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Name: Alec
Status: educator
Grade: 9-12
Location: CA
Country: USA
Date: Winter 2012-2013


Question:
How do you use and calculate an arc second in astronomy?


Replies:
Dear Alec,

That is a good question! I do not really calculate it, but the sky is naturally divided into a 360 degree circle. Thus, the space between the two "pointer" stars of the Big Dipper is about five degrees. Each degree is subdivided into 60 minutes of arc. Subsequently, each minute is subdivided into 60 seconds of arc, or 60 arc-seconds. So we can say that the separation between the two component stars of Beta, or Albireo, is about 30 arc seconds. If it is possible on a fine night to see objects separated by less than an arc-second, we call it sub-arc-second seeing.

This is not really a calculation, but I hope it helps! Thank you.

David H. Levy


An arc second is just an angle measurement; it is 1/3600 degree. The direction to something in the sky is an angle, and the difference between two directions is also an angle. Small-angle differences are given in arc seconds, milliarc seconds (1/1000 if an arc second), or even finer.

The directions to some of the closer stars from earth vary slightly over the course of a year, as the earth changes its position by orbiting the sun. The smaller the change, the farther away the star is. This idea is the basis of finding distances in parsecs (a contraction of ?parallax second?). If a star?s apparent direction changes by one arc second, the star is one parsec distant. If its direction changes by only half an arc second, the star is two parsecs distant. (A parsec is about 3.28 light years.) And so on. The distance in parsecs is the reciprocal of the direction difference in arc seconds.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


Hi Alec,

A degree is 3600 arc second. The arc second is used for positioning of objects that are very far away (galaxies) or for small features on a near object (a crater on our Moon).

Peter E. Hughes, Ph.D. Milford, NH



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