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). At any time, only half the sphere is visible: it is as if the flat ground on which we stand sliced the celestial sphere in half--the upper half is seen, the lower half is not. Because of that, only one pole is seen at any time, and for most of us, living north of the equator, that is the north pole.
(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. Click here to see such a picture.)
Just as the globe of the Earth has an equator around its middle, halfway between the poles, so the sphere of the sky is circled by the celestial equator, halfway between the celestial poles. As the sky rotates, stars on the equator trace a longer circle than any others.
Of course, we know well (as the priests in Babylon didn't) that the stars are not attached inside a huge hollow sphere. Rather, it is the Earth which rotates around its axis, while the stars are so distant that they seem to stand still. The final effect, however, is the same in both cases. Therefore, whenever that is convenient, we can still use the celestial sphere to mark the positions of stars in the sky.
Polaris, the Pole Star
By pure chance, a moderately bright star is seen near the northern celestial pole--Polaris, the pole star (or north star). Polaris is not exactly at the pole, but its daily circle is very small and for many purposes one can assume it is at the pole, a pivot around which the entire sky rotates.
![[IMAGE: Polaris, the pole star]](Sfigs/Spolaris.gif)
All this looks much clearer if one remembers that it is the Earth that rotates, not the sky. The axis around which the Earth spins points in a certain direction in the sky, and that is also the direction of the pole star (or more accurately, the northern celestial pole). As the Earth turns, even though the observer moves with it (for instance, from point B in the drawing to point A), that direction always makes the same angle with the horizon and is always to the north. Hence the pole star is always in the same spot--north of the observer, and the same height above the horizon.
If on a clear night you find yourself lost in the wilderness or at sea, the pole star can tell you where north is, and from that you easily deduce east, west and south. Any other star is unreliable for determining direction--it will move across the sky, and may even set--but not this one. For instructions on finding the pole star at night, click here.
The closer you are to the equator, the closer is the pole star to the horizon, and at the equator (point C) it is on the horizon, and probably not easy to see. Further south, at points such as D, it is no longer visible, but now you can see the southern pole of the sky. Unfortunately, no bright star comparable to Polaris marks that position. The existence of a bright star near the north celestial pole is just a lucky accident, and as will be seen, it wasn't always so, and will not be a few thousand years from now.