CHAPTER III. HIPPARCHUS AND PTOLEMY. Among the astronomers of antiquity there are two figures who stand out in full relief—Hipparchus and Ptolemy. The former, “the father of astronomy,” is especially the father of instrumental astronomy. As he was the first to place observation on a sure basis, and left behind him the germs of many of our modern instruments and methods, it is desirable to refer somewhat at length to his work and that of his successor, Ptolemy. Hipparchus introduced extra-meridional observations. He followed Meton, Anaximander, and others in observing on the meridian instead of on the horizon, and then it struck him that it was not necessary to keep to the meridian, and he conceived an instrument, called an Astrolabe, fixed on an axis so that the axis would point to the pole-star, like the one represented in Fig. 8. This engraving is of one of Tycho Brahe’s instruments, which is similar to but more elaborate than that of Hipparchus no drawing of which is extant. C, D, is the axis of the instrument pointed to the pole of the heavens; E, B, C, the circle placed North and South representing the meridian; R, Q, N, the circle placed at right angles to the polar axis, representing the equator, but in the instrument of Hipparchus it was fixed to the circle E, B, C, and not movable in its own plane as this one is. M, L, K, is a circle at right angles to the equator, and moving round the poles, being a sort of movable meridian. Thus, then, if the altitude of a star from the equator (or its declination) was required to be observed, the circle was turned round on the axis, and the sights, Q, M, moved on the circle till they, together with the sight A, pointed to the star; the number of degrees between one of the sights and the equator, was then read off, giving the declination required. The number of degrees, or hours and minutes, of Right Ascension, from K to E could be then read off along the circle R, Q, N, giving the distance of the object from the meridian. As the stars have an apparent motion, the difference in right ascension between two stars only could be obtained by observing them directly after each other, and allowing for the motion during the interval between the two observations. [Illustration: FIG. 8.—Astrolabe (Armillæ Æquatoriæ of Tycho Brahe) similar to the one contrived by Hipparchus. ]