 ce Persian astronomer
al-Sufi updates Ptolemy’s
star catalog.

1252 The Alfonsine Tables
are published in Toledo, Spain.
These list the positions of the
sun, moon, and planets based
on Ptolemy’s theories.

1543 Copernicus shows that
it is far easier to predict the
movement of the planets if the
sun is placed at the center of
the cosmos rather than Earth.

FROM MYTH TO SCIENCE 25

See also: The geocentric model 20 ■ Shifting stars 22 ■ The Copernican model
32–39 ■ The Tychonic model 44–47 ■ Elliptical orbits 50–55

Ptolemy produced a catalog stone, and its shadow gave a Claudius Ptolemy
of 1,022 star positions and listed precise indication of the height
48 constellations in the part of of the sun at noon. Ptolemy took Ptolemy was a polymath and
the celestial sphere known to the daily measurements to obtain produced works on a wide
Greeks—everything that could be accurate estimates of the time of range of topics, including
seen from a northern latitude of the solstices and equinoxes, which astronomy, astrology,
about 32o. Ptolemy’s constellations confirmed previous measurements geography, music, optics,
are still used today. Many of their showing that the seasons were and mathematics.
names can be traced even further different lengths. He believed that
back to the ancient Babylonians, the orbit of the sun around Earth Very little is known about
including Gemini (twins), Cancer was circular, but his calculations him, but he probably spent
(crab), Leo (lion), Scorpio (scorpion), led him to the conclusion that all his life in Alexandria, the
and Taurus (bull). The Babylonian Earth could not be at the exact Egyptian seaport with a
constellations are named on a center of that orbit. reputation for scholarship
cuneiform tablet called the Mul and a great library, where he
Apin, which dates back to the Ptolemy the astrologist was taught by the renowned
7th century bce, however, they are Like most thinkers of his day, mathematician Theon of
thought to have been compiled Ptolemy believed that the Smyrna. Many of his prolific
about 300 years earlier. movements of the heavenly bodies writings have survived. They
profoundly affected events on were translated into Arabic
Early quadrant Earth. His book on astrology, and Latin, disseminating his
To improve his measurements, Tetrabiblos, rivaled the Almagest ideas across the medieval
Ptolemy built a plinth. One of the in popularity over the following world. Geography listed the
earliest examples of a quadrant, 1,000 years. Ptolemy had not only locations of most of the places
his plinth was a huge rectangular provided a means to calculate in the known world, and
block of stone, one of whose vertical planetary positions, but he had was carried by Christopher
sides accurately aligned in the also produced a comprehensive Columbus on his voyages of
north–south plane. A horizontal interpretation of the ways those discovery in the 15th century.
bar protruded from the top of the movements affected humans. ■ The Almagest remained in
continual use in academia
Sun Sun’s height until about 1643, a century
after Ptolemy’s model of the
Horizontal bar Stone plinth universe had been challenged
by Copernicus.
0o
Key works
Sun’s
shadow c.150 ce Geography
c.150 ce Almagest
Ptolemy describes the 90o c.150 ce Tetrabiblos
design of his stone plinth in
the Almagest. It was a quadrant,
an instrument that measures
angles between 0° and 90°.

26

   STWHTEAESRUTSNWGMAOORVUDINNIGFORMLY

EARTH’S ROTATION

IN CONTEXT F rom the 4th century bce He was the father of the
to the 16th century ce, the Indian cyclic astronomy …
KEY ASTRONOMER prevailing view throughout
Aryabhata (476–550 ce) the Western world was that Earth that determines more
is stationary and located at the accurately the true positions
BEFORE center of the universe. Suggestions and distances of the planets.
350 bce Heraclides Ponticus, that Earth might be rotating were
a pupil of Plato, proposes that dismissed on the grounds that Helaine Selin
Earth rotates once a day on this would cause objects on Earth’s
its axis. The idea does not surface to fly off into space. In India, Historian of astronomy
become widespread because however, an astronomer named
it contradicts Aristotle, who is Aryabhata was convinced that
considered more authoritative. the movement of stars across the
night sky was due not to the stars
4th century bce Aristotle revolving in a distant sphere around
states that Earth is stationary Earth, but to Earth itself rotating.
in space.
An illusory movement Essentially a compendium of the
AFTER According to Aryabhata, the stars fundamentals of astronomy and
950 ce Iranian astronomer were stationary and their apparent relevant mathematics, it greatly
al-Sijzi supports the idea movement toward the west was an influenced Arabic astronomy.
that Earth rotates. illusion. His notion of a spinning
Earth was not widely accepted Among other achievements,
1543 Nicolaus Copernicus until the mid-17th century—a Aryabhata calculated the length
states that Earth rotates century after Nicolaus Copernicus of the sidereal day (the time it
as part of his heliocentric had endorsed the idea. takes Earth to rotate once in
(sun-centered) model of relation to the stars) to a high
the universe. Aryabhata’s achievements were degree of accuracy, and devised
considerable. His book Aryabhatiya original and accurate ways of
1851 The first demonstration was the most important work of compiling astronomical tables. ■
of Léon Foucault’s pendulum astronomy in the 6th century.
in Paris provides the final
scientific proof that Earth See also: The geocentric model 20 ■ The Copernican model 32–39 ■
is rotating. The Tychonic model 44–47 ■ Elliptical orbits 50–55

FROM MYTH TO SCIENCE 27

IANLTIHTETLNEIGCHLTOUSDKY

 MAPPING THE GALAXIES

IN CONTEXT A bd al-Rahman al-Sufi, once also consulted Arab merchants
better known in the West who traveled to the south and
KEY ASTRONOMER as Azophi, was a Persian east, and who saw more of the sky.
Abd al-Rahman al-Sufi astronomer who made the first His work centered on translating
(903–986 ce) record of what are now understood Ptolemy’s Almagest into Arabic.
to be galaxies. To al-Sufi, these In the process, al-Sufi tried to
BEFORE fuzzy, nebulous objects looked like merge the Hellenistic constellations
400 bce Democritus suggests clouds in the night’s sky. (which dominate star maps today)
that the Milky Way is made with their Arab counterparts, most
of a dense mass of stars. Al-Sufi made most of his of which were totally different.
observations in Isfahan and Shiraz,
150 ce Ptolemy records several in what is now central Iran, but he The fruit of this labor was
nebulae (or cloudy objects) Kitab suwar al-kawakib, or the Book
in the Almagest. The Large Magellanic Cloud, of Fixed Stars, published in 964 ce.
seen here above the ESO’s Paranal The work contained an illustration
AFTER observatory in Chile, can be easily of “a little cloud,” which is now
1610 Galileo sees stars observed with the naked eye from know to be the Andromeda Galaxy.
in the Milky Way using the southern hemisphere. This object was probably known
a telescope, confirming to earlier Persian astronomers, but
Democritus’s theory. al-Sufi’s mention is the earliest
record. Similarly, The Book of
1845 Lord Rosse makes the Fixed Stars includes the White
first clear observation of a Ox, another cloudy object. This is
spiral nebula, now known now named the Large Magellanic
as the Whirlpool Galaxy. Cloud and is a small galaxy that
orbits the Milky Way. Al-Sufi would
1917 Vesto Slipher discovers not have been able to observe
that spiral nebulae are rotating this object himself, but would
independently of the Milky Way. have received reports of it from
astronomers in Yemen and sailors
1929 Edwin Hubble shows who crossed the Arabian Sea. ■
that many spiral nebulae are
far beyond the Milky Way See also: Consolidating knowledge 24–25 ■ Examining nebulae 104–05 ■
and are galaxies themselves. Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77

28

FAONREWCHCINAALENDAR

THE SOLAR YEAR

IN CONTEXT T he traditional Chinese China’s calculations were ahead
calendar is a complex blend of the West’s: 50 years later, this
KEY ASTRONOMER of lunar and solar cycles, same period was used by Julius
Guo Shoujing (1231–1314) with 12 or 13 lunar months matched Caesar to create the Roman
up to the solar-derived seasons. Empire’s Julian system.
BEFORE It had first been formalized in the
100 bce Emperor Wu of 1st century bce during the Han By the time the Mongol leader
the Han Dynasty establishes Dynasty, and used a solar year of Kublai Khan conquered most of
the Chinese calendar based 365.25 days (365 days and 6 hours). China in 1276, a variant of the
on a solar year. original calendar, the Daming
calendar, was in use, but was
46 bce Julius Caesar reforms centuries old and in need of
the Roman calendar using a correction. The khan decided to
year-length of 365 days and impose his authority with a new,
6 hours, and adds a leap day more accurate calendar, which
every four years. became known as the Shoushi
(“well-ordered”) calendar. The task
AFTER of creating it was entrusted to
1437 The Timurid astronomer Guo Shoujing, the khan’s brilliant
Ulugh Beg measures the Chinese chief astronomer.
solar year as 365 days,
5 hours, 49 minutes, and A trained engineer, Guo Shoujing Measuring the year
15 seconds using a 164-ft invented a water-powered version Guo’s job was to measure the
(50-m) gnomon (the central of an armillary sphere, which is an length of the solar year, and to
column of a sundial). instrument used to model the this end he set up an observatory
positions of celestial bodies. in Khanbaliq (the “City of the
1582 Pope Gregory adopts Khan”), a new imperial capital
the Gregorian calendar as a that would one day become known
reform of the ancient Julian as Beijing. The observatory may
calendar by using a 365.25-day have been the largest anywhere
year, the same year as Guo’s in the world at the time.
Shoushi calendar.
Working with mathematician
Wang Chun, Guo began a series
of observations tracking the motion
of the sun throughout the year.

FROM MYTH TO SCIENCE 29

See also: Shifting stars 22 ■ Improved instruments 30–31 ■ Zu Chongzhi (Directory) 334

The two men traveled widely, The calendar has 365 days and
setting up another 26 observatories 6 hours in the year, but does not match the
across China. In 1279, the pair
announced that there were motion of the sun through the year.
29.530593 days to a month, and
that the true solar year was To measure the There is a need to
365.2524 days long (365 days, length of the year, better create a new calendar
5 hours, 49 minutes, and 12
seconds). This is just 26 seconds instruments must that matches the
longer than the current accepted be created. solar year.
measurement. Again, China was
ahead of the West. The same figure The solar year is found to be 365 days, 5 hours,
was not independently measured 49 minutes, and 12 seconds. There is a
and adopted for the universal new calendar for China.
Gregorian calendar in Europe
until 300 years later. measurements. This allowed Guo serving official calendar in Chinese
to measure the angle of the sun history. China officially adopted the
Enduring calendar with far greater accuracy. Gregorian calendar in 1912, but the
A great technological innovator, traditional calendar, today known
Guo invented several new The Shoushi calendar was as the rural or former calendar, still
observational devices and made widely regarded as the most plays a role in Chinese culture,
enhancements to the Persian accurate calendar in the world determining the most propitious
equipment that had begun to at the time. As a testament to its dates to hold weddings, family
arrive in China under Kublai success, it continued to be used celebrations, and public holidays. ■
Khan’s rule. Most importantly, he for 363 years, making it the longest-
built a giant gnomon to a height
of 44 ft (13.3 m), which was five
times taller than the previous
Persian design and featured a
horizontal crossbar marked with

Guo Shoujing Guo Shoujing was born into a poor Guo was tasked with building
family in the north of China, in the a canal to bring spring water
years when the Mongols were from the mountains to the new
consolidating their control over city. In the 1290s, Guo—by now
the region. A child prodigy who the khan’s chief science and
had built a highly advanced water engineering adviser—connected
clock by the age of 14, Guo was Khanbaliq to the ancient Grand
taught mathematics, astronomy, Canal system that linked to
and hydraulics by his grandfather. the Yangtze and other major
He became an engineer, working rivers. In addition to continuing
for the emperor’s chief architect his astronomical work, Guo
Liu Bingzhong. In the late 1250s, oversaw similar irrigation and
Kublai Khan took the throne canal projects across China, and
and chose the region around the his theoretical and technological
town of Dadu near the Yellow innovations continued to
River to build the new capital of influence Chinese society
Khanbaliq, now known as Beijing. for centuries after his death.

30

WALELHOAFVTEHREES-OTBASRESRIVNED
PTOLEMY’S CATALOG

  IMPROVED INSTRUMENTS

IN CONTEXT F or more than 1,000 years, to turn the city into a respected
Ptolemy’s Almagest place of learning, Ulugh Beg
KEY ASTRONOMER was the world’s standard invited scholars of many
Ulugh Beg (1384–1449) authority on star positions. disciplines from far and wide
Translated into Arabic, Ptolemy’s to study at his new madrasa,
BEFORE work was also influential in the an educational institution.
c.130 bce Hipparchus Islamic world up until the 15th
publishes a star catalog century, when the Mongol ruler Ulugh Beg’s own interest
giving the positions of more Ulugh Beg showed that a lot of was in astronomy, and it may
than 850 stars. the Almagest’s data were wrong. have been his discovery of serious
errors in the star positions of the
150 ce Ptolemy publishes A grandson of the Mongol Almagest that inspired him to
a star catalog in the conqueror Timur, Ulugh Beg was order the building of a gigantic
Almagest, which builds on just 16 years old when he became observatory, the largest in the
the work of Hipparchus and ruler of the family’s ancestral seat world at the time. Located on a
is seen as the definitive guide at Samarkand (in present-day hill to the north of the city, it took
to astronomy for more than Uzbekistan) in 1409. Determined five years to construct and was
a millennium.
Ulugh Beg regent, and by 1411, as he
964 ce Abd al-Rahman al-Sufi turned 18, his rule over the
adds the first references to The name Ulugh Beg means city was extended to include
galaxies in his star catalog. “Great Leader.” The sultan– the surrounding province.
astronomer’s birth name was
AFTER Mirza Muhammad Taraghay Ulugh Beg’s flair for
1543 Nicolaus Copernicus bin Shahrukh. He was born mathematics and astronomy
places the sun as the center on the move, as Timur’s army was not matched by his
of the universe, not Earth. traveled through Persia. leadership skills. When Shah
Rukh died in 1447, Ulugh Beg
1577 Tycho Brahe’s star His grandfather’s death assumed the imperial throne,
catalog records a nova, in 1405 brought the army to but he did not command enough
showing that the “fixed a halt in western China. The authority to keep it. In 1449, he
stars” are not eternal ensuing fight for control of his was beheaded by his own son.
and do change. lands was eventually won by
Ulugh Beg’s father, Shah Rukh. Key work
In 1409, Ulugh Beg was sent
to Samarkand as his father’s 1437 Zij-i Sultani

FROM MYTH TO SCIENCE 31

See also: Shifting stars 22 ■ Consolidating knowledge 24–25 ■ Mapping the galaxies 27 ■
The Copernican model 32–39 ■ The Tychonic model 44–47

The understanding of astronomy is based measured to within a few
on the study of the work of past scholars. hundredths of a degree, as
could the positions of the stars.
A precisely built sextant With better instruments,
in a protected location the work of past In 1437, Zij-i Sultani (“The
astronomers is Sultan’s Catalog of Stars”)
gives more accurate often found to was published. Of the 1,022
measurements. contain errors. stars included in the Almagest,
Ulugh Beg corrected the positions
completed in 1429. It was there, than a sixth), it is estimated to of 922. Zij-i Sultani also contained
with his team of astronomers and have had a radius of more than new measurements for the solar
mathematicians, that he set about 130 ft (40 m) and would have been year, planetary motion, and the
compiling a new star catalog. three stories high. The instrument axial tilt of Earth. These data
was kept underground to protect became very important, enabling
Giant instruments it from earthquakes and rested in the prediction of eclipses, the time
Ptolemy’s catalog had largely a curved trench along the north– of sunrise and sunset, and the
been derived from the work of south meridian. As the sun and altitude of celestial bodies, which
Hipparchus, and many of its star the moon passed overhead, their were needed to navigate. Ulugh
positions were not based on light focused into the dark trench, Beg’s work remained the definitive
fresh observations. and their positions could be star catalog until Tycho Brahe’s,
nearly 200 years later. ■
To measure accurately, Ulugh
Beg built the observatory on an All that remains of the Fakhri
immense scale. Its most impressive sextant is a 6½-ft (2-m) wide trench
instrument was the so-called gouged in a hillside. The observatory
Fakhri sextant. In fact, more like was destroyed after Ulugh Beg’s death
a quadrant (a quarter-circle rather in 1449 and not discovered until 1908.

The religions disperse,
kingdoms fall apart,
but works of science
remain for all ages.
Ulugh Beg

THE SUNFINALLY WE SHALL PLACE

HCIEMSNETLFEART OTHFE

THE UNIVERSE

THE COPERNICAN MODEL



34 THE COPERNICAN MODEL

IN CONTEXT To most people in mid-15th Of all discoveries and
century Europe, questions opinions, none may have
KEY ASTRONOMER about Earth’s place in exerted a greater effect on
Nicolaus Copernicus the cosmos had been answered the human spirit than the
(1473–1543) in the 2nd century by the Greco- doctrine of Copernicus.
Egyptian mathematician Ptolemy, Johann von Goethe
BEFORE who had modified ideas first put
c.350 bce Aristotle places Earth forward by Aristotle. These ideas Earth stayed in one place, while
at the center of the universe. placed Earth at the center of the everything else rose in the east,
cosmos, and they carried an official swung across the sky, and set in
c.270 bce Aristarchus proposes stamp of approval from the Church. the west. Furthermore, the Bible
a sun-centered (heliocentric) Yet the first convincing challenge seemed to state that the sun moves,
universe, with the stars a vast to this orthodoxy was to come from whereas Earth does not, so anyone
distance away. a figure within the Church, the who contradicted this view risked
Polish canon Nicolaus Copernicus. being accused of heresy.
c.150 ce Ptolemy publishes
the Almagest. A stationary Earth Nagging doubts
According to the version of the The Earth-centered, or geocentric,
AFTER universe described by Aristotle and model of the universe had never
1576 English astronomer Ptolemy, Earth was a stationary convinced everyone—in fact,
Thomas Digges suggests point at the center of the universe, doubts about it had surfaced
modifying the Copernican with everything else circling from time to time for more than
system, removing its outer around it, and stars were fixed 1,800 years. The most serious
edge and replacing it with in a large, invisible, distant
a star-filled unbound space. sphere, which rotated rapidly
around Earth. The sun, moon,
1605 Johannes Kepler discovers and planets also revolved at
that orbits are elliptical. different speeds around Earth.

1610 Galileo Galilei discovers This idea of the universe
the phases of Venus, and seemed like common sense.
Jupiter’s moons, strengthening After all, one only had to stand
the heliocentric viewpoint. outside and look up at the sky,
and it appeared obvious that

Nicolaus Copernicus Nicolaus Copernicus was born in developing his sun-centered
Torun, Poland, in 1473. From 1491 model of the universe. He did
to 1495, he studied mathematics, not complete this work until
astronomy, and philosophy at the 1530, although he did publish
University of Kraków, then from a summary of his ideas in
1496, canon (religious) law and 1514. Realizing that he risked
astronomy at the University of being ridiculed or persecuted,
Bologna, Italy. In 1497, he was Copernicus delayed publishing
appointed canon of the cathedral the full version of his theory
of Frombork, Poland, a post he until the last weeks of his life.
retained for life. From 1501 to
1505, he studied law, Greek, Key works
and medicine at the University
of Padua, Italy. Subsequently, 1514 Commentariolus
he returned to Frombork, where 1543 De revolutionibus orbium
he spent much of the rest of his coelestium (On the Revolutions
life. By 1508, he had begun of the Celestial Spheres)

FROM MYTH TO SCIENCE 35

See also: The geocentric model 20 ■ Early heliocentric model 21 ■ Consolidating knowledge 24–25 ■ The Tychonic
model 44–47 ■ Elliptical orbits 50–55 ■ Galileo’s telescope 56–63 ■ Stellar aberration 78 ■ Al-Battani (Directory) 334

concern related to predicting Ptolemy tried to fix some of the anomalies in Aristotle’s
the movements and appearances geocentric model by proposing that each planet moved in a
of the planets. According to the small circle called an epicycle. Each epicycle was embedded
Aristotelian version of geocentrism, in a sphere called a deferent. Each planet’s deferent rotated
the planets—like all other celestial around a point slightly displaced from Earth’s position in
bodies—were embedded in space. This point, in turn, continuously rotated around another
invisible concentric spheres point called an equant. Each planet had its own equant.
that revolved around Earth, each
rotating at its own steady speed. Center of
But if this were true, each planet epicycle
should move across the sky at
a constant pace and with an Planet
unvarying brightness—and this
wasn’t what was observed. Center of
deferent
Ptolemy’s fixes
The most glaring anomaly was Earth cycle
Mars, which had been carefully Epi
observed in ancient times by both
the Babylonians and the Chinese. Equant
It appeared to speed up and slow
down from time to time. If its eferent
movements were compared to D
those of the rapidly rotating outer
sphere of fixed stars, Mars usually address these problems, Ptolemy as further epicycles needed to
moved in a particular direction, but modified the original Aristotelian be added to keep prediction in
occasionally it reversed direction— geocentric model. In his revised line with observation.
a strange behavior described as model, the planets were attached
“retrograde motion.” In addition, its not to the concentric spheres Alternative views
brightness varied greatly over the themselves, but to circles attached From about the 4th century bce,
course of a year. Similar, but less to the concentric spheres. He a number of astronomers had
dramatic, irregularities were also called these circles “epicycles.” suggested theories refuting the
observed in the other planets. To These were suborbits around geocentric model. One of these
which the planets circled while ideas was that Earth spins on its
In so many and such important the central pivot points of these own axis, which would account
ways, then, do the planets bear suborbits were carried around the for a large proportion of the daily
witness to the Earth’s mobility. sun. These modifications, Ptolemy movements of celestial objects.
thought, sufficed to explain the The concept of a rotating Earth
Nicolaus Copernicus anomalies observed and matched had initially been put forward by
observational data. However, his a Greek, Heraclides Ponticus, in
model became hugely complicated, about 350 bce and later by various ❯❯

36 THE COPERNICAN MODEL

Ptolemy’s Earth- Copernicus’s Copernicus believes
centered model of the sun-centered model his model is more
explains the same data
universe relies upon elegant, and thus more
complex adjustments to with far fewer likely to be correct.
adjustments.
explain observed data.

Place the sun himself at
the center of the universe.

Arabic and Indian astronomers. Aristotelian ideas, but supporters of In the face of such an established
Supporters of geocentrism rejected geocentrism had also for centuries philosophical tradition with
his idea as absurd, believing a cited what seemed a scientifically little observational evidence to
spinning Earth would create huge valid reason for ruling it out—the contradict it, and the theological
winds, such that objects on Earth’s “lack of stellar parallax.” They arguments in favor of it, the
surface would simply fly off. argued that if Earth moved around geocentric view of the universe
Another idea, first proposed by the sun, it would be possible to went unchallenged for centuries.
Aristarchus of Samos in about observe some variation in the However, in about 1545, rumors
250 bce, was that Earth might relative positions of stars. No such began circulating in Europe of a
move around the sun. Not only did variation could ever be detected highly convincing challenge that
this go against deeply ingrained so, they said, Earth could not move. had appeared in the form of a book
entitled De revolutionibus orbium
coelestium (On the Revolutions of
the Celestial Spheres), by a Polish
scholar, Nicolaus Copernicus.

Copernican revolution
The work was extremely
comprehensive, and proposed
a new, detailed, mathematical,
and geometrical model of how the
universe works, based on years
of astronomical observations.

Copernicus’s theory was based
on a number of basic propositions.
First, Earth rotates on its axis daily,
and this rotation accounts for most
of the daily movements of the stars,
sun, and planets across the sky.

In his 1660 star atlas, German
mapmaker Andreas Cellarius illustrated
the cosmic systems of Ptolemy, Tycho
Brahe, and Copernicus (shown here).
All three still had their champions.

FROM MYTH TO SCIENCE 37

Copernicus thought it was just In the Ptolemaic model Sun
too unlikely that thousands of (top), Earth is at the center Mars
stars were spinning rapidly around and other celestial bodies
Earth every 24 hours. Instead, he go around Earth. In the Mercury Moon Venus
considered them to be fixed and Copernican system
immovable in their distant, outer (bottom), Earth together
sphere, and that their apparent with the moon have
movement was actually an illusion swapped position
caused by Earth’s spin. To refute with the sun; the
the idea that a spinning Earth sphere of the fixed
would create huge winds, and that stars is much
objects on its surface would fly off, farther out.
Copernicus pointed out that Earth’s
oceans and atmosphere were part Saturn Earth
of the planet and were naturally
part of this spinning motion. In his Embedded
own words: “We would only say “fixed” stars
that not merely the Earth and the
watery element joined with it have Jupiter
this motion, but also no small part
of the air and whatever is linked in Outer sphere
the same way to the Earth.” with embedded

Second, Copernicus proposed “fixed” stars
that it is the sun that is at the
center of the universe, not Earth,
which is simply one of the planets,
all of which circle the sun at
differing speeds.

Elegant solution Earth Moon
These two central tenets of Mercury Mars
Copernicus’s theory were of utmost
importance because they explained Saturn Sun
the movements and variation in
brightness of the planets without Venus
recourse to Ptolemy’s complicated
adjustments. If Earth and another Jupiter
planet, such as Mars, both circle
the sun and do so at different
speeds, taking a different amount
of time to complete each revolution,
they will sometimes be close to
each other on the same side as the
sun and sometimes far from each
other, on opposite sides to the sun.
This, at a stroke, explained the
observed variations in brightness
of Mars and the other planets. The
heliocentric system also elegantly
explained apparent retrograde
motion. In place of Ptolemy’s ❯❯

38 THE COPERNICAN MODEL

complicated epicycles, Copernicus Those things which I am of its implications for stellar
explained that such motion could saying now may be obscure, parallax. For centuries, supporters
be attributed to changes in yet they will be made clearer of geocentrism had argued that
perspective caused by Earth the absence of parallax could
and the other planets moving in their proper place. only be due to Earth not moving.
at different speeds. Nicolaus Copernicus Now, there was an alternative
explanation: the parallax was not
Distant stars never realized quite how far absent, but because of the great
Another of Copernicus’s tenets away the stars are—it is now distance to the stars, it was simply
was that the stars are much farther known that the very closest are too tiny to be measured with the
away from Earth and the sun than about 260,000 times more distant instruments of the time.
had previously been believed. He than the sun. But his assertion
said: “The distance between Earth was extremely important because Copernicus additionally
and the sun is an insignificant proposed that Earth is at the
fraction of the distance from center of the lunar sphere.
Earth and sun to the stars.” Earlier Copernicus maintained that
astronomers knew that the stars the moon circled Earth, as it did
were distant, but few suspected in the geocentric model. In his
just how far away they were, and heliocentric model, the moon
those who did, such as Aristarchus, moved with Earth as it circled the
had not managed to convince sun. In this system, the moon was
anyone. Even Copernicus probably the only celestial object that did
not primarily move around the sun.

In the Ptolemaic model (left), the occasional retrograde View as
(backward-moving) motion of Mars was regarded as due to loops that seen from
the planet makes in space. In the Copernican model (right), retrograde
motion was caused simply by changes in perspective because Earth Earth
and Mars orbit the sun at different speeds. Earth would from time to
time “overtake Mars on the inside” as shown here, causing Mars to Mars
reverse its apparent direction of movement for several weeks.

Mars Motion
of Mars

Epicycle Earth
Sun
Earth

Earth’s orbit

Mars’s deferent Mars’s orbit

FROM MYTH TO SCIENCE 39

Though Copernicus’s work was and that these movements must Mars’s apparent retrograde motion
widely circulated, it took a century be perfect circles. This therefore occurs about every 26 months and
or more before its basic ideas forced Copernicus to retain some lasts for 72 days. Its orbit is on a
were accepted by most other of Ptolemy’s epicycles in his model. slightly different plane from Earth’s,
astronomers, let alone the public The work of Johannes Kepler contributing to the apparent loop.
at large. One difficulty was that, later replaced the idea of circular
although it resolved many of the orbits with that of elliptical orbits, some of its propositions went
problems of the Ptolemaic system, eliminating most of the remaining against Biblical texts probably
his model also contained faults faults in Copernicus’s model. It led to the ban.
that had to be amended by later wasn’t until the 1580s and the
astronomers. Many of these faults work of Danish astronomer Tycho Viewed somewhat ambivalently
were due to the fact that, for Brahe that the idea of celestial at first by astronomers, and
philosophical reasons, Copernicus spheres was abandoned in favor prohibited by the Catholic Church,
clung to the belief that all the of free orbits. Copernicus’s heliocentric model
movements of celestial bodies therefore took considerable time
occurred with the objects Banned by the Church to catch on. Several centuries
embedded in invisible spheres De revolutionibus initially met passed before some of its basic
with little or no resistance from propositions were demonstrated
I am deterred by the fate of the Roman Catholic Church, to be true beyond dispute: that
our teacher Copernicus who, although some Protestants Earth moves in relation to the
although he had won immortal denounced it as heretical. In 1616, stars was eventually proved
fame with a few, was ridiculed however, the Catholic Church conclusively by English astronomer
and condemned by countless condemned Copernicus’s book and James Bradley in 1729. Proof that
people (for very great is the it remained proscribed reading for Earth spins came with the first
more than 200 years. The Church’s demonstration of Foucault’s
number of the stupid). decision coincided with a dispute it pendulum in 1851.
Galileo Galilei was having at the time with the
astronomer Galileo Galilei. Galileo Copernicus’s theory was
was an avid champion of the a serious blow to old ideas
Copernican theory and had made about how the world and wider
discoveries in 1610 that strongly universe work—many of them
supported the heliocentric view. dating from the time of Aristotle.
The dispute with Galileo caused As such, it is often cited as
the Church authorities to examine ushering in the “Scientific
De revolutionibus with intense Revolution”—a series of sweeping
scrutiny, and the fact that advances in many areas of science
that occurred between the 16th
and 18th centuries. ■

RTHEEVOTLEUL

1550–1750

ETSIOCNOPE

42 INTRODUCTION Dutch eyeglass-maker Johannes Kepler describes
Hans Lippershey the elliptical orbits of
Tycho Brahe builds
a large observatory applies for a patent for planets with his three laws
a telescope with of planetary motion.
on the island of three-times
Hveen, from where he magnification.
makes observations

for 20 years.

1576 1608 1619

1600 1610 1639

Italian friar Giordano Bruno is Using a telescope with English astronomer
burned at the stake as a heretic 33-times magnification, Jeremiah Horrocks
after expressing a view that the observes the transit
sun and Earth are not central Galileo Galilei of Venus across the
discovers four moons
or special in the universe. face of the sun.
orbiting Jupiter.

T he Dane Tycho Brahe was power, and they can resolve finer formulated his three laws of
the last great astronomer detail. The bigger the main lens planetary motion describing the
of the pre-telescope era. or mirror, the better the telescope geometry of how planets move.
Realizing the importance of trying on both counts. Starting in 1610,
to record more accurate positions, when Galileo made his first Kepler had solved the problem
Tycho built some high-precision telescopic observations of the of how planets move, but there
instruments for measuring angles. planets, the moon’s rugged surface, remained the problem of why
He accumulated an abundance of and the star clouds of the Milky they move as they do. The
observations, far superior to those Way, the telescope became the ancient Greeks had imagined
available to Copernicus. primary tool of astronomy, opening
up unimagined vistas. If I have seen further it
Magnifying the image is by standing on the
The realm of heavenly bodies still Planetary dynamics shoulders of giants.
seemed remote and inaccessible After Tycho Brahe died, the records
to astronomers at the time of of his observations passed to his Isaac Newton
Tycho’s death in 1601. However, assistant Johannes Kepler, who
the invention of the telescope was convinced by Copernicus’s
around 1608 suddenly brought arguments that the planets orbit
the distant universe into much the sun. Armed with Tycho’s data,
closer proximity. Kepler applied his mathematical
ability and intuition to discover
Telescopes have two important that planetary orbits are elliptical,
advantages over eyes on their own: not circular. By 1619, he had
they have greater light-gathering

THE TELESCOPE REVOLUTION 43

Dutch astronomer Dane Ole Rømer measures English astronomer
Christiaan Huygens the speed of light by Edmond Halley
observing eclipses of predicts the return of
correctly describes Jupiter’s moon Io. the comet that now
the shape of bears his name.

Saturn’s rings for
the first time.

1659 1676 1705
1675 1687 1725

Giovanni Domenico Isaac Newton publishes James Bradley
Cassini spots a gap in Principia, in which he lays proves that Earth

Saturn’s rings and out his universal law is moving by
concludes correctly that of gravitation. demonstrating
an effect called
they are not solid. stellar aberration.

that the planets were carried on about this force, Newton used the not suffer from the color problem.
invisible spheres, but Tycho had word gravitas, Latin for weight, from Reflecting telescopes of Newton’s
demonstrated that comets travel which we get the word gravity. design were widely used in the
unhindered through interplanetary 18th century, after English inventor
space, seeming to contradict this Improving telescopes John Hadley developed methods
idea. Kepler thought that some Newton not only created a for making large curved mirrors
influence from the sun impelled new theoretical framework for of precisely the right shape from
the planets, but he had no scientific astronomers with his mathematical shiny speculum metal. James
means to describe it. way of describing how objects Bradley, Oxford professor and
move, but he also applied his later Astronomer Royal, was one
Universal gravitation genius to practical matters. astronomer who was impressed
It fell to Isaac Newton to describe Early telescope makers found it and acquired a reflector.
the force responsible for the impossible to obtain images free
movement of the planets, with a from colored distortion with their There were also developments
theory that remained unchallenged simple lenses, although it helped in lens-making. In the early-18th
until Einstein. Newton concluded to make the telescope enormously century, English inventor Chester
that celestial bodies pull on each long. Giovanni Domenico Cassini, Moore Hall designed a two-part
other and he showed mathematically for example, used long “aerial” lens that greatly reduced color
that Kepler’s laws follow as a natural telescopes without a tube to distortion. The optician John
consequence if the pulling force observe Saturn in the 1670s. Dollond used this invention to build
between two bodies decreases much-improved refracting telescopes.
in proportion to the square of the In 1668, Newton designed and With high-quality telescopes
distance between them. Writing made the first working version of now widely available, practical
a reflecting telescope, which did astronomy was transformed. ■

44 IN CONTEXT

IUSANNTNAOUERSTWIUCAAELNDD KEY ASTRONOMER
Tycho Brahe (1546–1601)
 THE TYCHONIC MODEL
BEFORE
1503 The most accurate
star positions to date are
recorded by Bernhard
Walther at Nuremberg.

1543 Copernicus introduces
the idea of a sun-centered
cosmos, improving the
prediction of planetary
positions. These, however,
are still inaccurate.

AFTER
1610 Galileo’s use of the
telescope starts a revolution
that eventually supersedes
naked-eye astronomy.

1620 Johannes Kepler
completes his laws of
planetary motion.

1670s Major observatories
are established in all the
capitals of Europe.

I n the 16th century, the exact
orbits of the planets were a
mystery. Danish nobleman
Tycho Brahe realized that accurate
observations would need to be
taken over an extended period
of time if this problem were to be
solved. The need for better data
was underlined by the fact that a
conjunction of Jupiter and Saturn in
1562, when Tycho was 17, occurred
days away from the time predicted
by the best available astronomical
tables. Tycho undertook to take
measurements along the entirety
of the planets’ visible paths.

The astronomy of Tycho’s
time still followed the teachings
that Aristotle had laid down nearly

THE TELESCOPE REVOLUTION 45

See also: The geocentric model 20 ■ Consolidating knowledge 24–25 ■ The Copernican model 32–39 ■
Elliptical orbits 50–55 ■ Hevelius (Directory) 335

The appearance of a new star challenges Aristotle’s
insistence that the stars never change.

Careful measurement shows that the new star
is not an atmospheric phenomenon.

Further careful measurements of the Great Comet
show that it is much farther away than the moon.

Careful measurements are the Tycho used his immense wealth
key to accurate models of the solar system. to design and build fine instruments,
such as this armillary sphere, which
was used to model the night sky as
seen from Earth.

1,900 years earlier. Aristotle visible in the sky as Cassiopeia B. in 1576 he oversaw the building of a
had stated that the stars in the The observation of a new star large complex on the small island of
heavenly firmament were fixed, was an extremely rare event. Hven in the Øresund Strait, between
permanent, and unchanging. Only eight naked-eye observations what is now Denmark and Sweden.
In 1572, when Tycho was 26, a of supernovae have ever been This was one of the first research
bright new star was seen in the recorded. This sighting showed institutes of its kind.
sky. It was in the constellation that the star catalogs in use
of Cassiopeia and stayed visible at the time did not tell the whole Tycho carefully measured the
for 18 months before fading from story. Greater precision was positions of the stars and recorded
view. Influenced by the prevailing needed, and Tycho led the way. them on brass plates on a spherical
Aristotelian dogma, most observers wooden globe about 5 ft 3 in (1.6 m)
assumed that this was an object Precision instruments in diameter at his observatory
high in the atmosphere, but To accomplish his task, Tycho set on Hven. By 1595, his globe had
below the moon. Tycho’s careful about constructing a collection of around 1,000 stars recorded on it.
measurements of the new object reliable instruments (quadrants It could spin around a polar axis,
convinced him that it did not and sextants (p.31), and armillary and a horizontal ring was used
move in relation to nearby stars, spheres) that could measure the so that stars positioned above
so he concluded that it was not an position of a planet in the sky to the horizon at any given time
atmospheric phenomenon but a real an accuracy of about 0.5 arcminute could be distinguished from those
star. The star was later discovered (± 1⁄120º). He personally measured below the horizon. Tycho carried
to be a supernova, and the remnant planetary positions over a period of the globe with him on his travels,
of this stellar explosion is still around 20 years, and for this purpose but it was destroyed in a fire
in Copenhagen in 1728. ❯❯

46 THE TYCHONIC MODEL

Further evidence of a changing position that he had taken on Hven Tycho’s observations of the way the
cosmos came from Tycho’s with those that had been taken comet moved across the sky over
observation of the Great Comet at the same time by Bohemian the months also convinced him
in 1577. Aristotle had claimed astronomer Thaddaeus Hagecius that it was traveling through the
that comets were atmospheric in Prague. In both instances, the solar system. This overturned
phenomena, and this was still comet was observed in roughly another theory that had been
generally believed to be the case in the same place, but the moon believed for the previous 1,500
the 16th century. Tycho compared was not, suggesting that the years. The great Graeco-Egyptian
measurements of the comet’s comet was much farther away. astronomer Ptolemy had been
convinced that the planets were
embedded in real, solid, ethereal,
transparent crystalline spheres,
and that the spinning of these
spheres moved the planets across
the sky. However, Tycho observed
that the comet seemed to move
unhindered, and he concluded that
the spheres could not exist. He
therefore proposed that the planets
moved unsupported through space,
a daring concept at the time.

No parallax
Tycho was also very interested
in Copernicus’s proposition
that the sun, rather than Earth,
was at the center of the cosmos.
If Copernicus was right, the nearby
stars should appear to swing from
side to side as Earth traveled
annually on its orbit around the
sun—a phenomenon known as
parallax. Tycho searched hard,
but could not find any stellar
parallax. There were two possible
conclusions. The first was that the
stars were too far away, meaning
that the change in their position
was too small for Tycho to measure
with the instruments of the day.
(It is now known that the parallax
of even the closest star is about
100 times smaller than the typical
accuracy of Tycho’s observations.)
The second possibility was that

Tycho Brahe’s observatory complex
on the island of Hven attracted scholars
and students from all over Europe
between its founding in 1576 and
its closure in 1597.

THE TELESCOPE REVOLUTION 47

Copernicus was wrong and that The Tychonic model Mars
Earth did not move. This was kept Earth at the center
Tycho’s conclusion. of the cosmos as in the Jupiter
Ptolemaic model, but
The Tychonic model the five known planets Venus Mercury Saturn
In reaching this conclusion, Tycho were now orbiting the Sun
trusted his own direct experience. sun. Although he was
He did not feel Earth moving. In impressed by the Earth
fact, nothing that he observed Copernican model,
convinced him that the planet Tycho believed
was moving. Earth appeared to that Earth did
be stationary and the external not move.
universe was the only thing that
appeared to be in motion. This led Moon
Tycho to discard the Copernican
cosmos and introduce his own. In Outer ring
his model of the cosmos, all the of stars
planets except Earth orbited the
sun, but the sun and the moon to demonstrate that the planets’ realized that the bright stars Sirius,
orbited a stationary Earth. orbits are ellipses and to create a Arcturus, and Aldebaran had, by
model that would displace both the Tycho’s time, moved over half a
For many decades after his death Tychonic and Copernican models. degree away from the positions
in 1601, Tycho’s model was popular recorded by Hipparchus 1,850 years
among astronomers who were Tycho’s improved measurements earlier. Not only were the stars not
dissatisfied with Ptolemy’s Earth- would also allow English astronomer fixed in the sky, but the changing
centric system but who did not wish Edmond Halley to discover the positions of the closer stars could
to anger the Catholic Church by proper motion of stars (the change also be measured. Stellar parallax
adopting the proscribed Copernican in position due to the stars’ motion was not detected until 1838. ■
model. However, Tycho’s own through space) in 1718. Halley
insistence on observational accuracy
provided the data that would lead
to his idea being discredited
shortly after his death. His accurate
observations helped Johannes Kepler

Tycho Brahe Born a nobleman in 1546 in Rudolph II in Prague. There,
Scania (then Denmark, but now Tycho appointed Johannes
Sweden), Tyge Ottesen Brahe Kepler as his assistant.
(Tycho is the Latinized version
of his first name) became an Tycho was famed for his
astronomer after sighting a distinctive metal nose, the
predicted solar eclipse in 1560. legacy of a duel he fought as
a student. He died in 1601,
In 1575, King Frederick II allegedly of a burst bladder,
gave Tycho the island of Hven having refused out of politeness
in the Øresund Strait, where he to take a toilet break during
built an observatory. Tycho later a long royal banquet.
fell out with Frederick’s successor,
Christian IV, over the potential Key work
transfer of the island to his children
and closed the observatory. In 1588 Astronomiæ Instauratæ
1599, he was appointed Imperial Progymnasmata (Introduction
Mathematician to Emperor to the New Astronomy)

48

MVAIRRAIACBELTEISISTAAR

A NEW KIND OF STAR

IN CONTEXT The star Mira Ceti is B efore the work of German
observed to change in astronomer David Fabricius,
KEY ASTRONOMER brightness periodically. it was thought that there
David Fabricius (1564–1617) were only two types of star.
Mira Ceti is a The first were those of constant
BEFORE variable star. brightness, such as the 2,500 or
350 bce Greek philosopher so that can be seen with the naked
Aristotle asserts that the stars Some stars are eye above the horizon on a clear
are fixed and unchanging. variable. dark night. The second type were
the “new stars,” such as those
AFTER Aristotle was seen by Tycho Brahe in 1572
1667 Italian astronomer wrong when he asserted and Johannes Kepler in 1604.
Geminiano Montanari notes
that the star Algol varies that the stars are fixed The constant stars were
in brightness. and eternal. synonymous with the fixed,
permanent stars in the ancient
1784 John Goodricke discovers Greek cosmos—those that
Delta Cephei, a star that varies mapped out the patterns in the
in brightness over five days; constellations and never changed.
English astronomer Edward The new stars, by contrast, would
Pigott discovers the variable appear unexpectedly, apparently
Eta Aquilae. from nowhere, then fade away,
never to be seen again.
19th century Different kinds
of variable star are discovered, A third kind of star
including long-period variables, While observing the star Mira Ceti
cataclysmic variables, novae, (also called Omicron Ceti), in the
and supernovae stars. constellation of Cetus the whale,
Fabricius realized that there was
1912 Henrietta Swan Leavitt a third type of star in the sky—one
discovers a relationship that regularly varied in brightness.
between the periods and the He made his discovery in August
brightness of variable stars 1596 as he was plotting the
such as Delta Cephei. movement of Jupiter across the
sky in relation to a nearby star.