right ascension

The angular distance of a celestial body or point on the celestial sphere, measured eastward from the vernal equinox along the celestial equator to the hour circle of the body or point and expressed in degrees or hours.

astronomy A form of longitude that expresses the angular position of a star or other celestial body, referenced to the celestial extension of Earth's equatorial plane and axis, with zero value at the First Point of Aries, and representing the additional time until ascension. (Compare celestial latitude; longitude.) The sidereal day is the time of one rotation of Earth relative to the stars, which is ¹/_366.24~ less than the normal apparent solar day; its distinctive hour and minute are each the same proportion of the more familiar units. As Earth rotates, the stars repeat their longitudinal position every 24 sidereal hours, ascending from the eastern horizon just as the Sun does.

Observationally, the overhead position is a more convenient functional marker than the horizon. Right ascension is the time, expressed in sidereal units, from the passage of the First Point of Aries to that of the star being measured, and, unlike most forms of longitudinal measure, is unidirectional, the values going from 0 to virtually 24 (sidereal) hours. (For navigational usage, degrees rather than sidereal hours are used, with values 0° through to virtually 360°.)

The corresponding latitudinal measure is declination, which uses the angular degrees, etc., qualified by North else South (hemisphere).

As discussed under zodiac, the First Point of Aries is not fixed, so the specification data for a star position must refer to a year.

The twelve constellation names used notionally in the zodiac, to divide the year into twelve equal elements, are used even more notionally to divide the day likewise as Earth rotates; Aries is the segment of the sky from 0 to 2 h (0 to 30°), Taurus from 2 to 4 h (30 to 60°), et seq. through Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, and Aquarius to Pisces at 22 to 24 h (330 to 360°).

Greenwich Hour Angle (GHA) is a similar concept to right ascension, being the west longitude (0° to 360°) of the point directly below the celestial body at any instant. Since the GHA of any body equals the GHA of the First Point of Aries plus (360° minus RA of the body), the latter referred to as the Sidereal Hour Angle(SHA) of the body, which is virtually constant for stars, navigators can easily gain one GHA from the other. (Astronomers use the same terminology, but with the angles expressed in sidereal hours rather than degrees.)

While the pattern of the stars recurs virtually exactly every 24 sidereal hours, it advances every 24 normal hours by an amount corresponding to nearly 1°, specifically by ^360°/_365.24~, which figure apparently prompted our 360 degrees in a circle; see degree.

(RA): Celestial coordinate which is equivalent to the longitude of an object on Earth. The starting point for longitude on Earth is at Greenwich England, whereas in the celestial coordinate system it is at a point in the sky called the vernal equinox. The two great circles defined by the celestial equator and the ecliptic, which is defined as the plane of the solar system, cross each other at 2 points: at the vernal equinox (first day of spring) and the autumnal equinox (first day of autumn). The vernal equinox is the starting point for the RA coordinate system.

Equatorial Coordinates

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.

Explanation

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.

History

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).

See also

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

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