Man's physical universe



these observations he worked out a system of astronomy that dominated

the minds of men for the next thirteen centuries. His great contribution

was the compilation of his Almagest, or encyclopedia of

astronomy, which preserved the work of Hipparchus.

Ptolemy believed that the earth is the center of the universe. He

also erroneously believed that the earth is stationary, but he was correct

in his assumption that the earth is a sphere, balanced without supports

in space. He concluded that the fixed stars were fastened to the inside

of a vast dome that revolved about the earth once every twenty-four

hours. The planets supposedly moved in independent orbits between

the earth and this star-studded dome. His system was so satisfactory

that it was widely accepted and never seriously challenged until the

time of Copernicus , thirteen centuries later.

The thirteen hundred years following Ptolemy were a period of

retrogression, in which people returned to the old astrology, and, forgetting

the relatively advanced teachings of Ptolemy, generally considered

the earth to be fiat.

Copernicus Was the First of the Renaissance Astronomers.

Nikolaus Copernicus (1473-1543) was a skilled painter and student

of medicine, mathematics, and astronomy. He taught from the still

dominant Almagest and was greatly influenced by the teachings of

Aristarchus. Fortunately he did not accept traditional theories without

question but made observations of the stars for himself. He found so

many errors in the Ptolemaic tables that he came to distrust all of

Ptolemy's teachings.

In 1507, he wrote his famous book De Revolutionibus Orbium Caelestium

("Concerning the Revolutions of the Heavenly Bodies"). His

main thesis was that the sun is the center of the solar system and that

the earth is but one of the planets revolving around the sun. He also

advanced the modern concepts that the moon revolves around the

earth, accompanying it in its revolution around the sun, and that the

earth turns on its axis from west to east, thus accounting for day arid

night and the apparent motion of the stars.

Other Renaissance Astronomers. ,

Copernicus' follower, Bruno (1548-1600), went even farther and!

stated that the universe is infinite and that the stars are scattered

throughout space. He was regarded as a heretic, however, and was

burned at the stake in 1600.

Tycho Brake (1546-1601), whose interest in astronomy was aroused

by an eclipse of the sun coming at the exact time predicted in 1560,

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