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How does an equatorial telescope mount track a star across the sky?
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Telescope
Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.
An equatorial telescope mount tracks a star across the sky by aligning its physical structure with the rotation of the Earth. This is achieved through the following mechanism:
- Polar Alignment: The telescope’s polar axis is constructed parallel to Earth’s axis of rotation. This alignment allows the mount to mimic the apparent diurnal motion of the stars, which appear to rotate around the celestial poles due to Earth's rotation.12
- Coordinate Tracking: The mount utilizes two axes: the polar axis and the declination axis. By setting the telescope to the correct declination (the angular distance north or south of the celestial equator) and right ascension, the observer can pinpoint any object on the celestial sphere.234
- Sidereal Drive: To compensate for Earth's rotation, an electric motor drives the polar axis at a constant "sidereal rate." This rate is precisely matched to the speed at which Earth rotates relative to the stars, allowing the telescope to remain fixed on a celestial object for extended periods of observation.2