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 (unit of angle corresponding to 1/360 of a full circle) 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

(unit)
Further reading:
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

Referenced by pages:
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 (RH)
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)

Index