Celestial Sphere

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Celestial Sphere, imaginary sphere of the heavens, with the earth at its center. The sphere forms the basis for the coordinate systems used in assigning positions to objects observed in the sky. It is also used for designating time intervals and for navigation.

The equatorial system of coordinates establishes a grid in the celestial sphere that is based on the earth's equator and poles, projected outwards to intersect with the sphere. Because the earth is moving around the sun, the appearance of celestial objects such as stars changes on the sphere from day to day. Thus one particular moment of the year is assigned as the time when the celestial grid is established. This moment is the vernal equinox, when the sun's disk passes directly above the equator and marks the beginning of spring in the northern hemisphere (see Ecliptic). Celestial latitude is called declination, and celestial longitude is called right ascension in this equatorial system. Right ascension is measured from the zero-hour circle established by the vernal equinox. The yearly path traced by the sun across the celestial sphere forms a great circle, called the ecliptic, on the sphere. A coordinate system that establishes a grid on the celestial sphere using the ecliptic rather than the equator as its reference plane is also sometimes employed, as are other systems.

The apparent daily movement of the celestial sphere about the earth, caused by the earth's own rotation, is actually about four minutes shorter than the mean solar day.

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