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(arcsec, sec, second)
(a 3600th of an angular degree)

An arcsecond (arcsec or just 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 (unit of angle corresponding to 1/360 of a full circle) is divided into 60 arcminutes (arcmin or just 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 value.

Right ascension is commonly measured in hours (which might logically be called 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), which are not the same angular size as the arcsecond and arcminute associated with degrees. (As far as I can tell, the terms arcsecond and arcminute are unambiguous, but one must be careful with the terms second and minute.)

Astronomers sometimes avoid hours, minutes and/or seconds: radians may be used, or hours for right ascension may be avoided in favor of degrees, or degrees (and/or hours) may be expressed in decimal fashion when specifying more angular precision.


Referenced by:
Anglo-Australian Telescope (AAT)
Atacama Cosmology Telescope (ACT)
angular resolution
Australia Telescope Compact Array (ATCA)
Canada-France-Hawaii Telescope (CFHT)
Extended Groth Strip (EGS)
Einstein Telescope
equatorial coordinate system
fast radio burst (FRB)
Holmberg radius
Hubble Ultra-Deep Field (HUDF)
J designator
Kapteyn's Star
Lacaille 9352
Luyten Half-second Catalog (LHS)
Large Millimeter Telescope (LMT)
Luyten Two-Tenths Arcsecond Catalog (LTT)
gravitational microlensing
Ohio Radio Survey
Ooty Radio Telescope (ORT)
parsec (pc)
Plateau de Bure Interferometer (PdBI)
pointing error (PE)
plate scale
proper motion (PM)
Rho Ophiuchi Cloud Complex
solid angle (Ω)
spacetime diagram
2dF-SDSS LRG and QSO survey (2SLAQ)
Uppsala General Catalogue (UGC)
Very Large Array (VLA)