Page:Popular Science Monthly Volume 78.djvu/328

318 mastering these treasures was too great, perhaps his studies in anatomy and metaphysics prevented, but be that as it may, it must be acknowledged that his few pronouncements on physical science were for the most part erroneous and proved hindrances and not helps to those who followed him.

With the founding of the schools of Alexandria in the palmy days of the PtolemysPtolemy's [sic] astronomy became a science. It was during this period that simultaneous measurements were made upon the altitude of the sun at Alexandria and Syene. At the latter place the sun at the summer solstice was on the zenith and at the former place 7° 12' therefrom. From this and the known distance between stations the circumference of the earth was calculated as 250,000 stadia or 28,000 miles. This measurement was first made by Eratosthenes (276396) and places him in the first rank in the Hall of Fame.

The centuries immediately preceding and following the beginning of the Christian era mark the rise both of astronomy and of geometry. It was probably due to progress in the latter science that Hipparchus (190-120 B.C.) and, later, Ptolemy (A.D. 120-170), were led to propound the system that still bears their names. This was a far more ambitious system than any that had preceded it, and sought, for the first time perhaps, to describe the exact path of the heavenly bodies. From our vantage point of wider knowledge it is easy to see its absurdities. Hasty judgment, however, must not be passed either upon its founders or upon the system itself.

In the first place, it was based upon observations; in other words, it was a generalization from data.

Secondly, it satisfactorily explained the observations contained in the premises.

Thirdly, it made possible the forecasting of eclipses.

The characteristics of the system may be given as follows:

1. The earth is a globe set immovable at the center of the celestial sphere—which sphere carries the fixed stars and revolves once per day.

2. The size of the earth is insignificant in comparison with that of the celestial sphere.

3. Seven planets revolve around the earth in the following order—the moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn.

4. The moon and sun move in excentric circles, the rest in epicycles. Of the several considerations that must have induced the Alexandrian school to adopt this system we may note:

I. The Pythagorean system called for a moving earth, the Ptolemaic did not.

II. The observed motions of the planets were explained by the Ptolemaic system, and, while it is true that the Pythagorean system was capable of this also, it does not appear that the test by actual calculation had been made.