Right ascension anddeclination as seen on the inside of thecelestial sphere. The primary direction of the system is theMarch equinox, the ascending node of theecliptic (red) on thecelestial equator (blue). Right ascension is measured eastward up to 24h along the celestial equator from the primary direction.
An old term,right ascension (Latin:ascensio recta)[2] refers to theascension, or the point on the celestial equator that rises with anycelestial object as seen fromEarth'sequator, where the celestial equatorintersects thehorizon at aright angle. It contrasts withoblique ascension, the point on the celestial equator that rises with any celestial object as seen from mostlatitudes on Earth, where the celestial equator intersects thehorizon at anoblique angle.[3]
Right ascension (blue) anddeclination (green) as seen from outside thecelestial sphereVarioushour angles are depicted here. The symbol ♈︎ marks theMarch equinox direction. Assuming the day of the year is the March equinox: theSun 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 skyRayleigh scattering) the star's light for about six hours, before it sets on the western horizon. The Right ascension of the star is about 18h. 18h means it is a March early-hours star and inblue sky in the morning. If 12h RA, the star would be a March all-night star asopposite the March equinox. If 6h RA the star would be a March late-hours star, at its high (meridian) at dusk.
Right ascension is the celestial equivalent of terrestriallongitude. Both right ascension and longitude measure an angle from a primary direction (a zero point) on anequator. Right ascension is measured from the Sun at theMarch equinox i.e. theFirst Point of Aries, which is the place on thecelestial sphere where the Sun crosses thecelestial equator from south to north at the Marchequinox and is currently located in theconstellation 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.[4]
Anyangular unit could have been chosen for right ascension, but it is customarily measured in hours (h), minutes (m), and seconds (s), with 24h being equivalent to afull 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 theEarth rotates. The line which passes through the highest point in the sky, called themeridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24h of right ascension or 360° (degrees of arc),1/24 of a circle is measured as 1h of right ascension, or 15°;1/1440 of a circle is measured as 1m of right ascension, or15 minutes of arc (also written as 15′); and1/86400 of a circle contains 1s of right ascension, or15 seconds of arc (also written as 15″). A full circle, measured in right-ascension units, contains24 × 60 × 60 =86400s, or24 × 60 =1440m, or 24h.[5]
Because right ascensions are measured in hours (ofrotation of the Earth), they can be usedto time the positions of objects in the sky. For example, if a star with RA =1h 30m 00s is at its meridian, then a star with RA =20h 00m 00s will be on the/at its meridian (at its apparent highest point) 18.5sidereal hours later.
Sidereal hour angle, used incelestial 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 24h.[6] It is important not to confuse sidereal hour angle with the astronomical concept ofhour angle, which measures the angular distance of an object westward from the localmeridian.
The Earth's axis traces a small circle (relative to its celestial equator) slowly westward about thecelestial poles, completing one cycle in about 26,000 years. This movement, known asprecession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore,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 anepoch. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch.[7] 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 theproper motion of a star.) Over a precession cycle of 26,000 years, "fixed stars" that are far from theecliptic poles 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 of0h. The right ascension ofPolaris is increasing quickly—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. TheNorth Ecliptic Pole inDraco and theSouth Ecliptic Pole inDorado are always at right ascension 18h and 6h respectively.
The currently used standard epoch isJ2000.0, which is January 1, 2000 at 12:00TT. The prefix "J" indicates that it is aJulian epoch. Prior to J2000.0, astronomers used the successiveBesselian epochs B1875.0, B1900.0, and B1950.0.[8]
Howright ascension got its name. Ancient astronomy was very concerned with the rise and set of celestial objects. Theascension was the point on thecelestial equator (red) which rose or set at the same time as an object (green) on thecelestial sphere. As seen from the equator, both were on agreat 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.[9]
The concept of right ascension has been known at least as far back asHipparchus who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchus and his successors made theirstar catalogs inecliptic coordinates, and the use of RA was limited to special cases.
With the invention of thetelescope, 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 anequatorial 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 theEarth'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 ofsetting circles. The first star catalog to use right ascension and declination wasJohn Flamsteed'sHistoria Coelestis Britannica (1712, 1725).
The entire sky, divided into two halves.Right ascension (blue) begins at theMarch equinox (at right, at the intersection of theecliptic (red) and theequator (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°.
^U.S. Naval Observatory Nautical Almanac Office (1992). Seidelmann, P. Kenneth (ed.).Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA. p. 735.ISBN0-935702-68-7.
^Blaeu, Guilielmi (1668).Institutio Astronomica. Apud Johannem Blaeu. p. 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"
^see, for instance,U.S. Naval Observatory Nautical Almanac Office; U.K. Hydrographic Office; H.M. Nautical Almanac Office (2008). "Time Scales and Coordinate Systems, 2010".The Astronomical Almanac for the Year 2010. U.S. Govt. Printing Office. p. B2.
