202 PRINCIPLES OF THE CELESTIAL SPHERE.

Celestial sphere .—The sphere, divided into squares byparallels and meridians, is also made use of to distinguishin the heavens the position of the stars, as it is to distin-guish on earth the situation of places. It is assumed, 1stthat the heavens form a sphere, having the same centreand the same axis as the earth ; and, 2nd. that all thestars are situated on the surface of this sphere.

A great number of the heavenly bodies, viz. all thosecalled fixed stars, always remain at the same distance fromone another on the celestial sphere, and their relative po-sition does not alter.

If there were any one star placed on the celestialsphere, precisely in the direction of the axis, or on thepole, it would remain immovable, while all the otherswould revolve round it. The star, now called the polestar, is in fact, situated very near the celestial pole of ourhemisphere, and, consequently, describes only a very smallcircle.

All the heavenly bodies vary their position in relationto our globe. Astronomers measure the number of de-grees in latitude and longitude, which mark this position,at certain hours of the day; and on every day. Whenthey have thus distinguished in the heavens a successionof isolated points, indicating sufficiently the path followedby the star, they draw through these points a continuedcurve, which thus marks out the path traversed by thestar in its apparent motion on the surface of the celestialsphere.

From the study of the curves, traced on the celestialsphere by the motion of the heavenly bodies, astronomershave perceived that they are plane curves, and may bedrawn on a right lined circular cone or conical surface olrevolution, constituting that important study conic sec-tions. The planets describe ellipses by their movements,comets describe parabolas, and the sun is situated in oneof the focii of these curves. (See Lesson Thirteenth.)

The application of geometry to describe the course ofthe stars, is of so much importance, that but for it we