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right ascension

(direction coordinate along celestial equator)

Right ascension, one of the coordinates of the equatorial coordinate system, is the celestial sphere's analog to longitude. It is the angular distance to an hour circle (which is like a meridian) as an angle measured along the celestial equator eastward from the vernal equinox. It and declination (the analog of latitude) together specify a direction, i.e., a point in the celestial sphere.

Right ascension is sometimes specified in degrees, but (unlike declination) is often cited in angular hours, a unit equal to 15 degrees, i.e., the angular distance the Earth rotates in an hour. The angular hours are subdivided into angular 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 angular second is 15 arcseconds.) The hours (or degrees) 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, within the constellation Orion, Rigel has a right ascension of is about 5 hours and 15 minutes and Betelgeuse's is about 5 hours and 55 minutes

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 angular hours from the observer's zenith, and counts them westward, i.e., the opposite direction around the celestial sphere as does right ascension. Like the zenith, the a point in the sky's hour angle is constantly changing through the course of the day.

(coordinate,equatorial,celestial sphere,measure)
Further reading:

Referenced by pages:
AB Pictoris (AB Pic)
AD Leonis (AD Leo)
Algol (Beta Per)
Alpha Centauri (α Centauri)
arcsecond (arcsec)
AU Microscopii (AU Mic)
Baade's Window
Barnard's Star
Beta Centauri
Canis Major Dwarf Galaxy
celestial pole
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
ESO 137-001
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)
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)
Messier 74 (M74)
meridian circle
MWC 758
NGC 1600
NGC 253
NGC 2770
NGC 3314
Ohio Radio Survey (OSS)
Hulse-Taylor Binary (PSR B1913+16)
Rho Ophiuchi Cloud Complex
Ross 154
Ross 248
Sagittarius Dwarf Irregular Galaxy (SagDIG)
Sagittarius Dwarf Elliptical Galaxy (Sgr dE)
Scholz's Star
Small Magellanic Cloud (SMC)
solar apex
Teegarden's Star
3C 273
3C 279
3C 295
3C 48
Taurus Molecular Cloud 1 (TMC-1)
TOI 700 d
T Tauri
TW Hydrae (TW Hya)
Ursa Major II Dwarf
WISE 0855-0714 (W0855)
WISE 1506+7027
Wolf 359
WR 104
WR 140
ZTF J1539+5027