A Brief History of Astronomy 0

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A Brief History of Astronomy

From Creation Myths to BigBang Cosmology

4000 BC

Early peoples think that the

world is flat with a

crystalline sky overhead.The Sun is thought to be agod that rode across the sky

in a chariot, travellingbeneath the Earth at night.

The world is thought to

have been created in a smallamount of time by a deity or

deities.

Ancient views of a flat Earth


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Astronomical records

from Mesopotamia

2000 BC

Mesopotamian priests begin keeping systematic astronomical records.

Observations of the stars and planets made in India and China.

1500 BC

The Sumarians, Babylonians, Indians,Chinese and Egyptians develop

astronomy. The stars appear to formpatterns in the sky that are visible

every year. These patterns areconsidered fixed and are called

constellations. The Chinese divide thesky into 28 constellations; the Indians

into 27.

The length of the day, the month andthe year is known. The five naked eye

planets are known.

Chinese star map


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600 BC

In Greece, Anaximander notices that

the stars appear to rotate around a pole.He suggests that the sky is a completesphere around the Earth. He thinks that

the Earth's surface must be curvedafter hearing that travellers saw new

stars appearing when moving north orsouth. He pictures the Earth as a

cylinder.

The planets are known to move against

the background of the stars which

appear fixed to a crystal sphere. Theword planet means "wanderer".

Stars rotating around the pole

500 BC

Pythagoras and his followers teach that the Earth is a

sphere. The idea came about from observations ofLunar Eclipses - the Earth's shadow on the Moon is

always circular.

The Pythagoreans think that the motions of the planets

are mathematically related to musical sounds and

number. These ideas are called "The Music of theSpheres".

Pythagoras


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Rotating Earth

350 BC

Heracleides suggests that the daily motion of the Sun, Moon, planetsand stars around the Earth could be explained if the Earth rotated on

its axis once every day.

330 BC

Aristotle writes a series of books which contain ideas

that will influence humanity for 1800 years.

He talks about the four elements (earth, fire air andwater) which he says are only found on Earth. These

elements each have their own tendencies: earth isheavy and falls, fire is light and rises. Motion is in

straight lines. The heavier the object, the faster it falls.

A fifth element, the Aether, is only present in theobjects of the sky. Its natural motion is circular so

celestial objects travel around the Earth in perfectcircles. Aristotle assumes that light travels infinitelyfast.

The Earth and the heavens are, therefore, subject to

different natural laws. Things on Earth are corruptedand subject to change while the heavens are

incorruptible and unchanging.Aristotle


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Eratosthenes

250 BC

Eratosthenes measures of the size of the Earth from

observations of the Sun in different parts of the Earth. Onthe longest day of the year, the Sun is overhead in southern

Egypt but 7 from the vertical in northern Egypt.Eratosthenes takes the distance between these two points

and multiplies it by the ratio between a full circle (360)and the 7.

His measurement is within 1% of the correct value.

Measuring the size of the Earth from the altitude of the Sun at two locations


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Aristarchus

Aristarchus accurately measures of the distance to theMoon using trigonometry applied to Lunar eclipses. He

correctly shows that the moon is 25% as large as the Earth.


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Distance to the Moon

He makes the first attempt to find the distance to the Sun. His theory is good but themeasurements are difficult and his figure (19 times further than the Moon - 5% of the correct

value) is too low. Even so, the Sun is shown to be larger than the Earth.


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Distance to the Sun

Aristarchus even suggests that the Earth goes around the larger Sun. This idea does not take root

because of lack of evidence and will not become accepted for 1800 years.

140 BC

Hipparchus refines the distance between the Earthand Moon using Trigonometric Functions which he

had invented.

He thinks that he had observed positional changes

amongst the so called "fixed stars" but he is unsure.

He creates a very accurate map of the 1000 or sobrightest stars. This map will play an important role in

astronomical history 1800 years later.

During his research he discovers that there are two

types of year. The Tropical Year and the SiderealYear differ by 20 minutes. This causes the position of

the Celestial Pole to move in a circle taking 26,000years to complete one cycle. This phenomenon is

called the Precession of the Equinoxes.Hipparchus


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130 BC

Seleucus thinks that the Moon is somehow responsible for the tides. This idea would not be

proved for nearly 1800 years until the time of Newton.

115 BC

Poseidonius recalculates the Earth's circumference as 70% of the correct value.

This figure would become accepted until modern times. 1500 years later,Christopher Columbus would use this figure when searching for finance for his

expedition across the Atlantic.

Poseidonius also measures the distance between the Earth and the Sun to an

accuracy of 43%.

He also popularises astrology.

The astrologicalsymbol for

Scorpio

Ptolemy

100 AD

Ptolemywrites a book(known by its

Arabic name,The Almagest

- The Greatest)which

summarisesthe

astronomicalknowledge of

the ancients,especially that

ofAristotle.

The

cosmology is

based on Earthbeing the

centre of theUniverse with

the Sun,Moon, planets,

and stars (allset on crystal

spheres)revolving

around the

An Epicycle


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Earth in a

series ofcircles called

Epicycles. Theplanets,

Mercury andVenus always

lie close to theline joining the

Earth and theSun.

The system is

cumbersomebut could be

used to predict

the motions ofthe planets tonaked eye

accuracy.Tables are

created thatpredict the

positions ofthe planets in

the future. Herepublishes the

star map ofHipparchusand names the(48) classical

constellationswith the names

they are stillknown by in

the West.

Ptolemy writes

that the sphereof the stars is

200 timesfurther away

than the Moon.

The book alsocontains a


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summary of

geographicalknowledge

with estimatesof latitudes

and longitudesfor places in

Europe. Thesewould not be

improved for800 years.

The book is

one of the fewto survive the

chaos of the

European DarkAges. Afterthe fall of the

RomanEmpire, the

book would betranslated into

Arabic in theIslamic world,

and, later, intoLatin and will

play a part inEurope's

Renaissance


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The (Western) Constellations

500

In India, Aryabhatta, writes a book in which he states that the Sun is the centre of the SolarSystem. This idea would not be accepted for another 1000 years.


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Aryabhatta

600

Varahamihira writes that "Bodies fall towards theearth as it is in the nature of the earth to attract bodies",

1100 years before the idea would become accepted.

850

The Arab mathematician, al-Khwarizmi, adds andrefines Ptolemy's geographical knowledge, usingastronomical observations to give the latitudes and

longitudes of over 2400 localities in Europe and Asia.

He also champined the use of the Indian number

system working out the rules of arithmetic that wouldsimplify calculation. His numbers arrived in Europe

where they became known as Arabic Numerals.

al-Khwarizmi


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al-Battani

900

The length of the year is calculated as 365 days 5

hours 48 minutes 24 seconds by the Arabastronomer, al-Battani. At that time (and for thenext 600 years) Europe's calendar is based on a year

of 356 days 6 hours.

Al-Battani also updates the figures for the Precession

of the Equinoxes (54.5'' per year) and the tilt of the

Earth's axis (23 35').

His observations show that the Earth's distance to the

Sun varies, putting a doubt on the idea of perfect

circular orbits.


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964

al-Sufipublishesa book

aboutstars

listing anumber

of objectsthat are

hazy andfuzzy.

Theseturn out

to be the

starclustersand

galaxiesthat will

providethe

information to

enlargethe size

of theUniverse,

900 yearslater.

This is

the firstwritten

mentionof the

object

thatwouldlater be

known asthe

Andromeda

Galaxy.

al-Sufi's Book


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al-Biruni

1040

The Central Asian scientist, al-Biruni, develops the

experimental method of science including themodern mathematical treatment for handling errors.

His surveying techniques using triangulation allowhim to measure the radius of the Earth as 6339.6 km,

a value that would not be improved for 500 years.

He suggests that the velocity of light is immensecompared to that of sound. He theorises that the

appearance of the Milky Way is due to it being madeup of countless stars, an assertion that would not be

verified until the invention of the telescope 500 yearslater.

1050

al-Zarqali (known in the West as Arzachel) discovers that the point in

the year when the Earth is closest to the Sun moves forward at a rate of12.04'' per year. This is within 2% of the modern value. He also suggests

elliptical orbits for the planets.

1121

al-Khazini suggests that the centre of the Earth is the source of all

gravity.al-Zarqali


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The Ptolemaic Geo-centric (Earth Centred) System

1200

Many

Greek andArab books

aretranslated

into Latinincluding

Ptolemy'sAlmagest.

The influxof classical

knowledgehelps the

Renaissanc

e begin inEurope.

The

(Christian)

CatholicChurch

adoptsAristotle's

cosmology.In the

comingcenturies,

disagreement with this

cosmologywould

become aheresy.

The modelenlarges

Aristotle'sideas of the

corruptEarth and

the perfectheavens.

The mostcorrupt part


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of the

Universe isHell which

is situatedin the

centre ofthe Earth.

Both Earthand Hell are

imperfectand both

are subjectto change,

corruptionand decay.

Man's Sin

causes thecorruptionof the

Earth.Above the

Earth is theatmosphere.

This is lesssubject to

change butchanges

enough toproduce the

weather.Aurora,

meteors andcomets are

alsoconsidered

to beatmospheric

phenomena.

The Moonbeing

further fromthe Earth,

changesless. Itchanges its


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phases and

has ablotchy

appearancebut has the

perfectcircular

motion of acelestial

object. TheSun and

planetscome next.

They don'tchange and

also move

in circlesaround theEarth. Most

distant isthe crystal

spherecontaining

the stars.The stars

areunchanging

and eternal.God (the

mostperfect part

of theUniverse) is

on theoutside of

this finalcrystal

sphere. All

heavenlymotion is incircles (a

perfectshape) or

'circleswithin


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circles'.

Dante,

would laterwrite about

descendingthe nine

circles toHell and

ascendingthe celestial

spheres toGod.

Based onBiblical

chronologies, the Earth

and theUniverse

wereconsidered

to be only afew

thousandyears old.

1543

Polish priest, Mikolaj Kopernik(known in the West

as Nicholas Copernicus), publishes a booksuggesting that the Sun is the centre of the Universe

with the Earth orbiting around it and rotating daily onits axis. The idea does not explain all the astronomical

observations as he, also, insists on circular motion.

His opponents counter with several arguments. The

Earth could not carry the Moon around with it if itwas moving. Winds would blow us off if the Earth

was rotating. The stars should show a parallax (i.ethey would change relative position as our vantage

point changed).

Copernicus has few answers but suggests that the starsfail to show a parallax because they are very distant.

The book is later banned by the (Christian) Catholic Mikolaj Kopernik (Nicholas Copernicus)


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Church until the late 20th Century.

The new Sun-centred system is less cumbersome than

Ptolemy's and can be used for predicting the positionsand movements of the planets.

Helocentric (Sun-Centred) System


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Tycho Brahe

1572

The Danish astronomer, Tycho Brahe,

studies a brilliant new star that appears in thesky. The star is later known to be a type of

exploding star. Brahe names the star a nova.

Brahe finds no parallax indicating that it is a

real stellar object and not something close to

the Earth. The star fades after a couple ofyears. This is an indication that the starry

heavens do change.

He also studies a comet and shows that it ismoving in an elongated orbit amongst the

planets. This indicates that comets are not

atmospheric phenomena and that there are nocrystal spheres holding the planets since

objects can move freely between the planets.It also shows that not all heavenly motion is

circular.

This is the first observational evidence thatAristotle and Ptolemy's ideas may be

flawed. Brahe disagrees with Copernicus,however, and writes that the planets do

indeed go around the Sun but that the Sun

(carrying all the planets) orbits the Earth.This half-way idea is not taken seriously. Hemeasures the year to an accuracy of one

second. This helps promote the introductionof the Gregorian Calendar(now the

international standard) in 1582.

Tycho Brahe is the last of the Europeannaked-eye astronomers. His detailed and

accurate observations of the motion of Marswould lead to a better understanding of

planetary orbits after his death.


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1596

David Fabricius discovers that the star, Omicron Ceti, varies its

brightness over several months. This is another blow to the ideaof the unchanging heavens.

Omicron Ceti or Mira(Wonderful)

Giordano Bruno

1600

In Italy, Giordano Bruno believes and teaches that the Universe

is infinite, the Earth is moving around the Sun, the stars are othersuns with planets around them, and life is not confined to the

Earth.

He is eventually burnt at the stake for heresy!

1610

The Italian mathematician and astronomer, Galileo Galilei, points the newly invented telescopeto the sky and revolutionises astronomy.

On the first night, Galileo sees stars that are invisible to the naked eye. If these stars were being

seen for the first time, the 'ancients' could not have known everything! The Milky Way is seen tobe a vast collection of stars too numerous to be seen individually.

Observing the Sun, he sees sunspots, imperfections in the 'perfect' Sun. He watches them move

across the Sun as it rotates; the first time a celestial body has been observed to rotate on its axis.This leads to the thought that if the large Sun could rotate, why not the smaller Earth.

He sees Venus go through a complete cycle of phases. The planet appears to change its shapelike a miniature Moon from full to half to crescent. This could only happen if it was moving

around the Sun. If Venus was always between the Sun and the Earth (as Ptolemy thought) itwould only exhibit a crescent phase at all times.

Through the telescope, the Moon appears to have mountains and plains. This showed it to be a

world, no different to the Earth.


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Looking at Jupiter Galileo discovers its four large moons, resembling little stars. This proves thatnot everything is moving directly around the Earth. It is also an indication that it is possible for a

body to carry its moons with it as it moves around the Sun. If Jupiter could carry four moonswhy could the Earth not carry its single moon.

Galileo's observations support the Sun-centred Universe ofCopernicus and he advocates thissystem in all his writings. Unlike most academics of the time who only write in Latin, Galileowrites his books in the local language so that they could be read by everyone. The Catholic

Church is angered and forces him to deny that the Earth is moving around the Sun.

Although not the first to perform experiments, Galileo makes it fashionable and disproves someofAristotle's assertions. By dropping cannon balls from a tower (actually, the Leaning Tower of

Pisa) he proves that heavy objects fall to the Earth at the same rate as light objects. He shows thatfalling bodies accelerate as they fall to Earth. More interestingly, moving bodies could be subject

to two separate forces acting independently. This explains how objects could be carried on theEarth even if it was moving.

He also shows that the period of a pendulum swing is constant for a given length. This willeventually lead to accurate timepieces.

He attempts to measure the speed of light but fails due to lack of accurate equipment. Galileo's

physics experiments set the stage for Newton's work.

1610

Using a telescope, Simon Marius describes a fuzzy patch in the constellation of Andromeda.300 years later, this object (the Andromeda Galaxy) will dramatically expand humanity's notions

about the Universe.

1620

The German mathematician, John Kepler, uses the copious observations of Mars by Tycho

Brahe to show that the planets move in elliptical orbits. The circular motion of the ancients isfinally removed. This is the first major use of the newly discovered logarithms in a scientific

calculation.

In addition, Kepler shows that the closer a planet is to the Sun, the faster it moves. This is the

same effect that causes ballet dancers to rotate faster when they bring their arms in. The planets

are seen to be following mechanical laws similar to those on the Earth. This is a further blow tothe ancient idea of one law for the Earth, another for celestial objects.

Kepler discovers a simple mathematical relationship between the period of a planet to orbit the

Sun and its distance from the Sun. The square of the period is proportional to the cube of thedistance. This provides a scale for the Solar System. If any single distance in the Solar System

could be measured it would be possible to calculate all the others. Saturn, the furthest planet, isshown to be 10 times further from the Sun than the Earth.


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Kepler suggests that the Sun somehow pulls the planets around it. He correctly predicts thepassage of the planets Mercury and Venus in front of the Sun. These are called transits and they

would later help in accurately determining the distance from the Earth to the Sun.

1639

In England, Jeremiah Horrocks observes the first transit of Venus. He suggests thatobservations of this phenomenon from different parts of the Earth could be used to measure the

scale of the Solar System, hence the distance from the Earth to the Sun. Observations of an eventor object from two vantage points is called parallax.

He proves that the Moon's orbit around the Earth is an ellipse and suggests that the irregularities

in the orbit were due somehow to the Sun. He also suggests that Jupiter and Saturn affect eachother's orbits.

1640

Godefroy Wendelin measures the distance between the Earth and the Sun using the method firstused by Aristarchus. His result is 60% of the actual figure.

1650

Giovanni Riccioli discovers a double star with the telescope. This is another example of

properties of stars not visible to the naked eye.

1656

Christian Huygens (from the Netherlands) discovers a Moon around Saturn. Since this makessix planets (including the Earth) and six Moons, he declares the Solar System complete! More

bizarrely, he finds that Saturn itself is surrounded by a ring.

He discovers a new type of object, the Orion Nebula. This is a fuzzy cloud-like nebulous object

amongst the stars. Huygens guesses the distance to the brightest star, Sirius by assuming it is thesame luminosity as the Sun. He calculates the distance as being over 25,000 times the distance

between the Earth and Sun. This is a very large distance but is actually only one twentieth of thecorrect distance.

1666

Isaac Newton (England) begins work on his masterpiece, Principia. In this book, he explains the

motions of the Earth, Moon, and planets in terms of the same force of gravity that pulls objects(like apples) to the Earth.

He shows mathematically that two bodies that attract each other gravitationally will orbit each

other in an elliptical path (explaining Kepler's results). The more massive body will appear tomove less while the less massive body will appear to move more. By studying these motions it is


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possible to show that the Sun is far more massive than all the planets since they all appear tomove around it. The theory allows the motions of the Moon and planets to be calculated from

first principles. Most planetary orbits are shown to be almost circular apart from that of Mercury,which is strongly elliptical. Newton also confirmed that the planets affect each other's paths as

they orbit the Sun.

His equations of gravity show that all objects should fall to the Earth with the same acceleration,as Galileo found. Newton extends the experimental results ofGalileo into his three laws of

motion. These explain why we do not feel the rotation of the Earth on its axis or its motionaround the Sun. They also explain why the planets did not need to be pushed around the Sun and

remove the need for planetary 'crystal spheres'. According to Newton, the period of a pendulumcan be used to measure the force of gravity on the surface of the Earth.

Newton also explains the tides. They are caused mainly by the Moon (and to a lesser extent, the

Sun). His equations explain why there were two tides every day. The Moon is also shown to beresponsible for the Precession of the Equinoxes, discovered by Hipparchus.

For the first time, the laws in the heavens are shown to be the same as the laws on the Earth.

The motions of the Solar System (the Sun and planets) are now understood in detail. The stars,however, are still considered to be lights set on a distant crystal sphere beyond the planets.

Apart from his astronomical discoveries, Newton does important work on optics and

mathematics.

With the publication of Newton's work, the Age of Reason is considered to have begun.

1671

Jean Richer notices that a pendulum has a slower rate of swing at the equator than at higher

latitudes. He deduces that the Earth is not a perfect sphere but an oblate spheroid (a sphereflattened at the poles).

1672

Giovanni Cassini measures the parallax of Mars. The observations are made from Paris and

French Guiana. This gives a value of the Earth - Sun distance that is 93% of the actual value.

His discovery of four moons around Saturn destroys Huygens' view of Solar System perfection.

1676

Olaus Roemer observes Jupiter and its moons. He notes that the eclipses of the moons with the

planet were sometimes occurring later than predicted. This is because Jupiter's distance from theEarth varies as both planets orbit the Sun. He correctly deduces that the delay is caused by the

fact that light needs a few minutes to travel from the eclipses at Jupiter to the Earth.


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Using the best distance measurements available, Roemer calculates the speed of light. His figureis 75% of the correct value, an excellent value for the times. Aristotle's idea of an infinite speed

for light is shown to be wrong. The fact that light has a finite (though very large) speed meansthat the further we look into space, the further back in time we can see.

1718

By accurately comparing the positions of stars with those on Hipparchus' star map, Edmund

Halley shows that a small number of the stars had changed position in the 2000 year period sincethat map was made. The movement of one star is even noticeable when compared to maps made

by Tycho Brahe, 150 years earlier. This is now known as a star's Proper Motion. The amount ofthis motion is very small but this is the beginning of the end of the idea that the stars are fixed on

a crystal sphere.

Assuming that stars were moving at the same rate as planets, it is possible to make an estimate of

stellar distances. At the estimated distances, the stars had to be sun-like in their real brilliance

(luminosity). This is the first hint that the Sun is an ordinary star rather than the light at the centreof the Universe.

Halley also works out the orbit of the comet that bears his name. It is a highly elliptical orbit. Upto then, comets were thought to come and go at random. Halley shows that even comets follow

Newton's laws of gravity.

1728

James Bradley, attempts to determine stellar distances by observing stellar parallax during thecourse of the year. The idea is to use a baseline that is twice the distance between the Earth and

the Sun. Observations of stars are made to see if the stars' positions change.

They do, but not in the way expected. Bradley discovers a phenomenon called the Aberration of

Light. This is the first direct proof that the Earth is in motion but does not yield stellar distances.

It is caused by the fact that light has a finite speed. Bradley's observations give a value for thespeed of light which is close to the correct value.

Bradley also measures the diameter of Jupiter and finds that it is much larger than the Earth. Notonly is the Earth not the centre of the Solar System but it isn't even the largest of the planets.

1755

The German philosopher, Immanuel Kant speculates on the origin of the planets. He suggests a

nebula condensing around the Sun.

He thinks that the Milky Way is an "Island Universe" of stars arranged as a flat disk, and that

some of the nebulous objects in the sky may be other similar systems outside the Milky Way.

This idea would not be accepted for 170 years.


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1768

James Cookleads an expedition to the South Pacific to observe a transit of Venus. The

observations are not successful but the geographical discoveries made encourage others toexplore the world scientifically.

1780

William Herschel discovers Uranus, the first new planet since ancient times. This instantlydoubles the size of the Solar System.

He attempts to measure stellar parallax by looking at stars that are close together in the sky. He

assumes that one star may be closer than the other so that the parallax movement will be easier toobserve and measure. In many cases, he finds movement but this is independent of the Earth's

motion around the Sun. The stars are actually in orbit around each other. These are called BinaryStars. This demonstrates that the stars are not fixed to a crystal sphere and that Newton's law of

gravity also operates amongst the stars.

Herschel also discovers many stars that change their brightness. These are called Variable Stars.

Stars can no longer thought of as unchanging and uninteresting.

By counting stars, measuring their motions and applying statistics, Herschel makes the firstestimate of the size of the region occupied by the stars. This region is now called the Galaxy. The

observations indicate that the Solar System is a tiny speck within the Galaxy. It is apparentlysituated close to the galactic centre because the Milky Way appears symmetrical in the sky.

Herschel's estimate of the diameter of the Galaxy is enormous (9000 Light Years) but is actuallyless than 10% of the true value.

The Sun is shown to have a motion of its own relative to other stars. This motion is towards the

constellation of Hercules.

Herschel and others continue to speculate about the existence of other galaxies.

1798

Henry Cavendish applies Newton's equations to very accurate laboratory experiments to

measure the mass of the Earth.

1814

Joseph Fraunhofer passes light from the Sun through a high quality prism. This breaks downthe white light and displays a spectrum. He finds that the continuous rainbow of colours is

crossed by thousands of dark lines. These lines would unlock many mysteries of the Universe.

1838


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The first stellar distances are finally measured.

Friedrich Bessel measures the parallax of a faint star called 61 Cygni. It had been chosen

because it has a large Proper Motion and is therefore assumed to be nearby.

Even the nearest star is over 270,000 times further away than the Sun!

The stars are so far away that they must be Sun-like in luminosity to be visible from the Earth.The Sun is thus shown to be an ordinary star seen from close up. Copernicus had been correct

when he stated that the stars were too distant for a parallax to be easily visible.

1846

The French mathematician, Urbain Leverrier (France) studies anomalies in the motion ofUranus and predicts the existence of a new planet using the laws of gravity. This planet

(Neptune) is quickly detected and is considered another triumph forNewton.

The orbit of the planet Mercury is found to have an anomaly which Newton's laws of gravity

cannot explain. This has to wait forEinstein sixty years later.

1848

Armand Fizeau shows that lines in a spectrum change position when the light source is movingto or from the observer. When the source is moving away the lines are shifted towards the red

end of the spectrum (a Red Shift); when the source is moving closer the lines are shifted towardsthe blue end of the spectrum (a Blue Shift). This is called the Doppler Effect.

1851

Jean Foucault uses a large pendulum in a church to prove that the Earth is rotating on its axis.

This is now called a Foucault Pendulum and is the first direct proof of the Earth's rotationpostulated by Heracleides 2000 years previously.

Foucault also measures the speed of light in the laboratory to a high level of accuracy.

1854

Gustav Kirchhoffstudies the spectrum of glowing substances. He discovers that each type of

atom gives a different set of lines in the spectrum. This gives a method of identifying the atomspresent in glowing objects without needing a sample in the laboratory.

1863

William Huggins applies the technique of spectroscopy to astronomy. He studies thecomposition of the Sun, stars and planets. The same elements that exist on Earth are found in

space.


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He finds that the Sun and stars are mainly made of Hydrogen. He measures the Doppler Effect ofthe star, Sirius and finds that it is moving away from us. Comets are shown to contain glowing

carbon compounds. Many Nebulae produce spectra that show that they are glowing gases ratherthan stars. He uses photography to obtain spectra of very faint objects.

Aristotle's 2100 year old idea that the heavens are made of a different element (the Aether) isfinally proved to be wrong.

1864

Pietro Secchi photographs the spectra of over 4000 stars. He finds differences which wouldeventually lead to ideas of stellar evolution.

1868

Joseph Lockyer discovers a new element in the Sun's spectrum. It is named after the Greek

word for Sun, Helium. 40 years later, Helium would be found on the Earth.

1882

In the USA, Albert Michelson and Edward Morley measure the speed of light to a very high

level of accuracy. They attempt to measure the absolute motion of the Earth around the Sun byfinding a difference in the speed of light in different directions. No difference is found.

Michelson and Morley consider that the experiment has failed because it could not be reconciled

with the physics of the day. It turns out to be the most glorious failed experiment in the history ofscience, eventually laying the groundwork forEinstein'sTheory of Relativity.

1893

Wilhelm Wien studies radiation of energy and light from hot objects. He shows that the colourof a glowing body is related to its temperature in a definite mathematical way. This is called

Wien's Law and can be applied to the surfaces of stars.

Red stars are coolest. Orange stars are hotter; then come yellow stars; hotter still are white stars.

Blue stars are the hottest. Very cool stars give out their energy in the infra-red. The very hottest

stars shine mainly in the ultra-violet.

The Sun's surface temperature is shown to be around 6,000C. Some stars are hotter than theSun.

Theoretically, Wien's energy pattern could not be explained by the physics of the day. Theexplanation would have to await the development ofQuantum Theory.

1895


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Maximillian Wolfand Edward Barnard discover that dark areas in the Milky Way are darknebulae made up of gas and dust.

1900

The German physicist, Max Planck, develops the idea that energy exists in lumps (calledquanta) rather than continuous emissions. This idea explains Wien's work on radiation.

1906

Jacobus Kapteyn repeats William Herschel's statistical analysis of the stars. He discoversorder in the motions of the stars. The stars are not moving at random in the Galaxy. This is the

phenomenon of star streaming. His measurements of the size of the Galaxy increase its size butare still less than 60% of the correct figure. The presence of dark nebulae hinder accurate

measurements.

1912

Henrietta Leavitt studies thousands of variable stars. She finds that a particular type (calledCepheids) have regular periods and are easily distinguished by the way their brightness changes(the Light Curve) and their spectra. She observes examples of these stars in star clusters. Stars in

these clusters are all at the same distance from the earth. This allows her to discover a linkbetween the period and the luminosity. This is called the Period-Luminosity Law. The longer the

star's period, the more luminous the star.

These stars provide a yardstick for measuring distant objects in the Universe. The period gives

the luminosity; the luminosity can be compared with the apparent brightness of the star as seen

from the Earth; this gives the distance to the star. If the star is part of a group, cluster or nebula,the distance to that object is known.

1913

Walter Adams works out how to deduce a star's luminosity from its spectrum. Once the

luminosity is known, the distance can be calculated from the apparent brightness.

This new tool allows Ejnar Hertzsprung and Henry Russell to measure the distance to nearby

Cepheid variables thus providing the scale to Leavitt's cosmic yardstick.

Hertzsprung and Russell go on to find a relationship between the colour and luminosity of stars.Blue (hot) stars tend to be luminous, yellow (medium) stars tend to be less luminous, red (cool)

stars tend to be faint. More than 90% of stars fit this classification and are called Main Sequencestars. Some red stars are too luminous for their colour. These are called Red Giants because they

are very large. Some white stars are too dim for their colour. These are small and very densestars called White Dwarfs.


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A graph of these results is known as the Hertzsprung-Russell (or H-R) diagram. Special stars(like Cepheid variables) occupy distinct zones in the H-R diagram. The diagram is very

important in the study of stellar structure and provides a foundation for ideas about stellarevolution.

Russell studies the spectrum of the Sun to determine its chemical composition. The Sun is 90%Hydrogen, 9% Helium and 1% everything else. Most stars have a similar composition.

Niels Bohr applies Planck's quantum ideas to atoms helping to explain why and how atomicspectra form.

1915

Physics and astronomy are revolutionised by Albert Einstein.

He brings Planck's ideas of quanta into prominence by using them to explain a previously

mysterious effect when light shines on metals (the Photoelectric Effect).

He explains a strange movement of small particles in a liquid (called Brownian Motion) by

proving mathematically that it must be due to the random motions of atoms and molecules. Thisis the first direct proof of atomic theory and allows the size of these small particles to be

determined.

His Special Theory ofRelativity explains why the Michelson and Morley experiment had

apparently failed. Absolute motion cannot be measured: all motion is relative. This leads to theidea that the velocity of light is the maximum speed that any material body can have. No

information can travel faster than light. When we look into distant space we are looking at the

past! A further development leads to the famous equation

E = mc2

which shows that matter is a concentrated form of energy. This would allow future scientists to

explain the source of the energy of the stars. Time and space turn out to be changeable anddependent on the position and motion of the observer. This defies common sense but would be

found to be in accord with observation.

Einstein's General Theory of Relativity changes the way humans look at gravity. Newton had

envisaged gravity as a force between all matter. Einstein sees matter as distorting the very fabric

of space, causing it to curve. This curvature of space causes matter to move in non-linear paths.Under most conditions, the differences between the two theories of gravity are minimal.However, Einstein's theory explains the anomalies in the orbit of Mercury found by Leverrier

sixty years earlier.

General Relativity also predicts that light would be bent by a gravitational field. This would beproved during a total eclipse of the sun a few years later. Another prediction is that a strong

gravitational field would give a spectral red shift separate from that produced by the Doppler


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shift. This is proved when the spectrum of a very dense White Dwarf star is examined. The staris a companion of Sirius so the Doppler effect could be accounted for since the two stars move

together.

The General Theory of Relativity gives an overall view of the entire Universe indicating that it is

not static. Einstein thinks that the Universe is static and disregards this part of his equations. Hewould soon be proved wrong.

1917

An idea to explain the formation of the Solar System is postulated by James Jeans. He suggeststhat a passing star had drawn material from the Sun. This material had condensed to form the

planets (including the Earth).

If this idea is correct, the Sun's planetary system could be unique since stellar encounters are

very rare. Stars are too far apart to interact with others very frequently.

1918

Harlow Shapley applies Leavitt's Cepheid yardstick to Globular Clusters. These are largespherical groups of stars. Most types of object are distributed randomly in the sky. Globular

Clusters, however, are bunched up together. 70% of them occupy a 2% region of the sky.Shapley finds that these clusters are arranged in a sphere centred on a point a long way from the

Sun.

He assumes that the centre of these clusters is the centre of the Galaxy. If so, then that centre is

50,000 Light Years away from the Solar System. Not only is the Earth not the centre of the Solar

System; the Solar System is nowhere near the centre of the Galaxy. Shapley points out that theMilky Way looks symmetrical from the Earth because of the existence of dark nebulae

(interstellar clouds) blocking out distant stars.

Shapley's measurements to the centre of the Galaxy turn out to be an over-estimate, however.

This is the first time that the size of the Universe is over-estimated. The currently accepted figureis 30,000 Light Years. In 1930 Robert Trumpler would show that interstellar dust dims the

Globular Clusters making them look further than they actually were.

1924

Arthur Eddington uses gas theory to study the interiors of stars. He shows that stars are stablebecause there is a balance between two opposing tendencies. The energy and gas pressure

coming from the hot centre push the star outwards, tending to expand it. Gravity pulls the starinwards, tending to contract it.

He estimates the interior temperature of the Sun to be in the millions of degrees. This is so hot

that Jeans' idea of planetary formation would not work.


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Eddington discoveres the Mass-Luminosity Law for stars. More massive stars are moreluminous. His studies allow him to explain how Cepheid stars vary in brightness by pulsating.

1926

Edwin Schrdinger discovers a wave equation that puts Quantum Mechanics on a firmmathematical footing. This would lead to advances in the understanding of atomic and molecularspectra that would increase knowledge in astronomical objects.

1927

Jan Oort studies the star streaming discovered by Kapteyn. He shows that these stellar

movements are due to the stars in the Galaxy revolving about the centre. The stars closer to theGalactic centre travel faster than the stars further away. Oort uses the stellar motions to find the

location of the Galactic centre: its position agrees with Shapley's centre of Globular Clusters.

The centre of the Galaxy is confirmed to be 30,000 Light Years from the Sun's position. The Sunrequires 200 million years to orbit the Galactic centre. The Galaxy has enough matter to make100 thousand million stars like the Sun.

1929

Edwin Hubble studies the spiral nebulous object in the constellation of Andromeda (first noted

by Al-Sufi and Marius). Using the world's largest telescope, he manages to see stars in theobject. Some of the stars are Cepheids. This allows him to determine their distance and hence the

distance of the spiral. The distance of 800,000 Light Years he finds is far outside the domain ofour Galaxy even though it is an under-estimate.

The Andromeda spiral is in fact a galaxy outside our own and is now called the Andromeda

Galaxy.

Our Galaxy, with its thousands of millions of stars, is not unique.

More galaxies are quickly found; there are billions now known. The Universe is far, far larger

than previously thought. Hubble finds that there are three types of galaxies: spiral, elliptical andirregular. From its overall properties our Galaxy appeared to be a spiral.

Vesto Slipher had previously measured the velocities of many nebulae by taking photographs of

their spectra.

Hubble analyses the velocities of the ones now recognised as galaxies. He finds that the

overwhelming majority of galaxies are moving away from us. Their spectra show a Red Shift.He shows that there is a simple mathematical relationship between the distance of the galaxy and

its velocity away from us. This relationship is now called Hubble's Law.


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Hubble's Law provides another yardstick with which to measure distance. The Red Shift of agalaxy can be measured from its spectrum. This gives its velocity of recession from us. Hubble's

Law provides the distance.

The simplest way to explain these observations is to assume that the Universe is expanding.

Einstein's General Theory of Relativity had already predicted that the Universe would not stableif it was static. Hubble's work shows that the Universe is, indeed, not static.

Modern Cosmology (the study of the overall structure of the Universe) can be said to have begunwith Hubble's work.

1930

Abb Lematre and George Gamow explain the observation of the expanding Universe bypostulating that it began in a huge explosion. This is colourfully known as the Big Bang Theory.

Lematre suggests that all the matter in the Universe was once contained in a very dense "cosmicegg". This object exploded and the matter was spread out through space. We see the effects ofthis explosion when we observe the galaxies moving away from each other.

Gamow predicts that the echo of the explosion should be detectable as radiation with a

temperature of about 5 degrees above Absolute Zero. This radiation should permeate throughoutthe Universe. It would not be detected for over 30 years.

Using Hubble's Law and working backwards, they estimate that the age of the Universe is 2thousand million years. This figure is smaller than the age of the Earth as calculated by

geologists.

Alexander Friedman uses Einstein's equations of General Relativity to work out that there aretwo possible ends to the Big Bang Universe.

If the amount of matter in the Universe is above a certain critical level, then the expansion of the

Universe would eventually slow down and stop. The Universe would then contract with all thegalaxies and stars moving towards each other until they were back in a small area. This is known

as the Big Crunch.

If the amount of matter in the Universe is below the critical level, then the expansion would

continue forever. Eventually the Universe would expand so much that galaxies would not be

visible to each other. Cold, dark, and isolated embers would be all that was left of the galaxies.

To distinguish between these two scenarios requires a knowledge of how quickly the Universe is

expanding compared to how much matter it contains. This problem would not be solved for 70years.

1932


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Karl Jansky discovers radio waves from space. He finds that these signals come from the centreof the Galaxy, its position agreeing with Shapley's. Radio waves open a new window to the

Universe; a window that is not affected by gas and dust.

This marks the birth of Radio Astronomy.

1935

Otto Struve proves that invisible interstellar dust and gas exists by finding a spectral line ofCalcium.

He develops a new theory of planetary formation that is a normal part of stellar evolution ratherthan the rare stellar encounter ofJeans' model.

1938

Hans Bethe and Carl Weizscher work out the details of how the Sun produces its energy. It isby nuclear fusion, converting Hydrogen to Helium. Every second over 3 million tonnes of theSun's matter is converted into energy.

1942

Harold Jones measures the Astronomical Unit (the distance between the Earth and the Sun) to

an accuracy of over 99.99%.

Walter Baade studies the stars in the Andromeda Galaxy. He discovers that there are two

populations, each with different ages and chemical compositions. The Cepheids of each

population have a slightly different Period-Luminosity Law. This discovery corrects thedistances to the galaxies as measured by Hubble.

The distance to the Andromeda Galaxy is tripled to over 2 million Light Years.

These changes increase the age of the Universe to 6 thousand million years. This is longer thanthe geologists' estimate of the age of the Earth.

1948

Thomas Gold and Fred Hoyle suggest an alternative cosmology to explain the expanding

Universe. The Steady State Theory describes a Universe essentially unchanging in space andtime. As the Universe expands, new matter is created to fill in the gaps left. There was no BigBang.

Nobody can suggest how this new matter arises. For the idea to work a few hundred atoms would

need to be created per cubic kilometre every year.

1950


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Martin Ryle finds radio emissions from the Andromeda Galaxy. He finds that many (but not all)galaxies give out radio waves. These radio galaxies tend to be more abundant amongst the

further galaxies rather than those nearby. Looking at great distances implies looking at the past.This is the first hint that the Universe has changed with time. If so, the Steady State Theory could

not be correct.

William Morgan studies the distribution of luminous hot blue stars in our galactic

neighbourhood. He finds that they are arranged in parallel lines which mark out our Galaxy's

spiral arms. The arm that includes the Sun is called the Local Arm. Away from the centre is thePerseus Arm. Closer to the centre is the Sagittarius Arm.

These observations are later confirmed by studying the distribution and motions of glowing

nebulae. Using optical techniques, observations can only be made to a distance of about 10,000Light Years. This is only one third of the distance to the centre of the Galaxy. The Galaxy

contains dust and gas which block out light from the very distant stars.

Hendrik van de Hulst uses radio telescopes to map the positions of clouds of Hydrogen. Thisallows the Galaxy to be mapped over a larger area. He finds another spiral arm outside thePerseus. Radio waves travel through gas and dust better than light does.

1958

Allan Sandage calculates the age of the Universe by studying distances to nearby galaxies. His

age is 13 thousand million years. This is older than the Earth and the Sun. It is not as old as theoldest Globular Clusters.

1961

Yuri Gagarin becomes the first human being to orbit the Earth.

1963

Maarten Schmidt studies a group of radio objects that appear to be stars. These "stars" areshown to have very large Red Shifts. This indicates that they are further than most galaxies. They

are labelled as "quasi-stellar objects" (or, more commonly, Quasars).

Quasars are mysterious objects: highly luminous and very small. The nearest Quasar (called

3C273) is at a distance of 2 thousand million Light Years. This is over 800 times further than the

Andromeda Galaxy. It shines with the luminosity of 100 normal galaxies! Its brightness varies inperiods of about a month so it must be small compared to a galaxy. 3C273 has been estimated to

have a diameter of over 750,000 million kilometres. This is a million times smaller than ourGalaxy or 4800 times the distance between the Sun and the Earth.

No Quasars are found in the regions of space near our Galaxy. They are now considered to be

very young and active galaxies.


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Because light takes time to travel across space, Quasars show that the early Universe wasdifferent in the past. The Universe is therefore changing in time; it is an evolving Universe. This

contradicts the Steady State Theory.

Arno Penzias and Robert Wilson discover a Universal Background Radiation coming from all

directions equally. This is an effect of the Big Bang predicted by Gamow. The heat producedduring the explosion should have cooled down to a temperature of a few degrees above AbsoluteZero.

The new radiation indicates a temperature around 3 degrees above Absolute Zero. This

phenomenon cannot be explained by the Steady State Theory.

The Big Bang Theory is now accepted by most scientists. Speculation begins about how the

Universe will end. Will it expand forever or will it eventually contract back to nothingness? Thisdepends on the amount of matter in the Universe.

1965

Roger Penrose shows that very massive stars could collapse in on themselves. In theory, they

could form an object with a gravity so high that even light could not escape from them. Theseobjects are called Black Holes and are dismissed by most scientists. Black Holes have the

peculiar property of absorbing matter but never allowing any to escape. As the matter approachesthe Black Holes, it would radiate huge amounts of energy as it becomes compressed by the

gravitational forces.

At the centre of a Black Hole, there would be an object with an infinite density and zero size.

This is called a Singularity. As bizarre as they sound, Singularities are not precluded by the

General Theory of Relativity.

Stephen Hawking shows that if the Theory of Relativity is correct, then the Universe would

have begun as a Singularity rather than as Lematre's "cosmic egg". At the time of the BigBang, the Singularity would have exploded and the Universe would have come into being.

Space, time, and energy would have been created and would expanded together. The originalstate would have had an extremely high temperature. As the temperature dropped, matter would

form out of the energy and eventually, stars and galaxies would have formed out of the matter.

1969

Neil Armstrong and Edwin Aldrin become the first human beings to step on another world, theMoon.

1973

Paul Richards accurately measures the spectrum of the Universal Background Radiation. He

finds that it agrees with theoretical predictions for the Big Bang.


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The abundances of various isotopes of certain elements within galaxies also agree withtheoretical predictions for the Big Bang.

1975

Gustav Tammann refines the age of the Universe from Sandage's work. His figure of 18thousand million years is older than the oldest known objects in the Universe. It would later beshown to be an over-estimate.

1977

RBrent Tully, J Richard Fisher and others develop several new distance yardsticks with

which to measure the size (and age) of the Universe. These are described briefly below.

The luminosity of a spiral galaxy is related to the properties of a particular radio emission in its

spectrum.

The apparent light smoothness of elliptical galaxies is related to their distance.

Distant galaxies that give off X-rays affect the Universal Background Radiation lying betweenthem and us in a way dependent on the distance.

Distant Quasars passing close to a large galactic mass may have their light bent. This producesdouble or multiple images of the Quasar. There is a relation between the angle of the bending,

the time between light variation of the Quasar to be repeated in the duplicate images, and thedistance to the Quasar.

1979

Alan Guth studies the early history of the Universe in terms of particle physics. He suggests a

reason why the Universal Background Radiation appears to be so uniform. This leads to thedevelopment of Inflationary Big Bang theories.

The idea is that the early expansion of the Universe was very rapid for a short while before

settling down to the rate seen today. These theories explain several points in the Big BangTheory. However, there is no observational evidence for them.

1980

Margaret Geller and others discover structure in the Universe. The galaxies are arranged ingroups, clusters, clouds and superclusters.

Our galaxy is a member of a group (the Local Group) consisting of about 20 galaxies in a region

that is 5 million Light Years in diameter. Our galaxy, The Andomeda Galaxy and a third (calledM33) are all large spirals. The Andromeda galaxy is the dominant member of the group with 400

thousand million stars. Our galaxy and M33 contain about 100 thousand million stars.


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The spirals have a number of satellite galaxies. The Andromeda Galaxy has two ellipticalcompanions. Our Galaxy has five companions. Two are irregular galaxies called the Magellanic

Clouds. These are visible in the Southern Hemisphere and resemble detached portions of theMilky Way. Three are small almost-spherical ellipticals hidden behind the Galactic centre. The

rest of the galaxies of the group are small.

The Local Group is on the edge of a cloud of galaxies called the Coma-Sculptor Cloud. This isabout 25 million Light Years across. This cloud is part of the Virgo Supercluster. This

supercluster contains over 1000 galaxies that are mainly elliptical. The centre of the VirgoSupercluster is 60 million Light Years away from our Galaxy. Our Galaxy appears to be moving

towards the centre of the Virgo Supercluster at a speed of 600 kilometers per second.

1983

Andrei Linde suggests that an Inflationary Universe would be perfectly balanced between itsrate of expansion and the amount of matter it contains. Such a Universe would carry on

expanding forever.

1992

From satellite observations, George Smoot finds temperature variations (of the order of 10-5

degrees) in the Universal Background Radiation. These "wrinkles" could explain why theUniverse is clumpy with groups of galaxies rather than being perfectly smooth.

1995

The Hubble Space Telescope surveys the distant parts of the Universe. By doing so, it is looking

into the past.

It is found that spiral and elliptical galaxies are generally stable and unchanging. Irregular

galaxies are active and changing. Even when the Universe was only 30% of its current age,

galaxies had already formed. It appears that star formation was more active when the Universewas only 50% of its current age.

1996

Bruno Leibundgut observes a time delay in the way distant supernovas decay. This is anotherverification that the Universe is expanding.

Carlos Frenksimulates the early history of the Universe on a supercomputer to try and

reproduce the wrinkled structure of the Universe discovered by Smoot. The results only work ifthe expansion of the Universe increases with time.

1998


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J Richard Gott finds that Clusters and Superclusters of galaxies are linked to form filaments.They form "walls" or "sheets" up to 1,000 million Light Years long and enclosing enormous

voids. The Universe on the large scale has the appearance of a sponge.

The Universe resembles fractals produced by mathematical Chaos Theory. This has led to

speculation that this structure may have been caused by random quantum fluctuations during thevery early phase of the Universe.

Saul Perlmutter and his team complete a study of Supernovae (exploding stars) in othergalaxies. The luminosity of these stars can be calculated by studying the way their brightness

fades. The study looks at stars out to a distance of 7 thousand million Light Years. The resultsindicate that the expansion of the Universe is increasing.

Brian Schmidt confirms that the expansion of the Universe was 15% greater when the Universewas half its current age. There is speculation of a repulsive force present on the large scale. This

leads to ideas about the existance of dark energy.

2000

Observations of the variation of temperature in the Universe indicate that it will expand for ever(i.e it is flat and open).

2001

A type ofsub-atomic particle, the Nutrino, is shown to oscilate between three types and to have

mass. This mass, although small, could have an effect on the large scale structure and evolutionof the Universe.

New ideas, called M Theory, may explain the origin of the Big Bang as the collision of 11dimensional spaces.

2002, 2006 KryssTal

This essay is dedicated to Patrick Moore and Isaac Asimov.

Unanswered Questionsy What is the exact mechanism of the Big Bang?y How did the smooth Universe of the Background Radiation evolve into the wrinkled

sponge Universe?

y What part do quasars play in the evolution of galaxies?y Is there life elsewhere in the Universe?y Are there really other Universes?


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y Is there a purpose to the Universe? Is this a religious question?

Documents

A Brief History of Astronomy 0