Right ascension
Right ascension (abbreviated RA; symbol α) is the astronomical term for one of the two direction coordinates of a point on the celestial sphere in the equatorial coordinate system, usually combined with declination. Right ascension's angular distance is measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question.[1]
An old term, right ascension (latin, ascensio recta[2]) refers to the ascension, or the point on the celestial equator which rises with any celestial object, as seen from the Earth's equator, where the celestial equator intersects the horizon at a right angle. It is contrasted to oblique ascension, the point on the celestial equator which rises with a celestial object as seen from almost anywhere else on Earth, where the celestial equator intersects the horizon at an oblique angle.[3]
Explanation
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. For right ascension, the primary direction is known as the vernal equinox or 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. Right ascension is measured continuously in a full circle towards the east.[4]
Any units of angular measure can be used for right ascension, but it is customarily measured in hours ( h ), minutes ( m ), and seconds ( s ), with 24h being equivalent to a full circle. This is a result of the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates. Since a complete circle has 360°, 1h of right ascension is equal to 1⁄24 of this, or 15 degrees of arc, 1m of right ascension is equal to 15 minutes of arc, and 1s of right ascension is equal to 15 seconds of arc.[5]
Because right ascensions are measured in hours (of rotation of the Earth), they can be used to time the positions of objects in the sky. For example, if a star with RA = 01h 30m 00s is on the meridian, then a star with RA = 20h 00m 00s will be on the meridian 18.5 sidereal hours later.
Sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. It is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian.
Symbols and abbreviations
| Unit | Value | Symbol | Sexagesimal system | In radians |
|---|---|---|---|---|
| Hour | 1⁄24 circle | ( h ) | 15° | π⁄12 rad |
| Minute | 1⁄60 hour | ( m ) | 1⁄4°, 15' | π⁄720 rad |
| Second | 1⁄60 minute | ( s ) | 1⁄240°, 1⁄4', 15" | π⁄43200 rad |
Effects of precession
The Earth's axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, 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 an epoch. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch.[6]
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 J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, and B1950.0.[7]
History
 | This section needs additional citations for verification. (May 2012) |
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 catalogs in 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).
See also
Notes and references
- ^ 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. ISBN 0-935702-68-7.
- ^ Bleau, Guilielmi (1668). "Institutio Astronomica". p. 65., at Google books, "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 which rises together in a right sphere"
- ^ Lathrop, John (1821). "A Compendious Treatise on the Use of Globes and Maps". Wells and Lilly and J.W. Burditt, Boston. pp. 29, 39., at Google books
- ^ Moulton, Forest Ray (1916). "An Introduction to Astronomy". Macmillan Co., New York. pp. 125–126.; at Google books
- ^ Moulton (1916), p. 126.
- ^ Moulton (1916), pp. 92-95.
- ^
see, for instance,
U.S. Naval Observatory Nautical Almanac Office, Nautical Almanac Office (2008). "Time Scales and Coordinate Systems, 2010". The Astronomical Almanac for the Year 2010. U.S. Govt. Printing Office. p. B2,. ISBN 970-0-7077-40829.
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External links
- MEASURING THE SKY A Quick Guide to the Celestial Sphere James B. Kaler, University of Illinois
- Celestial Equatorial Coordinate System University of Nebraska-Lincoln
- Celestial Equatorial Coordinate Explorers University of Nebraska-Lincoln