The earth rotates about its axis once every day. When observing an object with a telescope, we have to compensate the rotation of the earth. An equatorial mount compensates this motion if the following 2 conditions are met:
- its polar axis is parallel with the polar axis of the earth;
- the mount rotates about its polar axis at exactly the same rate as the earth rotates about its axis
The first condition is met if the mount is correctly set up. The procedure of setting up an equatorial mount is called polar alignment.