### arcsecond

**(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.

(*unit*)
http://en.wikipedia.org/wiki/Minute_and_second_of_arc

http://en.wikipedia.org/wiki/Right_ascension

**Referenced by:**

Anglo-Australian Telescope (AAT)

Atacama Cosmology Telescope (ACT)

angular resolution

Australia Telescope Compact Array (ATCA)

C-BASS

Canada-France-Hawaii Telescope (CFHT)

CHARA

CRATES

Extended Groth Strip (EGS)

Einstein Telescope

EMPIRE Survey

equatorial coordinate system

fast radio burst (FRB)

Holmberg radius

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

gravitational microlensing

Ohio Radio Survey

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

SCUBA

seeing

solid angle (Ω)

spacetime diagram

SPHEREx

2dF-SDSS LRG and QSO survey (2SLAQ)

Uppsala General Catalogue (UGC)

Very Large Array (VLA)

index