Earth view from Apollo 11
The celestial sphere is an imaginary sphere that surrounds the Earth and acts like the Earth, but has an arbitrarily large radius. It has the same center, axis, and rotation as the Earth. It is used in astronomy and navigation. Conceptually, all objects in the sky are projected onto the celestial sphere. Projecting out from the Earth and onto the sphere are the celestial equator and the celestial poles. The celestial equator divides the sphere into the celestial Northern Hemisphere and the celestial Southern Hemisphere.
As the Earth rotates west to east around its axis(every 23 hours 56 minutes equals the sidereal day), the celestial sphere, and all objects on it, appear to rotate from east to west. This is called diurnal motion. This effect causes stars to seem rise in the east, culminate on the meridian(north-south line) and set in the west, unless it is a circumpolar star. The next night the same star will rise again, but, if the 24 hour day is applied, it will rise 4 minutes earlier each progressive day. The reason for this apparent discontinuity is that the Sun does not remain still on the celestial sphere, as the stars do, but moves about 1° eastward each day over the ecliptic (360° in one year to account for the annual motion of the Sun). An angle of 1° corresponds to 4 minutes in time (360° = 24 hours), so we need 4 additional minutes of diurnal motion to see the Sun back on the meridian again. So, the clocks in your home indicate solar time and the clocks used by astronomers display sidereal time.
You will find the celestial sphere referenced many times while you are doing research into aspects of astronomy. Today it is mainly a conceptual projection used for reference, at one time it was thought to be an actual phenomenon. You would be a wiser person for researching the concept further.
We have written many articles about the celestial sphere for Universe Today. Here’s an article about the celestial globe, and here’s an article about the ecliptic.
If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.
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