Movement of the sky

We are accustomed to the Sun, Moon and stars rising and setting, and most people hardly give this a thought. But to astronomers the way the sky moves is important, so as to know what will be appearing and when. We think of the sky moving, but of course it is the Earth that does the moving, by spinning once a day and also orbiting the Sun. But it’s much easier to think of the sky moving, and it is actually termed as the celestial sphere which surrounds the Earth and rotates, carrying the Sun Moon and planets with it, though they are not fixed to it as are the stars, and have their own motions. (Even the stars move through the sky, though even the nearest move so slowly across the celestial sphere that the stars we see today are in the same as those seen by our ancestors.)

So the daily spinning of the Earth, which is clockwise as seen from above the north pole, makes the sky move from left to right as seen from the northern hemisphere. If you could see the stars during the daytime you would be able to see the entire sky visible from your latitude over the course of a day. Earth’s movement around the Sun causes a slower left-to-right movement which takes a whole year to complete a full turn. The result is that if you were to look at the sky at the same time each night you’d find that the stars appear in one spot four minutes earlier each night, making a full turn of the starry sky every 23h 56m (called the sidereal day). But as we measure time as being from midnight to midnight, in practice the day is 24 hours long.

The sky appears to move from left to right as seen from the northern hemisphere. The Sun, Moon and stars therefore rise on the eastern horizon and set on the western horizon and reach their maximum height when due south (or, strictly speaking, when they are on your meridian wherever you are). In the southern hemisphere the direction of movement is reversed, so although objects rise in the east and set in the west, they reach their greatest height due north, and appear to move from right to left. In tropical regions, everything rises more or less vertically into the sky.

From the north pole you would see half the sky only, rotating around Polaris which would be above your head. As you move farther south you would see Polaris slipping down the sky and some more constellations appearing in the south for a short while. In mid latitudes you would see more of the southerly constellations, but still not the full celestial sphere. At the equator the whole sphere is visible at some time during the day, but as you move down into the southern hemisphere you start to lose Polaris and some of the northern constellations. When you reach the south pole you’ve lost not only Polaris but all the northern half of the sky.

celestial sphere