Now let us turn to the celestial globe.
What we call latitude and longitude on a terrestrial globe is called declination and right ascension on the celestial globe, because in the heavens there is a latitude and longitude which does not correspond to our latitude and longitude on the earth. If we imagine the lines of latitude and longitude on the earth to be projected, say as shadows thrown on the heavens by a light in the centre of the earth, the lines of right ascension (generally written R.A.) and declination (written Dec. or D.) will be perfectly depicted.
But there is another method of co-ordinating the stars, in which we have the words latitude and longitude used also, as we have said, for the heavens; meaning the distance of a star from the ecliptic instead of the equator, and its distance east or west measured by meridians at right angles to the ecliptic.
This premised, we are in a position to see the enormous advance rendered possible by the methods of observation introduced by Hipparchus and Ptolemy.
[4]. This instrument is also reported to have been used by the Chaldeans in 850 B.C.; the invention of it being attributed to Anaximander. This philosopher, says Diogenes Laertes, observed the revolution of the sun, that is to say, the solstices, with a gnomon; and probably he measured the obliquity of the ecliptic to the equator, which his master had already discovered.
[5]. 28,279 miles.
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.