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.
