Astrophysics (Index)About

arcsecond

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

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

(unit)
Further reading:
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

Referenced by pages:
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 (RH)
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)

Index