Dante and the early astronomers . circle could be read. This last and the eclipticcircle were both divided into 360 degrees, and asmany fractions of a degree as space and skill wouldallow. The equinoctial astrolabe was similar, but the fixedcircle was in the plane of the equator, instead of theecliptic. One of each of these is seen in the viewof the Pekin Observatory. But how did the old astronomers know how to findthe ecliptic and the equator in the sky, and set theircircles in those planes ? This they did by means ofthe suns motion. The gnomon told them the dayof the equinox (see p. 25), and
Dante and the early astronomers . circle could be read. This last and the eclipticcircle were both divided into 360 degrees, and asmany fractions of a degree as space and skill wouldallow. The equinoctial astrolabe was similar, but the fixedcircle was in the plane of the equator, instead of theecliptic. One of each of these is seen in the viewof the Pekin Observatory. But how did the old astronomers know how to findthe ecliptic and the equator in the sky, and set theircircles in those planes ? This they did by means ofthe suns motion. The gnomon told them the dayof the equinox (see p. 25), and on that day the sunwas in the equator: therefore, if a circle was set upso that the shadow of the upper part fell symmetricallyupon the lower, with a little line of light each side,it must be exactly in the plane of the equator. Inthe Square Porch such a circle was erected, a largeone of copper, and when once correctly adjusted itwas a standard plane, and also showed the date ofthe equinoxes, as accurately as the gnomon ^ -9 s -^ THE SCHOOL OF ALEXANDRIA. 117 Since the ecliptic is the path of the sun as seen inthe sky, it is obvious that it could be determined froma number of different observations of his position atdifferent times of the year. Finally, accurate solar tables were drawn up, showingthe suns position in the sky in degrees for differentdates, and then from these it was possible to find theplaces of planets and stars. They could not of coursebe compared directly, but the position of sun and moonwere compared during the day, when both were inthe sky, and then after dark the planets and starswere compared with the moon, allowing for her motionamong the stars in the meantime. Or secondly, whenthe moon was eclipsed, and therefore known to bein the ecliptic and exactly opposite the sun, the placesof stars could be found directly. This very brief description will give some idea ofthe chief instruments and methods used, and whenwe see how very rough and elementar
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