A Brief History of Astronomy

The Solar System Unravelled

(1671 to 1814)


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 flattened at the poles).


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.


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 the Earth 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.

Using the best distance measurements available, Roemer calculates the speed of light. His figure is 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 means that the further we look into space, the further back in time we can see.


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 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 of this 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 centre of the Universe.

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


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 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 the speed 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. Not only is the Earth not the centre of the Solar System but it isn't even the largest of the planets.


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.


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


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. 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 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 is 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 continue to speculate about the existence of other galaxies.


Henry Cavendish applies Newton's equations to very accurate laboratory experiments to measure the mass of the Earth.


Joseph Fraunhofer passes light from the Sun through a high quality prism. This breaks down the 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.

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An easy-to-understand scaling of the Universe in space. Distances in space are represented by the time light takes to travel there.

An easy-to-understand scaling of the Universe in time. The chronology of the Universe is compared to a real year.

A listing of the 20 brightest stars as well as explanations of the terms used.

Information about the planets and satellites of the Solar System with explanations of the terms used.

A historical account of the discovery of the electromagnetic spectrum and its uses in Astronomy. Radio waves, infra-red, visible light, ultra violet, X-rays and gamma rays are explained.

An account of how various properties of stars can be measured by studying starlight. Includes brightness, distance, luminosity, temperature, mass, radius, density and an introduction to the H-R Diagram.

An account of how stars evolve and change the chemistry of the Universe.

The force that moves apples and planets. A short introduction to the ideas of Kepler and Newton that culminated with the theory of Universal Gravitation.

This looks at the history of inventions and the various civilisations of the world.

Selected biographies of people from around the world including scientists and astronomers.

External Links

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History of Science A large collection of resources looking at the history of astronomy, physics, chemistry and mathematics.