Right ascension

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Right ascension and declination as seen on the inside of the celestial sphere. The primary direction of the system is the March equinox, the ascending node of the ecliptic (red) on the celestial equator (blue). Right ascension is measured eastward up to 24h along the celestial equator from the primary direction.

Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the Earth.[1] When paired with

astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system
.

An old term, right ascension (

oblique angle.[3]

Explanation

Right ascension (blue) and declination (green) as seen from outside the celestial sphere
opposite
the March equinox. If 6h RA the star would be a March late-hours star, at its high (meridian) at dusk.
Main article: Equatorial coordinate system

Right ascension is the celestial equivalent of terrestrial

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 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.[4]

As seen from Earth (except at the poles), objects noted to have 12h RA are longest visible (appear throughout the night) at the March equinox; those with 0h RA (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

15 seconds of arc (also written as 15″). A full circle, measured in right-ascension units, contains 24 × 60 × 60 = 86400s, or 24 × 60 = 1440m, or 24h.[5]

See also: Hour angle

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 = 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) 18.5 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 24h.[6] 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.

Symbols and abbreviations

Unit Value Symbol Sexagesimal system In radians
Hour 1/24 circle h 15° π/12 rad
Minute 1/60 hour, 1/1440 circle m 1/4°, 15
′
 π/720 rad
Second 1/60 minute, 1/3600 hour, 1/86400 circle s 1/240°, 1/4′, 15
″
 π/43200 rad

Effects of precession

Main article: Axial precession

The Earth's axis traces a small circle (relative to its celestial equator) slowly westward about the

South Ecliptic Pole in Dorado
are always at right ascension 18h and 6h respectively.

The currently used standard epoch is

Julian epoch. Prior to J2000.0, astronomers used the successive Besselian epochs B1875.0, B1900.0, and B1950.0.[8]

History

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.[9]

The concept of right ascension has been known at least as far back as

star catalogs in ecliptic coordinates
, and the use of RA was limited to special cases.

With the invention of the

Historia Coelestis Britannica
(1712, 1725).

The entire sky, divided into two halves. Right ascension (blue) begins at the March equinox (at right, at the intersection of the ecliptic (red) and the 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

Notes and references

  1. ISBN 0-935702-68-7
    .
  2. ^ 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"
  3. ^ Lathrop, John (1821). A Compendious Treatise on the Use of Globes and Maps. Wells and Lilly and J.W. Burditt, Boston. pp. 29, 39.
  4. ^ Moulton, Forest Ray (1916). An Introduction to Astronomy. Macmillan Co., New York. pp. 125–126.
  5. ^ Moulton (1916), p. 126.
  6. ^ Explanatory Supplement (1992), p. 11.
  7. ^ Moulton (1916), pp. 92–95.
  8. ^ 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.
  9. ^ Blaeu (1668), p. 40–41.

External links

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