The Earth's tilt (23.45º) causes the equatorial rotation of the Celestial Sphere. As a result, those of us in the northern hemisphere can see some of the constellations of the southern hemisphere, and visa versa. This sphere rotates about a virtual axis called the North and South Poles.

For the Northern Hemisphere, the axis of rotation is centered in Polaris, a star in the constellation Ursa Minor (or Little Bear) - sometimes called the Little Dipper. Polaris is not exactly at the axis of rotation, but its pretty close. The pole star is circled:

The southern hemisphere is not so lucky. This image has the south pole area circled, but there is no star associated with this region:

The motion of this virtual sphere is the reason for the changing positions of the night sky. In fact, the sky changes on a daily basis - with the exception of the poles. Because the Earth rotates and orbits the Sun at the same time, the constellations rise 15 minutes earlier every night.

If one were to stand at the north pole, the same constellations would be visible every night, although rotated (counter-clockwise). All of the constellations that lie below Earth's equator would never be visible. Likewise, standing at the south pole would make the northern constellations non visible - but the stars would rotate clockwise!

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