Daily motion

The universe is a mass of swirling motions, but most of the time, you can ignore all but one of them. That one is daily motion (diurnal motion), caused by the rotation of the Earth. You will see it immediately if you aim a 100× telescope at a star with the drive motor turned off.

As you know, celestial objects rise in the east, move across the sky, and set in the west. But as Figures 2.1 and 2.2 show, that is not the whole story. The motion is not directly from east to west; instead the whole sky rotates like a globe with Polaris at its north pole.

In the southern sky, each object rises somewhere on the eastern horizon (not necessarily due east!), passes across the sky, and sets somewhere in the west. Its path may be long or short. In the far southern sky, objects rise just east of south, climb only a short distance above the horizon, and set again a short time later, just west of south.

The most northerly celestial objects are circumpolar; that is, they do not rise or set at all.