North American Skies for January 1998

Telescopes and Sky Movement

("Telescopes 101," part II)

by Cathie Havens, S&S Optika


Click here for Part I


If you are thinking about a telescope, donít fail to consider the Earthís rotation. Because of the rotation, objects viewed will move out of your field of view quickly, especially at high powers. The easier it is to move the telescope along the axis of rotation of the Earth, the easier it will be to follow the planet or other object you are trying to observe.

The Earth rotates around an axis drawn between the North Pole and the South Pole. The extension of this axis into the sky defines the points (the Celestial Poles) around which the sky seems to rotate. In the Northern Hemisphere, that point is very near Polaris (the North Star). The North Celestial Pole (N.C.P.) is in the northern sky, at the same number of degrees above the horizon as your latitude. So at the North Pole, where latitude is 90 degrees, Polaris appears almost straight overhead. At the Equator, where latitude is 0 degrees, Polaris appears on the horizon. (Actually, since it currently is about 3/4ths of a degree from the N.C.P., it is slightly above the horizon half the time, just below it the other half.) In Denver, where the latitude is about 40 degrees north, Polaris appears about 40 degrees above the horizon.

Except at the Equator, stars do not move straight up from the eastern horizon. As they travel from East to West, they travel in an arc which traces an imaginary line similar to the lines of latitude on a terrestrial globe. In fact, the coordinate system used to identify star positions is very similar to the latitude and longitude system for Earth. In the sky, the North to South measurement (called latitude on Earth) is called Declination . The East to West measurement (called longitude on Earth) are called Right Ascension . These stellar coordinates can be used to map positions of stars and other heavenly bodies just as we map earthly positions in latitude and longitude. (Keep in mind that the entire coordinate system of the sky moves as the Earth turns.)

When choosing a telescope, the mount is just as important as the optics. For extended observation, or those at high power, an equatorial mounting is highly desirable. This mount matches the telescopeís axis to that of the Earth. The illustration at the top of this article shows a German Equatorial Mount. The axis is set at an angle, and can be adjusted to match the local latitude, to line it up with the N.C.P. By pointing it at the N.C.P. (that is, near Polaris), the telescope can be moved such that it will accurately track objects as the move across the sky. A manual slow motion control can make the tracking easy to do.

Another popular mounting is called the altitude-azimuth (Alt-azimuth). A very popular version of this is the Dobsonian mount. It is not aligned with the pole and requires two movements to following objects in the sky. With no slow motion controls, the Dobbie is moved by nudging it along gently.

In the case of either type of mounting, it needs to be sturdy and steady to keep the image still enough to observe.

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