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 coordinate systems and distances across the celestial sphere. 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 1/1296000 of a full circle, or 2π/1296000 of a radian. A milli-arcsecond (or mas) and micro-arcsecond are 1/1000 and 1/1000000 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. The only terms I've been able to find to avoid the ambiguity are minute of right ascension, second of right ascension, and hour of right ascension. It is significant that the angle measured in hours is between planes through Earth's axis, or equivalently, an angular distance along the celestial equator: two objects with coordinates differing by an hour are generally separated by less than an angular distance of 15 degrees; for example, a pair of stars within a degree of the celestial north pole are within two degrees of each other regardless of their hour-difference.

When describing patches of the sky (e.g., survey fields) astronomers often avoid units of right ascension: radians (1/2π of a full circle, the length of the circle's radius) may be used, or angular measures such as degrees, arcminutes, and arcseconds may be used. The same is true of small angles, such as the yearly proper motion of a star. Also, degrees are sometimes expressed in decimal fashion rather than using arcminutes and arcseconds, and arcminutes (and perhaps minutes of right ascension) 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

(These are hours, minutes, and seconds of right ascension, the table referring to their implied angles between the planes associated with meridians with that difference in right ascension, or equivalently, the angular distance at the equator.)


(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
https://astronomy.swin.edu.au/cosmos/a/Arcsecond
https://astronomy.swin.edu.au/cosmos/a/Arcminute
https://astronomy.swin.edu.au/cosmos/D/Degree+%28angular%29
https://astronomy.swin.edu.au/cosmos/R/Radian

Referenced by pages:
2dF-SDSS LRG and QSO survey (2SLAQ)
aberration
ACBAR
AD Leonis (AD Leo)
Alpha Centauri (α Centauri)
Anglo-Australian Telescope (AAT)
angular distance
angular resolution
apsis
Archeops
Arcminute Microkelvin Imager (AMI)
ASCA
ASTHROS
Atacama Cosmology Telescope (ACT)
Atacama Large Millimeter Array (ALMA)
ATHENA
Australia Telescope Compact Array (ATCA)
AXIS
Barnard's Loop
Barnard's Star
BINGO
C-BASS
Canada-France-Hawaii Telescope (CFHT)
Canis Major Dwarf Galaxy
Carina Nebula (NGC 3372)
CCAT
celestial pole
Centaurus A
Chandler wobble
CHARA
Circinus Galaxy (ESO 97-G13)
CONCERTO
Crab Nebula (M1)
CRATES
cross dispersion spectrograph
Cygnus Loop
declination (dec)
Einstein Telescope (ET)
elevation range
EMPIRE Survey
equatorial coordinate system (EQ)
equinox
ESO 137-001
Extended Groth Strip (EGS)
field of view (FOV)
Galactic All-sky Survey (GASS)
galactic coordinate system (GCS)
galactic north
GAMA
GOODS-North
GOODS-South
Gould's Belt
gravitational microlensing
Great Wall
Groth Strip
Hadley cell
HATNet
high-resolution imaging
Holmberg radius (RH)
Hubble Ultra-Deep Field (HUDF)
IC 342
ICRF
interferometer
International Celestial Reference System (ICRS)
J designator
J2000.0 equinox
Kapteyn's Star
KELT
LABOCA Extended Chandra Deep Field-South Submillimetre Survey (LESS)
Lacaille 9352
Large Magellanic Cloud (LMC)
Large Millimeter Telescope (LMT)
Luyten Half-second Catalog (LHS)
Luyten Two-Tenths Arcsecond Catalog (LTT)
Lynx
M64
M82
M87
Magellanic Stream
Messier 74 (M74)
Narrabri Stellar Intensity Interferometer (NSII)
NGC 1600
NGC 253
NGC 3314
NIRCam
north polar sequence (NPS)
nutation
obliquity
Ohio Radio Survey (OSS)
Ooty Radio Telescope (ORT)
opposition
parallax
parsec (pc)
PAWS
Perseus-Pisces Supercluster
plate scale
Plateau de Bure Interferometer (PdBI)
Pluto
pointing error (PE)
polarization modes
precession of the equinoxes
projected separation
proper motion (PM)
radiance
RELIKT-1
Rho Ophiuchi Cloud Complex
right ascension (RA)
Rossby radius of deformation
Rubin Observatory (VRO)
Sachs-Wolfe effect (SWE)
Sagittarius Dwarf Elliptical Galaxy (Sgr dE)
Sagittarius Dwarf Irregular Galaxy (SagDIG)
Scholz's Star
SCUBA
Sculptor Wall
seeing
Small Magellanic Cloud (SMC)
solar apex
solid angle (Ω)
spacetime diagram
Spektr-R
SPHEREx
Spitzer Space Telescope (SST)
standard ruler
STROBE-X
SuperBIT
SUSI
Teegarden's Star
Titan
Triangulum Galaxy (M33)
Uhuru
Uppsala General Catalogue (UGC)
Ursa Major II Dwarf
Very Large Array (VLA)
VLASS
WEAVE
WFC3
WISE 0855-0714 (W0855)
Wolf 359

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