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On a clear, dark night the sky looks like a gigantic dome studded with stars. We can easily see why the ancients believed that the starry sky was a huge sphere turning around Earth.

Today we know that stars are remote, blazing Suns racing through space at different distances from Earth. The Earth rotates, or turns, daily around its axis (the imaginary line running through its center between the North and South Poles).

But the picture of the sky as a huge, hollow globe of stars that turns around Earth is still useful. Astronomers call this fictitious picture of the sky the celestial sphere. "Celestial" comes from the Latin word for heaven.

Astronomers use the celestial sphere to locate stars and galaxies and to plot the courses of the Sun, Moon, and planets throughout the year. When you look at the stars, imagine yourself inside the celestial sphere looking out (Figure 1.1).

Why do the stars on the celestial sphere appear to move during the night when you observe them from Earth?_

Answer: Because the Earth is rotating on its axis inside the celestial sphere.

How It Seems:

A star here

Celestic sphere

How It Seems:

A star here

Celestic sphere

Figure 1.1. (a) To a stargazer on Earth, all stars appear equally remote. (b) We picture the stars as fixed on a celestial sphere that spins westward daily (opposite to Earth's actual rotation).

Figure 1.1. (a) To a stargazer on Earth, all stars appear equally remote. (b) We picture the stars as fixed on a celestial sphere that spins westward daily (opposite to Earth's actual rotation).

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