4000 BC Early peoples thought that the world is flat with a crystalline sky overhead. The Sun was a god that rode across the sky in a chariot, travelling beneath the Earth at night.

The world was created in a small amount of time by whichever deity was worshipped.

A small number of people still believe this today.

1500 BC The Sumarians, Babylonians, and Egyptians develop astronomy. The length of the day, month and year is known. The five naked eye planets are known.

600 BC Anaximander notices that the stars appear to rotate around a pole. He suggests that the sky is a complete sphere around the Earth. He thought that the Earth's surface must be curved after hearing that travellers saw new stars appearing when moving north or south. He pictured the Earth as a cylinder.

The planets are known to move against the background of the stars which appear fixed to a crystal sphere. The word planet means "wanderer".

500 BC Pythagoras and his followers taught that the Earth was a sphere. The idea came about from observations of Lunar Eclipses - the Earth's shadow on the Moon is always circular.

The Pythagoreans thought that the motions of the planets were mathematically related to musical sounds and number. These ideas were called "The Music of the Spheres".

350 BC Heracleides suggests that the daily motion of the Sun, Moon, planets and 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 and water) which he says are only found on Earth. These elements each have their own tendencies: earth is heavy 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 the objects of the sky. Its natural motion is circular so celestial objects travel around the Earth in perfect circles. Aristotle assumes that light travels infinitely fast.

The Earth and the heavens were, therefore, subject to different natural laws. Things on Earth were corrupted and changed while the heavens were incorruptable and unchanging.

250 BC Aristarchus accurately measures of the distance to the Moon using trigonometry applied to Lunar eclipses. He correctly shows that the moon is 25% as large as the Earth.

He makes the first attempt to find the distance to the Sun. His theory is good but the measurements 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.

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.

240 BC Eratosthenes measures of the size of the Earth from observations of the Sun in different parts of the Earth. On the longest day of the year, the Sun was overhead in southern Egypt but 7° from the vertical in northern Egypt. Eratosthenes took the distance between these two points and multiplied it by the ratio between a full circle (360°) and the 7°.

His measurement was within 1% of the correct value.

140 BC Hipparchus refines the distance between the Earth and Moon using Trigonometric Functions which he had invented.

He thought he had observed changes amongst the so called "fixed stars" but wasn't sure. He created a very accurate map of the 1000 or so brightest stars. This map will play an important role in astronomical history 1800 years later.

During his research he discovered that there were two types of year. The Tropical Year and the Sidereal Year differ by 20 minutes. This phenomenon is called the Precession of the Equinoxes.

130 BC Seleucus thought that the Moon was somehow responsible for the tides. This idea would not be proved for nearly 1800 years.

115 BC Poseidonius recalculated the Earth's circumference as 70% of the correct value. This figure was accepted until modern times. 1500 years later, Columbus would use this figure when financing his expedition to the Americas.

He measured the distance between the Earth and the Sun to an accuracy of 43%.

He also popularised astrology.

100 AD Ptolemy wrote a book (The Almagest) which summarised the astronomical knowledge of the ancients (especially that of Aristotle).

The cosmology was based on Earth being the centre of the Universe with the Sun, Moon, planets, and stars (all set on crystal spheres) revolving around the Earth in a series of circles. The system was cumbersome but could be used to predict the motions of the planets to naked eye accuracy. He republished the star map of Hipparchus and named the classical constellations with the names they are still known by in the West. Ptolemy thought that the sphere of the stars was 200 times further away than the Moon.

The book was one of the few to survive the chaos of the Middle Ages. After the fall of the Roman Empire, the book was translated into Arabic in the Islamic world, and, later, into Latin and played a part in Europe's Renaissance

1200 The Catholic Church adopts Aristotle's cosmology when the Ptolemy's Almagest was translated into Latin. Later, disagreement with this cosmology became a heresy.

The model enlarges Aristotle's ideas of the corrupt Earth and the perfect heavens. The most corrupt part of the Universe is Hell situated in the centre of the Earth. Both Earth and Hell are imperfect and both subject to change, corruption and decay. Man's Sin causes the corruption of the Earth. Above the Earth is the atmosphere. This is less subject to change but changes enough to produce the weather. Aurora, meteors and comets are also atmospheric phenomena.

The Moon is further from the Earth and therefore changes less. It changes its phases and has a blotchy appearance but has the perfect circular motion of a celestial object. The Sun and planets come next. They don't change and also move in circles around the Earth. Most distant is the crystal sphere containing the stars. The stars are unchanging and eternal. God (the most perfect part of the Universe) is on the outside of this final crystal sphere. All heavenly motion is in circles (a perfect shape) or 'circles within circles'.

Dante, later wrote about descending the nine circles to Hell and ascending the celestial spheres to God.

Based on Biblical chronologies, the Earth and the Universe were considered to be only a few thousand years old.

People who are fundamentalist in their religion believe a similar model to this.

1543 Nicholas Copernicus publishes a book suggesting that the Sun is the centre of the Universe with the Earth orbiting around it and rotating daily on its 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 it was moving. Winds would blow us off if the Earth was rotating. The stars should show a parallax (i.e they would change relative position as our vantage point changed).

Copernicus has few answers but does suggest that the stars fail to show a parallax because they are very distant. The book is later banned by the Church until the late 20th Century. The new Sun-centred system is less cumbersome than Ptolemy's and can be used for predicting the positions and movements of the planets.

1572 Tycho Brahe studies a new star, actually a nova, or exploding star. He finds no parallax indicating that it is a real stellar object. The star fades after a couple of years. The indication is that the starry heavens do change.

He also studies a comet and shows that is moving in an elongated orbit amongst the planets. This indicates that comets are not atmospheric and that there are no crystal 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 that Aristotle 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.

Tycho Brahe was the last of the great naked-eye astronomers. His detailed and accurate observations of the motion of Mars led to a better understanding of planetary orbits after his death. He measured the year to an accuracy of one second. This helped promote the introduction of the Gregorian Calendar in 1582.

1596 David Fabricus discovers that the star, Omicron Ceti, varies its brightness over several months. This is another blow to the idea of the unchanging heavens

1600 Bruno believes and teaches that the Universe is infinite, the Earth is moving around the Sun, the stars are other suns with planets around them, and life was not confined to the Earth.

He was burnt at the stake for heresy!

1610 Galileo points the newly invented telescope to 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! Indeed, the Milky Way is found to be 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 do so. 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 shape like a miniature Moon from full to half to crescent. This could only happen if it was moving around the Sun rather than the Earth. If Venus was always between the Sun and the Earth it would only exhibit a crescent phase at all times.

The Moon appeared to have mountains and plains. This showed it to be a world, no different to the Earth.

Looking at Jupiter he discovers its four large moons, resembling little stars. This proved that not everything is going around the Earth. It was also an indication that it is possible for a body to carry its moons with it as it goes around the Sun. If Jupiter could carry four moons why could the Earth not carry its single moon?

Galileo's observations supported the Sun-centred Universe of Copernicus and he advocated this system in all his writings. Unlike most academics of the time who only wrote in Latin, Galileo wrote his books in the vernacular so that they could be read by everyone. The Catholic Church was angered and made him deny that the Earth moved around the Sun.

Galileo made experimentation fashionable and disproved some of Aristotle's assertions. By dropping cannon balls from a tower (actually, the Leaning Tower of Pisa) he proved that heavy objects fall to the Earth at the same rate as light objects. He showed that falling bodies accelerated as they fell to Earth. More interestingly, moving bodies could be subject to two separate forces acting independently. This explained how objects could be carried on the Earth even if it was moving.

He also showed that the period of a pendulum is constant for a given length. This will eventually lead to accurate timepieces.

He attempted to measure the speed of light but failed. However, his physics experiments set the stage for Newton's work.

1610 A fuzzy patch is noted in the constellation of Andromeda by Simon Marius. 300 years later, this object will dramatically expand humanity's notions about the Universe.

1620 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 is finally removed. This was the first major use of the newly discovered logarithms in a scientific calculation.

He showed that the closer a planet is to the Sun, the faster it moved. This is the same effect that causes ballet dancers to rotate faster when they bring their arms in. The planets were following mechanical laws similar to those on the Earth. This was a further blow to the ancient idea of one law for the Earth, another for the celestial objects.

Kepler discovered 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 was proportional to the cube of the distance. This provided a scale for the Solar System. If any single distance in the Solar System could be measured all the others could be calculated. Saturn, the furthest planet, was shown to be 10 times further from the Sun than the Earth.

Kepler suggested that the Sun somehow pulled the planets around it. He correctly predicted the passage of the planets Mercury and Venus in front of the Sun. These are called transits and they would later help in determining the distance from the Earth to the Sun.

1639 Jeremiah Horrocks observes the first transit of Venus. He suggests that observations of this phenomenon from different parts of the Earth could yield the scale of the Solar System, hence the distance to the Sun. Observations of an event or object from two vantage points is called parallax.

He proved that the Moon's orbit around the Earth is an ellipse and suggested that the irregularities in the orbit were due somehow to the Sun. He also suggested that Jupiter and Saturn affected each other's orbits.

1640 Godefroy Wendelin measures the distance between the Earth and the Sun using the method first used by Aristarchus. His result was 60% of the actual figure.

1650 Giovanni Riccioli discovers a double star with the telescope. This is a further case of properties of stars not visible to the naked eye.

1656 Christian Huygens discovers a Moon around Saturn. Since this makes six 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 guessed the distance to the brightest star, Sirius by assuming it was the same luminosity as the Sun. His calculated the distance as being over 25,000 times the distance between the Earth and Sun. As large as this distance was, it is actually one twentieth of the correct distance.

1666 Isaac Newton 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 showed 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 to move less while the less massive body will appear to move more. By studying these motions it was possible to show that the Sun was far more massive than all the planets since they all appeared to move around it. The theory allowed the motions of the Moon and planets to be calculated from first principles. Most planetary orbits were almost circular apart from Mercury's. The planets affected each other's paths as they orbited the Sun.

His equations of gravity showed that all objects should fall to the Earth with the same acceleration, as Galileo found. Newton extended the experimental results of Galileo into his three laws of motion. These explained why we could not feel the rotation of the Earth on its axis or its motion around the Sun. They also explained why the planets did not need to be pushed around the Sun and removed the need for planetary 'crystal spheres'. According to Newton, the period of a pendulum can be used to measure the force of gravity on the surface of the Earth.

Newton also explained the tides. They were caused mainly by the Moon (and to a lesser extent, the Sun). His equations explained why there were two tides every day. The Moon was also responsible for the Precession of the Equinoxes, discovered by Hipparchus.

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

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

Apart from his astronomical discoveries, Newton did important work on optics and mathematics.

With the publication of Newton's work, the Age of Reason had 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 sphere flatted 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 destroyed Huygens' view of Solar System perfection.

1676 Olaus Roemer observed Jupiter and its moons. He noted that the eclipses of the moons with the planet were sometimes occurring later than predicted. This was because Jupiter's distance from the Earth varied as both planets orbited the Sun. The delay was caused by the fact that light needed a few minutes to travel from the eclipses to the Earth.

Using the best distance measurements available, Roemer calculated the speed of light. His figure was 75% of the correct value, an excellent first attempt. Aristotle's idea of an infinite speed for light was wrong. The fact that light had a finite (though very large) speed meant that the further we looked into space, the further back in time we could 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 since that map was made. The movement of one star was 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 of this motion was very small but this was the beginning of the end of the idea that the stars were fixed on a crystal sphere.

Assuming that stars were moving at the same rate as planets, it was 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 centre of the Universe.

Halley also worked out the orbit of the comet that bears his name. It was a highly elliptical orbit. Up to then, comets were thought to come and go at random. Halley showed that even comets followed Newton's laws of gravity.

1728 James Bradley, attempts to determine stellar distances by observing stellar parallax during the course of the year. The idea is to use a baseline that is twice the distance between the Earth and the Sun. Observations of stars were made to see if the stars' positions changed.

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 gave a value for the speed of light which was close to the correct value.

Bradley also measured the diameter of Jupiter and found that it was much larger than the Earth. Not only was the Earth not the centre of the Solar System but it wasn't even the largest of the planets.

1755 The 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.

1768 James Cook leads an expedition to the South Pacific to observe a transit of Venus. The observations were not successful but the geographical discoveries made encouraged others to explore the world scientifically.

1780 William Herschel discovers Uranus, the first new planet since ancient times. This instantly doubles 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 to observe 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 Binary Stars. It 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 made the first estimate 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 was apparently situated 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 actually less 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 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 down the white light and displays a spectrum. He found that the continuous rainbow of colours was crossed by thousands of dark lines. These lines would unlock many mysteries of the Universe.

1838 The first stellar distances are finally measured.

Friedrich Bessel measured the parallax of a faint star called 61 Cygni. It had been chosen because it had a large Proper Motion and was 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 Two mathematicians, Urbain Leverrier and John Couch Adams study anomalies in the motion of Uranus and predict the existence of a new planet using the laws of gravity. This planet (Neptune) is quickly detected and is considered another triumph for Newton.

The orbit of the planet Mercury is found to have an anomaly which Newton's laws of gravity cannot explain. This has to wait for Einstein sixty years later.

1848 Armand Fizeau shows that lines in a spectrum change position when the light source is moving to 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 towards the 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 was the first direct proof of the Earth's rotation postulated by Heracleides 2000 years previously.

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

1854 Gustav Kirchhoff studies 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 atoms present in glowing objects without needing a sample in the laboratory.

1863 William Huggins applies the technique of spectroscopy to astronomy. He studies the composition of the Sun, stars and planets. The same elements that exist on Earth are found in space.

He found that the Sun and stars are mainly made of Hydrogen. He measured the Doppler Effect of the star, Sirius and found that it was moving away from us. Comets were shown to contain glowing carbon compounds. Many Nebulae produced spectra that showed that they were glowing gases rather than stars. He used 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) was finally proved to be wrong.

1864 Pietro Sechi photographs the spectra of over 4000 stars. He finds differences which would eventually lead to ideas of stellar evolution.

1868 Joseph Lokyer discovers a new element in the Sun's spectrum. It is named after the Greek word for Sun, Helium. 40 years later, Helium is found on the Earth.

1882 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 by finding a difference in the speed of light in different directions. No difference was found.

Michelson and Morley thought that the experiment had failed because it could not be reconciled with the physics of the day. It turned out to be the most glorious failed experiment in the history of science, eventually laying the groundwork for Einstein's Theory of Relativity.

1893 Wilhelm Wien studies radiation of energy and light from hot objects. He shows that the colour of a glowing body is related to its temperature in a definite mathematical way. It 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 was shown to be around 6,000°C. There were stars that were hotter than the Sun.

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

1895 Maximillian Wolf and Edward Barnard discover that dark areas in the Milky Way are dark nebulae made up of gas and dust.

1900 Max Planck develops the idea that energy exists in lumps (called quanta) rather than continuously. The idea explains Wien's work on radiation.

1906 Jacobus Kapteyn repeats William Herschel's statistical analysis of the stars. He discovers order 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 but are 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 (called Cepheids) 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 link between 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 luminosity is known, distance can be calculated. 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. 90% of stars fit this classification and are called Main Sequence stars. 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 dense stars called White Dwarfs.

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 stellar evolution.

Russell studied 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.

Neils Bohr applied Planck's quantum ideas to atoms helping to explain why and how atomic spectra 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. This is the first direct proof of atomic theory and allows the size of these small particles to be determined.

His Special Theory of Relativity explained why the Michelson and Morley experiment apparently failed. Absolute motion cannot be measured: all motion is relative. This led to the idea 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 led to the famous equation

which showed that matter was a concentrated form of energy. This would allow future scientists to explain the source of the energy of the stars. Time and space turned out to be changeable and dependent on the position and motion of the observer. This defied common sense but was found to be in accord with observation.

Einstein's General Theory of Relativity changed the way humans look at gravity. Newton had envisaged gravity as a force between all matter. Einstein saw matter as distorting the very fabric of space, causing it to curve. This curvature of space caused matter to move in non-linear paths. Under most conditions, the differences between the two theories of gravity were minimal. However, Einstein's theory explained the anomalies in the orbit of Mercury found by Leverrier sixty years earlier.

General Relativity also predicted that light would be bent by a gravitational field. This was proved during a total eclipse of the sun a few years later. Another prediction was that a strong gravitational field would give a spectral red shift separate to the Doppler shift. This was proved when the spectrum of a very dense White Dwarf star was examined. The star was a companion of Sirius so the Doppler effect could be accounted for.

The General Theory of Relativity gave an overall view of the entire Universe indicating that it was not static. Einstein thought that the Universe was static and disregarded this part of his equations. He would soon be proved wrong.

1917 An idea that would explain the formation of the Solar System was postulated by James Jeans. He suggested that a passing star had drawn material from the Sun. This material had condensed to form the planets (including the Earth).

If this idea was correct, the Sun's planetary system could be unique since stellar encounters are very rare. Stars are too far apart to interact with others.

1918 Harlow Shapley applies Leavitt's Cepheid yardstick to Globular Clusters. These are large spherical 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 was 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 pointed out that the Milky 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 were an over-estimate, however. This was the first time that the size of the Universe had been over-estimated. The currently accepted figure is 30,000 Light Years. In 1930 Robert Trumpler showed that interstellar dust had dimmed the Globular Clusters making them look further than they actually were.

1924 Arthur Eddington used gas theory to study the interiors of stars. He showed that stars were stable because there was a balance between two opposing tendencies. The energy and gas pressure coming from the hot centre pushed the star outwards, tending to expand it. Gravity pulled the star inwards, tending to contract it.

He estimated the interior temperature of the Sun to be in the millions of degrees. This was so hot that Jeans' idea of planetary formation would not work.

Eddington discovered the Mass-Luminosity Law for stars. More massive stars are more luminous. His studies allowed him to explain how Cepheid stars varied in brightness by pulsating.

1926 Edwin Schrödinger discovered a wave equation that put Quantum Mechanics on a firm mathematical footing. This would lead to advances in the understanding of atomic and molecular spectra 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 the Galactic 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 Sun requires 200 million years to orbit the Galactic centre. The Galaxy has enough matter to make 100 thousand million stars like the Sun.

1929 Edwin Hubble studies the spiral nebulous object in the constellation of Andromeda (first noted by Marius). Using the world's largest telescope, he managed to see stars in the object. Some of the stars are Cepheids. This allowed him to determine their distance and hence the distance of the spiral. The distance of 800,000 Light Years he found was far outside the domain of our Galaxy even though it was 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 found that there were three types of galaxies: spiral, elliptical and irregular. 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 analysed the velocities of the ones now recognised as galaxies. He found that the overwhelming majority of galaxies are moving away from us. Their spectra showed a Red Shift. He showed that there was a simple mathematical relationship between the distance of the galaxy and its velocity away from us. This relationship is now called Hubble's Law.

Hubble's Law provides another yardstick with which to measure distance. The Red Shift of a galaxy 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 was to assume that the Universe was expanding. Einstein's General Theory of Relativity had already predicted that the Universe was not stable if it was static. Hubble's work had shown that the Universe was not static.

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

1930 Abbé Lemaître and George Gamow explain the observation of the expanding Universe by postulating that it began in a huge explosion. This is colourfully known as the Big Bang Theory.

Lemaître suggested that all the matter in the Universe was once contained in a very dense "cosmic egg". This object exploded and the matter was spread out through space. We see the effects of this explosion when we observe the galaxies moving away from each other.

Gamow predicted that the echo of the explosion should be detectable as radiation with a temperature of 5 degrees above Absolute Zero. This radiation should permeate throughout the Universe. It would not be detected for over 20 years.

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

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

If the amount of matter in the Universe was above a certain critical level, then the expansion of the Universe would eventually slow down and stop. The Universe would then contract with all the galaxies 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 was 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 would require a knowledge of how quickly the Universe was expanding compared to how much matter it contained. This problem would not be solved for 70 years.

1932 Karl Jansky discovers radio waves from space. He finds that these signals come from the centre of the Galaxy, its position agreeing with Shapley's. Radio waves open a new window to the Universe. A window that is not stopped 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 of Calcium.

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

1938 Hans Bethe and Carl Weizsächer work out the details of how the Sun produces its energy. It is by nuclear fusion, converting Hydrogen to Helium. Every second over 3 million tonnes of the Sun'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 the distances 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 than the 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 and time. As the Universe expands, new matter is created to fill in the gaps left. There was no Big Bang.

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 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 the Perseus Arm. Closer to the centre is the Sagittarius Arm.

These observations were later confirmed by studying the distribution and motions of glowing nebulae. Using optical techniques, observations can only be made to a distance of about 10,000 Light 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.

Van De Hulst used radio telescopes to map the positions of clouds of Hydrogen. This allowed the Galaxy to be mapped over a larger area. He found another spiral arm outside the Perseus. 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 the oldest 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" are shown 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 in periods 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 our Galaxy 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.

Because light takes time to travel across space, Quasars show that the early Universe was different in the past. The Universe is therefore changing in time; it is an evolving Universe. This contradicted 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 produced during the explosion should have cooled down to a temperature of a few degrees above Absolute Zero.

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? This depends 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. These objects 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 approaches the 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 showed that if the Theory of Relativity is correct, then the Universe would have begun as a Singularity rather than as Lemaître's "cosmic egg". At the time of the Big Bang, 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 original state 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, the Moon.

1973 Paul Richards accurately measured the spectrum of the Universal Background Radiation. He finds that it agrees with theoretical predictions for the Big Bang.

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

1975 Gustav Tammann refines the age of the Universe from Sandage's work. His figure of 18 thousand million years is older than the oldest known objects in the Universe. It would later be shown to be an over-estimate.

1977 Brent Tully, 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 between them and us in a way dependent on the distance.

Distant Quasars passing close to a large galactic mass may have their light bent. This produces double 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 the distance 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 the development 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 Bang Theory. However, there is no observational evidence for them.

1980 Margaret Geller and others discover structure in the Universe. The galaxies are arranged in groups, clusters, clouds and superculsters.

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 (called M33) 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.

The spirals have a number of satellite galaxies. The Andromeda Galaxy has two elliptical companions. 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 the Milky 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 is about 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 Virgo Supercluster 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 its rate 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 the Universe 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 Universe was only 50% of its current age.

1996 Carlos Frenk simulates 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 if the expansion of the Universe increases with time.

1998 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 the very early phase of the Universe.

A type of sub-atomic particle, the Nutrino, may have mass. This mass, although small, could have an effect on the large scale structure and evolution of the Universe.

Saul Perlmutter and his team complete a study of Supernovae (exploding stars) in other galaxies. The luminosity of these stars can be calculated by studying the way their brightness fades. The study looked at stars out to a distance of 7 thousand million Light Years. The results indicated that the expansion of the Universe was increasing.

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

2000 Recent observations of the variation of temperature in the Universe indicates that it will expand for ever (i.e it is flat and open).

Unanswered questions include:

• What is the exact mechanism of the Big Bang?
• Why did the Big Bang occur?
• How did the smooth Universe of the Background Radiation evolve into the wrinkled sponge Universe?
• What part do quasars play in the evolution of galaxies?
• Is there life elsewhere in the Universe?
• Are there other Universes? Is this a meaningful question?
• Is there a purpose to the Universe? Is this a religious question?

© 2000 Kryss Katsiavriades

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