Astronomy Answers
From the Astronomical Dictionary


[Astronomy Answers] [Dictionary] [AnswerBook] [Universe Family Tree] [Science] [Starry Sky] [Planet Positions] [Calculate] [Colophon]

The description of the word you requested from the astronomical dictionary is given below.

the angle

In geometry, an angle is a difference between directions. Angles are commonly measured in degrees. A full circle is equivalent to 360 degrees: If you turn around completely and end up looking in the same direction as you did before, then you've turned over 360 degrees. A right angle is equal to 90 degrees, and the angles between the sides of an equilateral triangle (one with three sides of equal length) is 60 degrees.

A degree is also written as °, so 31° is 31 degrees. A degree is further divided into 60 arcminutes (also written as 60' or 60 minutes of arc), and an arcminute into 60 arcseconds (also written as 60" or 60 seconds of arc), so one degree is equal to 3600 arcseconds. Astronomical objects often appear so small in the sky that their apparent angular sizes are expressed in arcminutes or arcseconds. For instance, the Sun and Moon have angular diameters of about 30 arcminutes, the planet Jupiter has an angular diameter of about 40 arcseconds, and the planet Pluto of 0.1 arcseconds.

The smallest detail a telescope can possible see (which is called its resolution) is also often measured in arcseconds. It is about equal to 0.13 arcseconds divided by the diameter of the primary entrance of the telescope in meters, or to 130 arcseconds over the diameter in centimeters, or to 5.0 arcseconds divided by the diameter in inches. With a perfect 6-inch telescope, you may be able to see details as small as 0.8 arcseconds - so in such a telescope Jupiter would appear as a small disk, but Pluto would look like a point, just like the stars. Often, the resolution of telescopes is worse than what this formula yields, because the mirrors or lenses are not perfect and because the atmosphere of the Earth tends to blur the images.