2.3: The Mounting of a Telescope

As the drawing above makes clear, during the night we view the pole star from different positions (such as A and B). This however makes no noticeable difference in its place in the sky, because it is so distant from us. If the Earth rotated not around its axis but along a parallel line through A or B, the sky would not appear any different.

Figure \(\PageIndex{1}\): The equatorial mounting of a telescope. To track a star it is only necessary to rotate the telescope around its polar axis.

To the eye the rotation of the sky is very, very slow (it is most noticeable when the Sun or Moon are rising or setting). A telescope however greatly magnifies the rotation rate, and any star observed with it quickly drifts to the edge of the field of view and then disappear, unless the direction of the telescope is constantly adjusted. That is usually done automatically, by turning the telescope around an axis parallel to the Earth's rotation, for as explained above, a parallel shift does not change the apparent rotation of the stars.

To make such an adjustment easy, an astronomical telescope (pictured above) is mounted very differently from a surveyor's telescope (a “theodolite," pictured below). A theodolite usually has two axes--one allows it to scan all horizontal directions over 360 degrees, while the other adjusts its elevation and allows it to set its sights on reference marks higher than the viewer, such as mountaintops. On the other hand, a telescope for viewing stars (above) also has two perpendicular axes, but the main one (the “equatorial axis") is slanted to point at the pole star and is therefore parallel to the Earth's axis. As the celestial sphere rotates, a clockwork (or in cheap telescopes, the hand of the observer on a suitable knob) turns the telescope at a matching rate, keeping the same stars in the field of view.

Figure \(\PageIndex{2}\): An old surveyor's telescope (theodolite).