Dante and the early astronomers . n centre of gravity enables us to determine the gravitationalforce they exercise on each other, and thus their combined mass ; andtheir spectra give some idea of their density. For instance, the mass ofthe double star Alpha Centauri is nearly twice that of our sun ; and asthe components appear to be about equal to each other, and both show aspectrum resembling that of the sun, we may conclude that AlphaCentauri consists of two stars, each-of which has about the same diameteras our sun. Arcturus has a diameter far greater, some say ten times,some not less than
Dante and the early astronomers . n centre of gravity enables us to determine the gravitationalforce they exercise on each other, and thus their combined mass ; andtheir spectra give some idea of their density. For instance, the mass ofthe double star Alpha Centauri is nearly twice that of our sun ; and asthe components appear to be about equal to each other, and both show aspectrum resembling that of the sun, we may conclude that AlphaCentauri consists of two stars, each-of which has about the same diameteras our sun. Arcturus has a diameter far greater, some say ten times,some not less than twenty-five times as great as the sun ! ^ The mean length of Earths shadow (which varies a little with herdistance from the sun,) is 857,000 miles, or 216 times her semi-diameter. 192 ARAB ASTRONOMY. this—since the size was deduced from the distance—that he also thought it much smaller, and the length ofa shadow thrown by any dark body is longer the nearerit is to the light-source, but shorter, the smaller is Earth small and nearifstant Fig. 37. Earths Shadow. The two last chapters are devoted to eclipses, lunarand solar, and Alfraganus points out that, unlike lunareclipses, eclipses of the sun vary in duration and magni-tude according to the place on Earth from which theyare viewed. The book concludes: Enough having now been saidconcerning the eclipses of sun and moon, by the good-ness of God we have been enabled to bring this writingto an end; and for this Deo Laus et Gloria. The time at which Alfraganus lived is not preciselyknown, but it seems to have been in the first half ofthe 9th century, since from internal evidence he wroteafter, but probably not much after, the death of AlMamun. His name indicates that he was a native ofthe beautiful and fertile country, shut in by loftymountains, which lies on either side of the ancient riverJaxartes. He was surnamed the Calculator, and wroteon sun dials and the astrolabe, but we do not know ofany observational
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