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An **arcsecond** (**arcsec** or **second of arc**, or in context,
**second** or **sec**) is a unit of
**angle** or **arc**,
commonly used in astronomy for celestial sphere coordinates
and distances across the sky. A **degree** (**angular degree**,
unit of angle corresponding to 1/90 of a right angle)
is divided into 60 **arcminutes** (**arcmin**,
**minute of arc**, or in context, **minute** or **min**),
which, in turn, is divided into 60
arcseconds, making an arcsecond 1/3600 of a degree or 1/1,296,000 of
a full circle, or 2π/1,296,000 of a **radian**. A **milli-arcsecond**
(or **mas**) and **micro-arcsecond** are 1/1,000 and 1/1,000,000 of this unit.

Right ascension is traditionally measured in **hours** (aka
**angular hours**), 1/24 of a full circle, corresponding to 15 degrees.
Fractions of an hour are also expressed as minutes
(1/60 hour) and seconds (1/60 minute).

Astronomers sometimes avoid angular hours, minutes and/or seconds:
**radians** (1/2π of a full circle, the length of
the circle's radius) may be used, or right ascension may be cited
in *degrees*. Also, degrees (and others of these units) are
sometimes expressed in decimal fashion rather than using minutes
or seconds.

Relations:

unit | degrees | hours | radians |

degree | 1 | 1/15 | π/180 |

arcminute | 1/60 | 1/900 | π/10800 |

arcsecond | 1/3600 | 1/54000 | π/648000 |

hour | 15 | 1 | π/12 |

minute | 1/4 | 1/60 | π/720 |

second | 1/240 | 1/3600 | π/43200 |

radian | 180/π | 12/π | 1 |

full circle | 360 | 24 | 2π |

http://en.wikipedia.org/wiki/Minute_and_second_of_arc

http://en.wikipedia.org/wiki/Degree_(angle)

http://en.wikipedia.org/wiki/Right_ascension

http://en.wikipedia.org/wiki/Radian

Anglo-Australian Telescope (AAT)

aberration

ACBAR

Atacama Cosmology Telescope (ACT)

angular resolution

Archeops

ASCA

Australia Telescope Compact Array (ATCA)

C-BASS

Canada-France-Hawaii Telescope (CFHT)

CHARA

Circinus Galaxy (ESO 97-G13)

CONCERTO

CRATES

Extended Groth Strip (EGS)

Einstein Telescope

EMPIRE Survey

equatorial coordinate system

ESO 137-001

fast radio burst (FRB)

Holmberg radius (R

Hubble Ultra-Deep Field (HUDF)

interferometer

J designator

Kapteyn's Star

Lacaille 9352

Luyten Half-second Catalog (LHS)

Large Millimeter Telescope (LMT)

Luyten Two-Tenths Arcsecond Catalog (LTT)

Lynx

M64

M82

M87

gravitational microlensing

NGC 1600

NGC 3314

nutation

Ohio Radio Survey (OSS)

Ooty Radio Telescope (ORT)

Pan-STARRS

parallax

parsec (pc)

PAWS

Plateau de Bure Interferometer (PdBI)

pointing error (PE)

plate scale

proper motion (PM)

Rho Ophiuchi Cloud Complex

Sagittarius Dwarf Irregular Galaxy (SagDIG)

Sagittarius Dwarf Elliptical Galaxy (Sgr dE)

SCUBA

seeing

solid angle (Ω)

spacetime diagram

SPHEREx

standard ruler

Teegarden's Star

2dF-SDSS LRG and QSO survey (2SLAQ)

Uppsala General Catalogue (UGC)

Uhuru

Ursa Major II Dwarf

Very Large Array (VLA)