Arcsecond - Index

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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	2π

(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) Colibrì CONCERTO Crab Nebula (M1) CRATES cross dispersion spectrograph Cygnus Loop declination (dec) du Pont Telescope Einstein Telescope (ET) elevation range EMPIRE Survey equatorial coordinate system (EQ) equinox ESO 137-001 ExoGRAVITY 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 magnitude 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) Polaris (Alpha UMi) polarization modes precession of the equinoxes projected separation proper motion (PM) radiance RATAN-600 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 SPICE Spitzer Space Telescope (SST) standard ruler STROBE-X SuperBIT SUSI Teegarden's Star Titan TRACE 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 Notes to myself regarding this page Index