Use in Astronomy
In astronomy the sizes of objects in the sky are often given in terms of their angular diameter as seen from Earth, rather than their actual sizes. Since these angular diameters are typically small, it is common to present them in arcseconds. An arcsecond is 1/3600th of one degree, and a radian is 180/ degrees, so one radian equals 3600*180/ arcseconds, which is about 206265 arcseconds. Therefore, the angular diameter of an object with visual diameter d at a distance D, expressed in arcseconds, is given by:
 = 206265 d / D arcseconds.
The angular diameter of Earth's orbit around the Sun, from a distance of one parsec, is 2″ (two arcseconds).
The angular diameter of the Sun, from a distance of one lightyear, is 0.03″, and that of the Earth 0.0003″. The angular diameter 0.03″ of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth.
This table shows the angular sizes of noteworthy celestial bodies as seen from the Earth:
Celestial body  Angular diameter  Relative size (10 pixels per arcsecond) 

Sun  31.6′ – 32.7′  28.7 – 29.7 times the maximum value for Venus (orange bar below) / 1896″ – 1962″ 
Moon  29.3′ – 34.1′  26.6 – 31.0 times the maximum value for Venus (orange bar below) / 1758″ – 2046″ 
Venus  9.565″ – 66.012″  Read more about this topic: Angular Diameter Other articles related to "use in astronomy":Equatorial Coordinate System  Spherical Coordinates  Use in Astronomy
... Telescopes equipped with equatorial mounts and setting circles employ the equatorial coordinate system to find objects ... Setting circles in conjunction with a star chart or ephemeris allow the telescope to be easily pointed at known objects on the celestial sphere ... Famous quotes containing the word astronomy:“Awareness of the stars and their light pervades the Koran, which reflects the brightness of the heavenly bodies in many verses. The blossoming of mathematics and astronomy was a natural consequence of this awareness. Understanding the cosmos and the movements of the stars means understanding the marvels created by Allah. There would be no persecuted Galileo in Islam, because Islam, unlike Christianity, did not force people to believe in a “fixed” heaven.” Main Site Subjects
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