Man's physical universe

xanabras

50 THE UNIVERSE A VAST SYSTEM OF PARTS

knowledge for nearly two thousand years.

One of his greatest contributions

was the creation of the inductive method of reasoning and the

working out of the principles of logic.

Aristotle improved nearly every

field of learning he touched except those of physics and astronomy. He

considered the earth to be a sphere in the center of the universe and

taught that the planets were supernatural beings entirely unlike the

the earth. Aristotle added a fifth element/ ether, which was supposed

to move in circles. He rejected the ideas of previous philosophers that

matter could be divided into ultimate particles called atoms.

Of all the Greeks, Aristarchus (died 230 h.c.) held the most modern

ideas of astronomy. He taught that the spherical earth and the planets

revolved about the sun. He concluded that the sun was about twenty

times more distant from the earth than the moon. The actual ratio

is about 1 to 400, but his errors were due to inadequate instruments;

the principles of his experiments were sound. He accounted for the

relative immobility of the stars by rightly assuming that they were

very much farther away than the sun. Unfortunately the experiments

of Aristarchus were rejected for the teaching of Aristotle.

Eratosthenes (died about 195 B.C.) was the first man to measure the

size of the earth on the assumption that it was a sphere. He noted that

at noon on June 21 at Syene, Egypt, the sun was directly overhead

and that exactly a year later at Alexandria the sun was a little more

than 7 degrees south of the line passing directly overhead. He concluded

that this was due to the fact that the earth was round. Inasmuch

as the distance between these two cities (about 480 miles by his

measurements) represented 7/360 of the entire circumference of the

earth, he figured the circumference to be about 24,000 miles, which is

within about 900 miles of the present accepted value for the circumference

at the equator.

Hipparchus (died 125 B.C.) was the last of the great Greek astronomers.

He catalogued about 1080 stars, classified according to their

brightness, and mapped the course of the sun among the stars. His

measurements of the size of the moon and its distance from the earth

were within about 10 per cent of present-day values. He discovered

the precession of the equinoxes (see page 98) and determined its

amount.

The next great man was Claudius Ptolemy, a Greek born in Egypt,

who, between the years 127 and 151 a.d., made observations in Alexandria,

the center of Hellenic culture for centuries. On the basis of

1

When the ancients used the word "element," it meant "elementary quality" rather

than "elementary substance," which is the modern meaning of the word, first clearly

defined by Lavoisier.

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