Editing Right ascension (section)

==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]].