### 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**
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

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