Page:Astronomy for Everybody.djvu/103

It is commonly supposed that the principal work of an astronomer is to study the stars as he sees them in his telescope. This is true only in the sense that a telescope is a necessary part of almost every astronomical instrument. But the mere studying of a star with a telescope is a very small part of the astronomer's work. The most important practical use of astronomy to our race consists in the determination of the latitudes and longitudes of points on the earth's surface, so that we may know where towns and cities are situated and be able to make a map of a state or country. This requires a knowledge of the exact positions of the stars in the heavens, that is to say, of their right ascension and declination. We have shown in a former chapter how these quantities correspond to longitude and latitude on the earth's surface. Through that correspondence an observer may determine his latitude by the star's declination and his longitude by its right ascension, combined with a knowledge of the sidereal time at a place of known longitude.

The figures and dimensions of the planets, the motions of the satellites, the orbits of planets and comets, the structure of nebulae and clusters of stars—all these offer fields of astronomical investigation to which there is