Right ascension

RAR.A.αR. A.α=21:54:50.7010657.39ascensionascensionalascensional times.astronomical longitude
Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.wikipedia
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Declination

DecDec.declinations
When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.
A star's direction remains nearly fixed due to its vast distance, but its right ascension and declination do change gradually due to precession of the equinoxes and proper motion, and cyclically due to annual parallax.

Equinox (celestial coordinates)

equinoxvernal equinoxMarch equinox
Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question. Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is currently located in the constellation Pisces.
Since the right ascension and declination of stars are constantly changing due to precession, astronomers always specify these with reference to a particular equinox.

Celestial coordinate system

celestial longitudecelestial coordinatesaltitude
When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.

Equatorial coordinate system

equatorial coordinatesequatorialCoordinates
When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.
A star's spherical coordinates are often expressed as a pair, right ascension and declination, without a distance coordinate.

Celestial sphere

celestialcelestial hemispherehemisphere
When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system. Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is currently located in the constellation Pisces.
For instance, the Astronomical Almanac for 2010 lists the apparent geocentric position of the Moon on January 1 at 00:00:00.00 Terrestrial Time, in equatorial coordinates, as right ascension 6 h 57 m 48.86 s, declination +23° 30' 05.5". Implied in this position is that it is as projected onto the celestial sphere; any observer at any location looking in that direction would see the "geocentric Moon" in the same place against the stars. For many rough uses (e.g. calculating an approximate phase of the Moon), this position, as seen from the Earth's center, is adequate.

Angular distance

angular separationapparent distanceangular separations
Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.
Given two angular positions, each specified by a right ascension (RA), ; and declination (dec), the angular distance between the two points can be calculated as,

Longitude

WestlongitudinalE
Right ascension is the celestial equivalent of terrestrial longitude.
The vertical north–south plane still intersects the plane of the Greenwich meridian at some angle; that angle is the astronomical longitude, calculated from star observations.

Hour circle

Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.
The location of stars, planets, and other similarly distant objects is usually expressed in the following parameters, one for each of the three spatial dimensions: their declination, right ascension (epoch-fixed hour angle), and distance.

First Point of Aries

sun passes into the constellation Aries
Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is currently located in the constellation Pisces.
The First Point of Aries is considered to be the celestial "prime meridian" from which right ascension is calculated.

Equinox

autumnal equinoxautumn equinoxequinoxes
Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is currently located in the constellation Pisces. Hour angle still1.png direction.

Sidereal time

sidereal daysidereallocal sidereal time
For example, if a star with RA = 1 h 30 m 00 s is at its meridian, then a star with RA = 20 h 00 m 00 s will be on the/at its meridian (at its apparent highest point) 18.5 sidereal hours later.
For this reason, to simplify the description of Earth's orientation in astronomy and geodesy, it was conventional to chart the positions of the stars in the sky according to right ascension and declination, which are based on a frame that follows Earth's precession, and to keep track of Earth's rotation, through sidereal time, relative to this frame as well.

Minute and second of arc

masarcsecondarc second
Since a complete circle contains 24 h of right ascension or 360° (degrees of arc), 1⁄24 of a circle is measured as 1 h of right ascension, or 15°; 1⁄1440 of a circle is measured as 1 m of right ascension, or 15 minutes of arc (also written as 15′); and 1⁄86400 of a circle contains 1 s of right ascension, or 15 seconds of arc (also written as 15″).
The principal exception is right ascension (RA) in equatorial coordinates, which is measured in time units of hours, minutes, and seconds.

Meridian (astronomy)

meridiancelestial meridianastronomical meridian
The line which passes through the highest point in the sky, called the meridian, is the projection of a longitude line onto the celestial sphere. It is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian.
An object's right ascension and the local sidereal time can be used to determine the time of its culmination (see hour angle).

Hour angle

Greenwich hour anglelocal hour angleSidereal hour angle
It is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian.
where LHA object is the local hour angle of the object, LST is the local sidereal time, is the object's right ascension, GST is Greenwich sidereal time and is the observer's longitude (positive east from the prime meridian).

Equatorial mount

equatorialGerman equatorial mountequatorial telescope
The easiest way to do that is to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earth's axis.
In astronomical telescope mounts, the equatorial axis (the right ascension) is paired with a second perpendicular axis of motion (known as the declination).

Dorado

Dorado constellationDorDorado (constellation)
The North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18 h and 6 h respectively.

Setting circles

Digital Setting Circlesdivided circlessetting circle
Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of setting circles.
Setting circles consist of two graduated disks attached to the axes – right ascension (RA) and declination (DEC) – of an equatorial mount.

Axial precession

precession of the equinoxesprecessionprecession of equinoxes
This movement, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly.

Ecliptic coordinate system

ecliptic coordinatesecliptic longitudeecliptic latitude
But Hipparchus and his successors made their star catalogs in ecliptic coordinates, and the use of RA was limited to special cases.

Orbital pole

ecliptic polenorth ecliptic poleecliptic north pole
The North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18 h and 6 h respectively.

Star catalogue

star catalogNLTTLHS
But Hipparchus and his successors made their star catalogs in ecliptic coordinates, and the use of RA was limited to special cases.
Stars numbered 1–225300 are from the original catalogue and are numbered in order of right ascension for the 1900.0 epoch.

Celestial equator

equatorialequatorial planeequatorial sky
Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question. Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is currently located in the constellation Pisces.

Sun

solarSolThe Sun
Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.

Astronomical object

celestial bodiescelestial bodycelestial object
An old term, right ascension (ascensio recta ) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle.

Earth

Earth's surfaceterrestrialworld
An old term, right ascension (ascensio recta ) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle.