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In Renaissance Europe, the ancient Greeks were still regarded as the ultimate authorities on scientific matters, as demonstrated by this 15th-century German painting of astronomers on Mount Athos.

FAR EAsTERN AsTRoNoMY

Other cultures developed constellations quite different from those of the Greeks. The Chinese, for example, recognized a total of 283 constellations, many of them small and faint. Whereas the Greeks pictured mythological beasts and heroes in the sky, Chinese constellations represented scenes from court and social life. East Asian astronomers kept a particular lookout for unexpected phenomena termed "guest stars," which we now know as comets, novae, and supernovae. Among the events they chronicled was the Crab Nebula supernova in ad 1054.

kyongju observatory

This stone tower in Korea was reportedly used by astronomers on every clear night of the year. Dating from ad 634, it is the world's oldest surviving astronomical observatory.

crystalline spheres all nested within each other, rotating on different axes and at different speeds, which carried the celestial bodies around the spherical Earth. Later Greeks modified his system, but the principles of perfect circular motion and an Earth-centered (geocentric) universe remained entrenched in astronomical thinking until the 17th century

The greatest observational astronomer of the Greeks was Hipparchus, who compiled the first accurate catalog of the naked-eye stars in the 2nd century bc. As well as measuring their positions, Hipparchus also classified stars into six categories of brightness, establishing the magnitude scale we use today

In the second century ad, Ptolemy presented a summary of Greek astronomical knowledge in a work usually known as the Almagest, meaning "greatest," a name given to it by later Arabic astronomers. This included an updated version of Hipparchus's catalog, expanded from 850 stars to over 1,000 and arranged into 48 constellations— the foundation of our present-day constellation system.

Ptolemy also offered a new model for the motions of celestial bodies. The basic orbit of each body consisted of a large circle, called the deferent, with its center offset from Earth. As each object moved along the deferent, it also traced out a smaller circle, known as an epicycle.

ARABic AsTRoNoMY

After the decline of Greek and Roman civilization, the center of astronomical research moved east to Baghdad, where Ptolemy's work was translated into Arabic. Shortly before ad 1000, an Arab astronomer named al-sufi produced a revised version of Ptolemy's star catalog, called the Book of the Fixed Stars. As well as the star catalog, al-Sufi's book contained drawings of each constellation. Widely copied and reissued with various illustrations, this became one of the most popular Arab books of astronomy. Between the 10th and 13th centuries, the ancient Greek works were reintroduced to Europe via Arab-dominated Spain.

TuRKisH astronomers

This 16th-century illustration of an observatory founded by Suleyman the Magnificent shows the great traditions of Arab astronomy being carried on by their successors, the Ottoman Turks.

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THE REBIRTH oF wEsTERN AsTRoNoMY

European astronomy was awoken from its dormancy in the 16th century by a Polish clergyman and astronomer, Nicolaus Copernicus (1473-1543), who revived the Sun-centered or heliocentric theory proposed by the Greek philosopher Aristarchus in the 3rd century bc. Such an arrangement explained why Mercury and Venus never strayed far from the Sun, because their orbits were now recognized to be closer to the Sun than Earth's. It also explained why Mars, Jupiter, and Saturn took occasional backward, or "retrograde," loops in the sky, because Earth was overtaking them on its faster, smaller orbit.

Tycho Brahe (1546-1601), a Danish nobleman, realized the need for new and improved observations against which theories of planetary motion could be judged. Between 1576 and 1586, he built two observatories, called Uraniborg and Stjerneborg, on the island of Hven, between Denmark and Sweden, where he built up a detailed series of observations of the motions of the planets. Tycho could never bring himself to accept the heliocentric theory. Instead, he developed his own ingenious compromise in which Earth remained stationary at the center, orbited by the Moon and Sun, while the planets orbited the moving Sun.

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