Page:A short history of astronomy(1898).djvu/58

14 arose twelve zodiacal constellations, the names of which have come down to us with unimportant changes from early Greek times Owing, however, to an alteration of the position of the equator, and consequently of the equinoctial points, the sign Aries, which was defined by Hipparchus in the second century B.C. (see chapter ., § 42) as beginning at the vernal equinoctial point, no longer contains the constellation Aries, but the preceding one, Pisces; and there is a corresponding change throughout the zodiac. The more precise numerical methods of modern astronomy have, however, rendered the signs of the zodiac almost obsolete; but the first point of Aries (♈︎), and the first point of Libra (♎︎), are still the recognised names for the equinoctial points.

In some cases individual stars also received special names, or were called after the part of the constellation in which they were situated, e.g. Sirius, the Eye of the Bull, the Heart of the Lion, etc.; but the majority of the present names of single stars are of Arabic origin (chapter ., § 64).

14. We have seen that the stars, as a whole, retain invariable positions on the celestial sphere, whereas the sun and moon change their positions. It was, however, discovered in prehistoric times that five bodies, at first sight barely distinguishable from the other stars, also changed their places. These five—Mercury, Venus, Mars, Jupiter, and Saturn—with the sun and moon, were called planets, or wanderers, as distinguished from the fixed stars.