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Estimating Distances in the Sky

Observers become accustomed to measuring distances in the sky by degrees. The distance from the point directly overhead (or zenith) to the horizon is 90º. Half of this distance is 45º. One-third of the distance between the zenith and the horizon is 30º, and so on.

Certain stars can be used as measuring sticks to help in gauging distances. For example, the distance between the two brightest stars in the bowl of the Big Dipper (the "pointers" to Polaris) is about 5º. In the southern sky, distance between the two stars which form the leg of Crux, the Southern Cross, is also about 5º.

Binoculars can also make a good measuring stick. Binoculars with a 7º field, for instance, show a circle of sky with a diameter of 7º. We can measure long distances across the sky by steps of 7º each. In telescopes, the size of the field of view varies according to the magnification (or power) of the eyepiece being used. For example, a 20mm eyepiece might have a field of about 1/2º, or a 32mm eyepiece might show a 1º field of view.

To determine the actual field of view of any eyepiece, point the telescope toward any bright, concentrated group of stars near the celestial equator. Look through the chosen eyepiece and note the stars at opposite sides of the field. Then check a star atlas (either yours or a friend's) to determine the distance in degrees between these stars. This distance is the field of view for that eyepiece.

The field of an eyepiece can also be determined by noting how long it takes a star near the equator to cross the field. The rate of movement is 1/4º per minute. For example, a time of 8 minutes equals a distance of 2º.

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