The Celestial Sphere :

How do astronomers keep track of the sky when the earth is constantly moving?

The celestial sphere is an imaginary sphere of gigantic radius with the earth located at its center. The poles of the celestial sphere are aligned with the poles of the Earth. The celestial equator lies along the celestial sphere in the same plane that includes the Earth's equator.

An astronomer can only see half the sky at a time, that is, only half the sky is above the horizon at any time. But the sky keeps moving as the earth rotates. Just as the sun rises and sets every day, so does every star in the sky each night. The celestial sphere is a large sphere surrounding the earth and with it we can keep references to where celestial bodies lie in the sky.

We can locate any object on the celestial sphere by giving it two coordinates, called the Right Ascension and the Declination. These are called celestial coordinates.

Stars and galaxies have (almost) fixed positions in Right Ascension and Declination. The Sun and planets, on the other hand, move among the distant stars so that their coordinates change throughout the year. Because of the Earth's yearly orbital motion, the Sun appears to circle the ecliptic.

From Astronomy 201 Cornell University

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