Astrophysics (Index)About

right ascension

(RA)
(direction coordinate along celestial equator)

Right ascension is the distance to an hour circle as an angle measured along the celestial equator eastward from the vernal equinox. It and declination together specify a direction, i.e., a point in the celestial sphere. Declination and right ascension are analogous to latitude and longitude, respectively.

Right ascension (but not declination) is commonly cited in angular hours, a unit equal to 15 degrees, i.e., the angular distance the Earth rotates in an hour. The hours are subdivided into minutes and seconds that are 1/60 and 1/3600 of an angular hour. (A degree is similarly divided into the smaller arcminutes and arcseconds, i.e., an arcsecond is 1/15 of an angular second.) Right ascension is also sometimes cited in degrees. The hours are counted from a line from the Sun through the Earth's position at the time of the vernal equinox (March equinox), counting to a higher number of hours as you go eastward.

For example, the constellation Orion has a right ascension in the region of 5 hours and 30 minutes (it covers a portion of the sky, so its area includes right ascensions from less than 5 hours to more than 6 hours).

Hour angle is another type of measure of the placement of an hour circle: there is more than one kind of hour angle, but a common type counts hours from the observer, and westward, i.e., the opposite direction around Earth as does right ascension.


(coordinate,equatorial,celestial sphere,measure)
Further reading:
http://en.wikipedia.org/wiki/Right_ascension

Referenced by pages:
Achernar
AD Leonis (AD Leo)
Algol (Beta Per)
Alpha Centauri
arcsecond (arcsec)
Arcturus
AU Microscopii (AU Mic)
Baade's Window
Barnard's Star
Beta Centauri
Betelgeuse
Canis Major Dwarf Galaxy
Canopus
celestial sphere
Circinus Galaxy (ESO 97-G13)
declination (dec)
Extended Groth Strip (EGS)
Elias 2-27
Epsilon Eridani
Epsilon Indi (ε Indi)
equatorial coordinate system
equinox
ESO 137-001
G239-25
galactic north
GG Tau
Gliese 436 b (GJ 436 b)
HD 189733 b
HD 209458 b
Hubble Deep Field (HDF)
Hubble Deep Field South (HDF-S)
HL Tau
HR 8799
Hubble Ultra-Deep Field (HUDF)
J1407
J2000.0 equinox
J designator
Kapteyn's Star
Kepler Telescope
Lacaille 9352
Lalande 21185
LHS 1140
LHS 3844 b
Large Magellanic Cloud (LMC)
Lockman hole
Luhman 16
Luyten 726-8
Crab Nebula (M1)
Andromeda (M31)
Triangulum Galaxy (M33)
M64
M82
M87
meridian circle
NGC 1600
NGC 253
NGC 2770
NGC 3314
Ohio Radio Survey (OSS)
Procyon
Hulse-Taylor Binary (PSR B1913+16)
Rho Ophiuchi Cloud Complex
Rigel
Ross 154
Ross 248
Sagittarius Dwarf Irregular Galaxy (SagDIG)
Sagittarius Dwarf Elliptical Galaxy (Sgr dE)
Scholz's Star
Sirius
Small Magellanic Cloud (SMC)
solar apex
Teegarden's Star
3C 273
3C 295
3C 48
Taurus Molecular Cloud 1 (TMC-1)
TOI 700 d
TRAPPIST-1
T Tauri
TW Hydrae (TW Hya)
Ursa Major II Dwarf
Vega
WISE 0855-0714
WISE 1506+7027
Wolf 359

Index