The Celestial Sphere
The Sun rules by daytime sky, but at night, especially if the Moon does not shine, the show belongs to the stars. Bright and dim, randomly distributed across the sky, with odd formations that catch the eye, their number seems huge. To ancient observers it seemed as if Earth was at the center of a giant star-studded "celestial sphere," which reinforced the belief, held for thousands of years, that we are at the center of the universe.
If you watch stars throughout the night, you will see that most of them also rise to the east of you and set west of you, like the Sun and Moon. Indeed, the entire celestial sphere seems to rotate slowly--one turn in 24 hours--and since half of it is always hidden below the horizon, this rotation constantly brings out new stars on the eastern horizon, while others to disappear beneath the western one.
We of course know that it is not the universe that rotates around us from east to west, but our Earth is the one rotating, (from west to east). But it is still convenient to talk about "the rotation of the celestial sphere." That could also make the sky rotate the way it is observed to do.
Most stars keep fixed positions relative to each other, night after night. The eye naturally groups them into patterns or constellations, to which each culture has given its own names. The names we use come from the ancient Greeks and the Romans.
Like the globe in the drawing, the sphere of the sky has two points around which it turns, points that mark its axis --the celestial poles. Stars near those poles march in daily circles around them, and the closer they are, the smaller the circles (they do not rise and set).
If you mount a camera on a dark night in a way that the pole is in the middle of its field of view, open the shutter and take a time exposure, the image of each star will be smeared into part of a circle, and all the circles will be centered on the pole.
