Celestial Sphere

The earth is surrounded by stars at various distances, but when the night sky is viewed on a clear night from anywhere on the earth, it appears that all the stars (and other celestial objects) are fixed onto a huge sphere surrounding the observer. From any given place at any given time, roughly half of the sphere is visible above the horizon, provided that there are clear views unobscured by things like trees or buildings. The other half is below the horizon and invisible.

The earth rotates in space once a day, and the stars are effectively motionless. (They are actually in motion with respect to one another, but generally the effects of these proper motions are so negligible that they will not affect the configurations of the stars in a way detectable with the naked eye over many tens of thousands of years or more.) This means that from the viewpoint of an observer on the earth, the celestial sphere appears to rotate around him or her once a day, with all the stars affixed to it. (The fact that the observer is not at the center of the earth is of little consequence except in the case of the moon, for which lunar parallax must be taken into account before we can determine the position of the moon in the sky, either now or in the past.)

If we are interested in how human societies perceive the sky, as opposed to what modern physics and astronomy can tell us about the universe, then it is how the sky appears rather than how it actually is that is important. Thinking in terms of the celestial sphere enables us to define a number of fundamental concepts that are extremely useful in discussing ancient skies.

For example, on this rotating sphere we can identify an equator and two poles. The latter are called the north and south celestial poles, and they are directly overhead as viewed from the north and south poles respectively on the earth. More generally, and most usefully, we can define lines of latitude (declination) and longitude (right ascension), which allow us to specify the position of any star on the sphere.

See also:

Declination; How the Sky Has Changed over the Centuries; Lunar Parallax; Precession.

References and further reading

Aveni, Anthony F. Skywatchers, 49-57. Austin: University of Texas Press, 2001.

Krupp, Edwin C. Echoes of the Ancient Skies, 3-6. Oxford: Oxford University Press, 1983.

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