## Glossary |
## Arcsecond |
updated: 2021-08-109 |

A second of arc, arcsecond (arcsec), or arc second, denoted by the symbol ″, is

- 1/60 of an arcminute,
- 1/3600 of a degree,
- 1/1296000 of a turn,
- and π/648000 (about 1/206181.8) of a radian.

One arcsecond therefore equals

Since antiquity, the arcminute and arcsecond have been used in astronomy: in the ecliptic coordinate system as latitude (β) and longitude (λ); in the horizon system as altitude (Alt) and azimuth (Az); and in the equatorial coordinate system as declination (δ). All are measured in degrees, arcminutes, and arcseconds. The principal exception is right ascension (RA) in equatorial coordinates, which is measured in time units of hours, minutes, and seconds.

The arcsecond is also often used to describe small astronomical angles such as the angular diameters of planets (e.g. the angular diameter of Venus which varies between 10″ and 60″); the proper motion of stars; the separation of components of binary star systems; and parallax, the small change of position of a star or solar system body as the Earth revolves about the Sun. These small angles may also be written in milliarcseconds (mas), or thousandths of an arcsecond. The unit of distance called the parsec, abbreviated from the parallax angle of one arc second, was developed for such parallax measurements. The distance from the Sun to a celestial object is the reciprocal of the angle, measured in arcseconds, of the object's apparent movement caused by parallax.

Reference: By Cmglee - Own work, CC BY-SA 3.0

For more information see: Minute and second of arc - Wikipedia

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