Right ascension

Right ascension (abbrev. RA; symbol α) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.


RA is the celestial equivalent of terrestrial longitude. Both RA and longitude measure an east-west angle along the equator; and both measure from a zero point on the equator. For longitude, the zero point is the Prime Meridian; for RA, the zero point is known as the First Point of Aries, which is the place in the sky where the Sun crosses the celestial equator at the March equinox.

RA is measured eastward from the March equinox. Any units of angular measure can be used for RA, but it is customarily measured in hours, minutes, and seconds, with 24 hours being equivalent to a full circle. The reason for this choice is that the earth rotates at an approximately constant rate (see sidereal time). Since a complete circle has 360 degrees, an hour of right ascension is equal to 1/24 of this, or 15 degrees of arc, a single minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc. Sidereal Hour Angle, used in celestial navigation, is similar to RA, but increases westward rather than eastward. It is important not to confuse SHA with the concept of hour angle as it is usually used in astronomy, which is how far west an object is from one's local meridian.

RA can be used to determine a star's location and to determine how long it will take for a star to reach a certain point in the sky. For example, if a star with RA = 01:30:00 is at a location's meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later.

Since the right ascension (and declination) of stars are constantly changing due to precession, astronomers always specify these with reference to a particular epoch. The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. Prior to this astronomers used the successive Besselian Epochs B1875.0, B1900.0 and B1950.0.


The concept of right ascension has been known at least as far back as Hipparchos who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchos and his successors made their star catalogs in ecliptical coordinates, and the use of RA was limited to special cases.

With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do that is to use an equatorial mount for the telescope, which allows the telescope to rotate at the same rate as the earth. As the equatorial mount became widely adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of setting circles. The first star catalog to use right ascension and declination was John Flamsteed's "Historia Coelestis Britannica" (1712, 1725).

ee also

*Declination, Celestial coordinate system
*geographic coordinates, ecliptic
*Setting circles

External links

* [http://www.astronomynotes.com/nakedeye/radcspin.htm Right ascension and declination visualized]

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Look at other dictionaries:

  • Right ascension — Right Right (r[imac]t), a. [OE. right, riht, AS. riht; akin to D. regt, OS. & OHG. reht, G. recht, Dan. ret, Sw. r[ a]tt, Icel. r[ e]ttr, Goth. ra[ i]hts, L. rectus, p. p. of regere to guide, rule; cf. Skr. [.r]ju straight, right. [root]115. Cf.… …   The Collaborative International Dictionary of English

  • Right ascension — Ascension As*cen sion, n. [F. ascension, L. ascensio, fr. ascendere. See {Ascend}.] 1. The act of ascending; a rising; ascent. [1913 Webster] 2. Specifically: The visible ascent of our Savior on the fortieth day after his resurrection. ( Acts i.… …   The Collaborative International Dictionary of English

  • right ascension — ► NOUN Astronomy ▪ position measured along the celestial equator, expressed in hours, minutes, and seconds …   English terms dictionary

  • right ascension — n. Astron. the angular distance of the hour circle of a celestial body from the vernal equinox, measured eastward along the celestial equator and expressed in degrees (from 0 to 360) or, more commonly, in hours (from 0 to 24), minutes, and… …   English World dictionary

  • right ascension — Astron. the arc of the celestial equator measured eastward from the vernal equinox to the foot of the great circle passing through the celestial poles and a given point on the celestial sphere, expressed in degrees or hours. [1585 95] * * * ▪… …   Universalium

  • right ascension — Refraction Re*frac tion (r?*fr?k sh?n), n. [F. r[ e]fraction.] 1. The act of refracting, or the state of being refracted. [1913 Webster] 2. The change in the direction of ray of light, heat, or the like, when it enters obliquely a medium of a… …   The Collaborative International Dictionary of English

  • right ascension — noun (astronomy) the equatorial coordinate specifying the angle, measured eastward along the celestial equator, from the vernal equinox to the intersection of the hour circle that passes through an object in the sky; usually expressed in hours… …   Useful english dictionary

  • right ascension — right′ ascen′sion n. astron. the arc of the celestial equator measured eastward from the vernal equinox to the foot of the great circle passing through the celestial poles and a given point on the celestial sphere, expressed in degrees or hours • …   From formal English to slang

  • right ascension — noun Date: 15th century the arc of the celestial equator between the vernal equinox and the point where the hour circle through a given body intersects the equator reckoned eastward commonly in terms of the corresponding interval of sidereal time …   New Collegiate Dictionary

  • Right ascension — …   Википедия

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