To keep an astronomical object within the field of view, telescopes need to be able to compensate for the Earth's rotation. Some telescopes use an equatorial mount, which allows the telescope to track an object along a parallel to the celestial equator. This kind of telescope mount is particularly useful for astrophotography.
The polar axis (green) is pointed at the north celestial pole; its angle to the horizon is the same as the observer's latitude. Once the telescope mount is aligned to the pole, the right ascension (r.a.) and declination (dec.) are adjusted to match the celestial coordinates of the object.
As a result of the Earth's rotation, the right ascension coordinate drifts across the sky along a path parallel to the celestial equator. Once the telescope is adjusted to point at an object's sky coordinates (right ascension and declination), it is simply a matter of driving the right ascension axis with a motor to keep up with the object as it moves.
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