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.
Analogous to the longitude on Earth, the Right Ascension of an object on the celestial sphere is measured along the celestial equator in units of hours, minutes and seconds of time, as the angular distance to some fiducial direction for with R.A. = 0h0m0s. 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). Since the earth rotates 360 degress in 24 hours, 1 h represents 15 degrees of rotation. Rotation of 1 degree represents 4 minutes of time.
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. To further refine positions indeicated by the declination, each degree is divided into 60 minutes and each minute is divided into 60 seconds.
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.This page was designed by L Stirling Churchman and Martha Haynes, and can be found on the Astro 201 web page.
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... Last modified 2May00