View source for Right ascension

{{short description|Astronomical equivalent of longitude}}

[[File:Ra and dec demo animation small.gif|right|thumb|350px|
'''Right ascension''' and [[declination]] as seen on the inside of the [[celestial sphere]]. The primary direction of the system is the [[equinox (celestial coordinates)|March equinox]], the ascending node of the [[ecliptic]] (red) on the [[celestial equator]] (blue). Right ascension is measured eastward up to 24<sup>h</sup> along the celestial equator from the primary direction.]]

'''Right ascension''' (abbreviated '''RA'''; symbol '''{{mvar|α}}''') is the [[angular distance]] of a particular point measured eastward along the [[celestial equator]] from the [[Sun]] at the [[equinox (celestial coordinates)|March equinox]] to the ([[hour circle]] of the) point in question above the Earth.<ref
>{{cite book
 | author = U.S. Naval Observatory Nautical Almanac Office
 | editor-last = Seidelmann
 | editor-first = P. Kenneth
 | title = Explanatory Supplement to the Astronomical Almanac
 | publisher = University Science Books, Mill Valley, CA
 | date = 1992
 |page=735
 | isbn = 0-935702-68-7}}</ref> 
When paired with [[declination]], these [[celestial coordinate system|astronomical coordinates]] specify the location of a point on the [[celestial sphere]] in the [[equatorial coordinate system]].

An old term, ''right ascension'' ({{langx|la|ascensio recta}})<ref
>{{cite book
|url=https://archive.org/details/guilielmiblaeui00hortgoog
|title=Institutio Astronomica
|publisher=Apud Johannem Blaeu
|date=1668
|first=Guilielmi
|last=Blaeu
|author-link=Willem Blaeu
|page=65
}}, "''Ascensio recta'' Solis, stellæ, aut alterius cujusdam signi, est gradus æquatorus cum quo simul exoritur in sphæra recta"; roughly translated, "''Right ascension'' of the Sun, stars, or any other sign, is the degree of the equator that rises together in a right sphere"</ref> 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 [[perpendicular|intersects]] the [[horizon]] at a [[right angle]]. It contrasts with ''oblique ascension'', the point on the celestial equator that rises with any celestial object as seen from most [[latitude]]s on Earth, where the celestial equator intersects the [[horizon]] at an [[oblique angle]].<ref
>{{cite book
|url=https://archive.org/details/bub_gb_Z1QmAAAAMAAJ
|title=A Compendious Treatise on the Use of Globes and Maps
|date=1821
|first=John<!-- Hiram? -->
|last=Lathrop
|publisher=Wells and Lilly and J.W. Burditt, Boston
|pages=[https://archive.org/details/bub_gb_Z1QmAAAAMAAJ/page/n39 29], 39
}}</ref>

==Explanation==
[[File:Ra and dec on celestial sphere.png|thumb|300px|'''Right ascension''' (blue) and [[declination]] (green) as seen from outside the [[celestial sphere]]]]
[[File: Hour angle still1.png|thumb|Various [[hour angle]]s are depicted here. The symbol ♈︎ marks the [[equinox|March equinox]] direction.
<br />Assuming the day of the year is the March equinox: the [[Sun]] lies toward the grey arrow, the star marked by a green arrow will appear to rise somewhere in the east about midnight (the Earth drawn from "above" turns anticlockwise). After the observer reaches the green arrow, dawn will over-power (see blue sky [[Rayleigh scattering]]) the star's light for about six hours, before it sets on the western horizon. The Right ascension of the star is about 18<sup>h</sup>. 18<sup>h</sup> means it is a March early-hours star and in [[diffuse sky radiation|blue sky]] in the morning. If 12<sup>h</sup> RA, the star would be a March all-night star as [[opposition (planets)|opposite]] the March equinox. If 6<sup>h</sup> RA the star would be a March late-hours star, at its high (meridian) at dusk.]]
{{main|Equatorial coordinate system}}
Right ascension is the celestial equivalent of terrestrial [[longitude]]. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an [[equator]]. Right ascension is measured from the Sun at the [[Equinox (celestial coordinates)|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 [[Pisces (constellation)|constellation Pisces]]. Right ascension is measured continuously in a full circle from that alignment of Earth and Sun in space, that equinox, the measurement increasing towards the east.<ref
>{{cite book
|url=https://archive.org/details/anintroductiont06moulgoog
|title=An Introduction to Astronomy
|last=Moulton
|first=Forest Ray
|author-link=Forest Ray Moulton
|date=1916
|publisher=Macmillan Co., New York
|pages=[https://archive.org/details/anintroductiont06moulgoog/page/n156 125]–126
}}</ref>

As seen from Earth (except at the poles), objects noted to have 12{{Abbreviation|<sup>h</sup> RA|Hours right ascension}} are longest visible (appear throughout the night) at the March equinox; those with 0{{Abbreviation|<sup>h</sup> RA|hours right ascension}} (apart from the sun) do so at the September equinox. On those dates at midnight, such objects will reach ("culminate" at) their highest point (their meridian). How high depends on their declination; if 0° declination (i.e. on the [[celestial equator]]) then at Earth's equator they are directly overhead (at [[zenith]]).

Any [[angular unit]] could have been chosen for right ascension, but it is customarily measured in hours (<sup>h</sup>), minutes (<sup>m</sup>), and seconds (<sup>s</sup>), with 24<sup>h</sup> being equivalent to a [[full circle (unit)|full circle]]. Astronomers have chosen this unit to measure right ascension because they measure a star's location by timing its passage through the highest point in the sky as the [[Earth's rotation|Earth rotates]]. The line which passes through the highest point in the sky, called the [[Meridian (astronomy)|meridian]], is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24<sup>h</sup> of right ascension or 360° ([[Degree (angle)|degrees of arc]]), {{sfrac|1|24}} of a circle is measured as 1<sup>h</sup> of right ascension, or 15°; {{sfrac|1|1440}} of a circle is measured as 1<sup>m</sup> of right ascension, or [[arcminute|15 minutes of arc]] (also written as 15′); and {{sfrac|1|86400}} of a circle contains 1<sup>s</sup> of right ascension, or [[arcsecond|15 seconds of arc]] (also written as 15″). A full circle, measured in right-ascension units, contains {{nobr|24 × 60 × 60 {{=}} {{val|86400}}<sup>s</sup>}}, or {{nobr|24 × 60 {{=}} {{val|1440|fmt=gaps}}<sup>m</sup>}}, or 24<sup>h</sup>.<ref>Moulton (1916), p. 126.</ref>

{{see also|Hour angle}}

Because right ascensions are measured in hours (of [[Earth's rotation|rotation of the Earth]]), they can be used
to time the positions of objects in the sky. For example, if a star with RA&nbsp;= {{nowrap|1<sup>h</sup> 30<sup>m</sup> 00<sup>s</sup>}} is at its meridian, then a star with RA&nbsp;= {{nowrap|20<sup>h</sup> 00<sup>m</sup> 00<sup>s</sup>}} will be on the/at its meridian (at its apparent highest point) 18.5 [[Sidereal time|sidereal hours]] later.

Sidereal hour angle, used in [[celestial navigation]], is similar to right ascension but increases westward rather than eastward. Usually measured in degrees (°), it is the complement of right ascension with respect to 24<sup>h</sup>.<ref>''Explanatory Supplement'' (1992), p. 11.</ref> It is important not to confuse sidereal hour angle with the astronomical concept of [[hour angle]], which measures the angular distance of an object westward from the local [[Meridian (astronomy)|meridian]].

==Symbols and abbreviations==
{| class="wikitable" style="margin: 0 auto"
|-
! scope="col" | Unit !! scope="col" | Value !! scope="col" | Symbol !! scope="col" | [[Sexagesimal]] system !! scope="col" | In [[Radian|radians]]
|-
! Hour
| {{sfrac|1|24}} circle || align="center"|<sup>h</sup> || 15[[degree symbol|°]]|| align="right"|{{sfrac|{{pi}}|12}} rad
|-
! Minute
| {{sfrac|1|60}} hour, {{sfrac|1|{{val|1440|fmt=gaps}}}} circle || align="center"|<sup>m</sup> || {{sfrac|1|4}}°, 15[[Minute of arc|′]]||align="right"|{{sfrac|{{pi}}|720}} rad
|-
! Second
| {{sfrac|1|60}} minute, {{sfrac|1|{{val|3600|fmt=gaps}}}} hour, {{sfrac|1|{{val|86400}}}} circle || align="center"|<sup>s</sup> || {{sfrac|1|240}}°, {{sfrac|1|4}}′, 15[[Second of arc|″]]||align="right"| {{sfrac|{{pi}}|{{val|43200}}}} rad
|}

== Effects of precession ==
{{main|Axial precession}}

The Earth's axis traces a small circle (relative to its celestial equator) slowly westward about the [[celestial pole]]s, completing one cycle in about 26,000 years.  This movement, known as [[Axial precession|precession]], causes the coordinates of stationary celestial objects to change continuously, if rather slowly.  Therefore, [[Equatorial coordinate system|equatorial coordinates]] (including right ascension) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an [[Epoch (astronomy)|epoch]]. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch.<ref>Moulton (1916), pp. 92–95.</ref> Right ascension for "fixed stars" on the equator increases by about 3.1 seconds per year or 5.1 minutes per century, but for fixed stars away from the equator the rate of change can be anything from negative infinity to positive infinity. (To this must be added the [[proper motion]] of a star.) Over a precession cycle of 26,000 years, "fixed stars" that are far from the [[ecliptic pole]]s increase in right ascension by 24h, or about 5.6' per century, whereas stars within 23.5° of an ecliptic pole undergo a net change of{{nbsp}}0h. The right ascension of [[Polaris]] is increasing quickly{{mdash}}in AD 2000 it was 2.5h, but when it gets closest to the north celestial pole in 2100 its right ascension will be 6h. The [[North Ecliptic Pole]] in [[Draco (constellation)|Draco]] and the [[South Ecliptic Pole]] in [[Dorado]] are always at right ascension 18<sup>h</sup> and 6<sup>h</sup> respectively.

The currently used standard epoch is [[J2000.0]], which is January 1, 2000 at 12:00 [[Terrestrial Time|TT]]. The prefix "J" indicates that it is a [[Julian epoch]]. Prior to J2000.0, astronomers used the successive [[Epoch (astronomy)#Besselian epoch|Besselian epochs]] B1875.0, B1900.0, and B1950.0.<ref>see, for instance, {{cite book
 | author1 = U.S. Naval Observatory Nautical Almanac Office
 | author2 = U.K. Hydrographic Office
 | author3 = H.M. Nautical Almanac Office
 | title = The Astronomical Almanac for the Year 2010
 | publisher = U.S. Govt. Printing Office
 | date = 2008
 | page=B2
 | chapter=Time Scales and Coordinate Systems, 2010 }}</ref>

==History==
{{Refimprove section|date=May 2012}}

[[File:Old RA diagram.png|thumb|400px|How '''right ascension''' got its name. Ancient astronomy was very concerned with the rise and set of celestial objects. The ''ascension'' was the point on the [[celestial equator]] (red) which rose or set at the same time as an object (green) on the [[celestial sphere]]. As seen from the equator, both were on a [[great circle]] from pole to pole (left, ''sphaera recta'' or right sphere). From almost anywhere else, they were not (center, ''sphaera obliqua'' or oblique sphere). At the poles, objects did not rise or set (right, ''sphaera parallela'' or parallel sphere). An object's right ascension was its ascension on a right sphere.<ref>Blaeu (1668), p. 40–41.</ref>]]

The concept of right ascension has been known at least as far back as [[Hipparchus]] who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchus and his successors made their [[star catalog]]s in [[Ecliptic coordinate system|ecliptic 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]], which allows the telescope to be aligned with one of its two pivots parallel to the Earth's axis. A motorized clock drive often is used with an equatorial mount to cancel out the [[Earth's rotation]]. 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).
{{-}}
[[File:Stars and ra.png|thumb|500px|center|The entire sky, divided into two halves. '''Right ascension''' (blue) begins at the [[Equinox|March equinox]] (at right, at the intersection of the [[ecliptic]] (red) and the [[celestial equator|equator]] (green)) and increases eastward (towards the left). The lines of right ascension (blue) from pole to pole divide the sky into 24 hours, each equivalent to 15°.]]

==See also==
{{columns-list|colwidth=30em|
* [[Celestial coordinate system]]
* [[Celestial pole]]
* [[Declination]]
* [[Ecliptic]]
* [[Equatorial coordinate system]]
* [[Equinoctial colure]]
* [[Geographic coordinate system]]
* [[Hour angle]]
* [[Right ascension of the ascending node|Right ascension of the ascending node (RAAN)]]
* [[Setting circles]]
* [[Sidereal time]]
}}

==Notes and references==
{{reflist}}

== External links ==
* [http://stars.astro.illinois.edu/celsph.html MEASURING THE SKY A Quick Guide to the Celestial Sphere] James B. Kaler, University of Illinois
* [http://astro.unl.edu/naap/motion1/cec_units.html Celestial Equatorial Coordinate System] University of Nebraska-Lincoln
* [http://astro.unl.edu/naap/motion1/cec_both.html Celestial Equatorial Coordinate Explorers] University of Nebraska-Lincoln
* {{cite web|last=Merrifield|first=Michael|title=(α,δ) – Right Ascension & Declination|url=http://www.sixtysymbols.com/videos/declination.htm|work=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]}}
* [http://www.funsci.com/fun3_en/sider/sider.htm Sidereal pointer] ([[Torquetum]]) – to determine '''RA'''/[[Declination|'''DEC''']].

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