# 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.

• North Celestial Pole (NCP) and the South Celestial Pole (SCP) - these are just the north and south poles extended into space.
• Celestial Equator - The earth's equator, but at a much greater radius. If the earth's equator was a rubber band, then the celestial equator is the same rubber band just stretched away from the earth.
• Horizon - The horizon changes depending on your position on earth.
• Zenith- The point on the celestial sphere directly overhead.
• Meridian- The line that extends from the north point on the horizon upwards through the zenith and then downward to the south point on the horizon.

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.

• Analogous to the longitude on Earth, the Right Ascension of an object on the celestial sphere is measured along the celestial equator, as the angular distance to some fiducial direction for with R.A. = 0 degrees. By convention, this fiducial direction is the point on the celestial where the Sun is found on the first day of spring (the vernal equinox).

• Analogous to the latitude on Earth, the Declination of an object on the celestial sphere is measured northward or southward from the plane containing the equator. The declination of the equator is 0 degrees, the North Celestial Pole, +90 degrees, the South Celestial Pole, -90 degrees.

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.