From his home on the island of Rhodes in the Aegean, Hipparchus, the greatest of the ancient Greek astronomers, drew up a catalogue of the positions and motions of the objects in the sky. He interpreted the observations as meaning that the Earth was at the centre of everything, and that the planets revolved around the Earth in circles. Claudius Ptolemaeus (more usually called simply Ptolemy), a Greek living in Alexandria in Egypt, observed that the planets did not precisely follow their predicted paths. However, since the circle was regarded as 'perfect' he proposed an 'epicycle' scheme in which each planet pursued a smaller circle about its mean position as it progressed around its orbit.
Having studied mathematics at the University of Cracow, the Polish astronomer Nicolaus Copernicus realised in 1507 that the complexity of Ptolemy's scheme could be banished if it was assumed that the planets revolved around the Sun, with only the Moon going around the Earth. Although Copernicus worked out the consequences of this 'heliocentric' theory and informally circulated it to colleagues, it was not formally published until his death in 1543, as De Revolutionibus Orbium Coelestium.
The Danish nobleman Tycho Brahe noted the appearance of a 'new star' in 1572 in the constellation of Cassiopeia. It appeared as bright as the planet Venus for three weeks, but slowly faded and finally disappeared from sight a year later. When Brahe reported his observations in a short book De Nova Stella, King Frederick II of Denmark was so impressed that Brahe was assigned the small island of Hven in the channel between Copenhagen and Helsingfors to enable him to establish an 'observatory' to undertake a systematic study of the motions of the planets. As optics had not yet been invented, Brahe developed and refined a wide variety of instruments designed to give accurate measurements of the positions of objects in the sky. In the observatory, which Brahe had named Urania, he had a live-in staff of technicians to assist him with observations. As the work progressed, Brahe received a stream of visiting dignitaries and fellow celestial observers, but when Christian IV assumed the throne support for the project ended. In 1599 Brahe relocated to Prague and continued his work under the patronage of Emperor Rudolph II of Germany.
When Brahe died in 1601, his priceless archive of observations passed to Johannes Kepler, who had, towards the end, served as his chief assistant. At that time, there was no clear distinction between astronomy and astrology, and to supplement his mathematical study of empirical laws of planetary motion Kepler earned his living by casting horoscopes. As an unfortunate sign of the times, Kepler's mother was tried as a witch!
In 1687 Isaac Newton published Philosophiae Naturalis Principia Mathematica, promoting his law of Universal Gravitation, from which Kepler's laws followed as a consequence. Brahe had therefore produced a catalogue of exceptionally accurate observations without making any attempt to interpret them, Kepler had provided the empirical analysis without understanding why the planets moved as they did, and Newton had identified the motivating force. Overall, these were remarkable achievements for naked-eye astronomy.
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