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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 1/360 of a full circle
which is 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 indicated by a different
set of units, **hours** (aka **angular hours**), 1/24 (15 angular
degrees) of the circle around the sky at the given declination,
which corresponds to the shift in the sky over the course of an
hour (1/24 of a day). Fractions of an (angular) hour are also
expressed as (angular) **minutes** (1/60 angular hour) and (**seconds**
(1/60 minute). (These minutes and seconds are not the same angular size
as arcminutes and arcseconds, creating the possibility of ambiguity.)

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 are sometimes expressed in decimal
fashion rather than using arcminutes and arcseconds, and arcminutes
(and perhaps minutes) are also sometimes expressed in decimal fashion.

Equivalences:

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 |

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

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

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

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

Anglo-Australian Telescope (AAT)

aberration

ACBAR

Atacama Cosmology Telescope (ACT)

AD Leonis (AD Leo)

Atacama Large Millimeter Array (ALMA)

angular distance

angular resolution

Archeops

ASCA

Australia Telescope Compact Array (ATCA)

ATHENA

AXIS

Barnard's Star

BINGO

C-BASS

celestial pole

Canada-France-Hawaii Telescope (CFHT)

Chandler wobble

CHARA

Circinus Galaxy (ESO 97-G13)

CONCERTO

CRATES

Extended Groth Strip (EGS)

Einstein Telescope

EMPIRE Survey

equatorial coordinate system

ESO 137-001

field of view (FOV)

Galactic All-sky Survey (GASS)

high-resolution imaging

Holmberg radius (R

Hubble Ultra-Deep Field (HUDF)

ICRF

interferometer

J designator

Kapteyn's Star

Lacaille 9352

Luyten Half-second Catalog (LHS)

Large Magellanic Cloud (LMC)

Large Millimeter Telescope (LMT)

Luyten Two-Tenths Arcsecond Catalog (LTT)

Lynx

Triangulum Galaxy (M33)

M64

Messier 74 (M74)

M82

M87

gravitational microlensing

NGC 1600

NGC 3314

Narrabri Stellar Intensity Interferometer (NSII)

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

Pluto

precession of the equinoxes

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

Spektr-R

SPHEREx

standard ruler

Teegarden's Star

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

Uppsala General Catalogue (UGC)

Uhuru

Very Large Array (VLA)

WISE 0855-0714 (W0855)