Right ascension (abbreviated RA; symbol ) is the
angular distance of a particular point measured eastward along the
celestial equator from the
Sun at the
March equinox
The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the ve ...
to the (
hour circle of the) point in question above the earth.
When paired with
declination
In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of th ...
, these
astronomical coordinates specify the location of a point on the
celestial sphere
In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphe ...
in the
equatorial coordinate system.
An old term, ''right ascension'' ( la, ascensio recta)
[, "''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"] refers to the ''ascension'', or the point on the celestial equator that rises with any
celestial object as seen from
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's
equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...
, where the celestial equator
intersects the
horizon
The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...
at a
right angle
In geometry and trigonometry, a right angle is an angle of exactly 90 degrees or radians corresponding to a quarter turn. If a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles. Th ...
. It contrasts with ''oblique ascension'', the point on the celestial equator that rises with any celestial object as seen from most
latitude
In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north ...
s on Earth, where the celestial equator intersects the
horizon
The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...
at an
oblique angle
In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle.
Angles formed by two rays lie in the plane that contains the rays. Angles ...
.
Explanation
Right ascension is the celestial equivalent of terrestrial
longitude
Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek let ...
. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an
equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...
. Right ascension is measured from the Sun at the
March equinox
The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the ve ...
i.e. the
First Point of Aries, which is the place on the
celestial sphere
In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphe ...
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.
As seen from Earth (except at the poles), objects noted to have 12 are longest visible (appear throughout the night) at the March equinox; those with 0 (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
The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "high ...
).
Any units of angular measure could have been chosen for right ascension, but it is customarily measured in hours (
h), minutes (
m), and seconds (
s), with 24
h being equivalent to a 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 rotates. The line which passes through the highest point in the sky, called the
meridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24
h of right ascension or 360° (
degrees of arc), of a circle is measured as 1
h of right ascension, or 15°; of a circle is measured as 1
m of right ascension, or
15 minutes of arc (also written as 15′); and of a circle contains 1
s of right ascension, or
15 seconds of arc (also written as 15″). A full circle, measured in right-ascension units, contains , or , or 24
h.
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 = is at its meridian, then a star with RA = 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 24
h. It is important not to confuse sidereal hour angle with the astronomical concept of
hour angle
In astronomy and celestial navigation, the hour angle is the angle between two planes: one containing Earth's axis and the zenith (the '' meridian plane''), and the other containing Earth's axis and a given point of interest (the ''hour circle' ...
, which measures the angular distance of an object westward from the local
meridian.
Symbols and abbreviations
Effects of precession
The Earth's axis traces a small circle (relative to its celestial equator) slowly westward about the
celestial poles, completing one cycle in about 26,000 years. This movement, 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
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided ...
. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. 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 the
proper motion of a star.) Over a precession cycle of 26,000 years, "fixed stars" that are far from the
ecliptic 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 of
Polaris
Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude th ...
is increasing quicklyin AD 2000 it was 2.5h, but when it gets closest to the north celestial pole in 2100 its right ascension will be 6h. The
North Ecliptic Pole
An orbital pole is either point at the ends of an imaginary line segment that runs through the center of an orbit (of a revolving body like a planet, moon or satellite) and is perpendicular to the orbital plane. Projected onto the celestial sphe ...
in
Draco
Draco is the Latin word for serpent or dragon.
Draco or Drako may also refer to:
People
* Draco (lawgiver) (from Greek: Δράκων; 7th century BC), the first lawgiver of ancient Athens, Greece, from whom the term ''draconian'' is derived
* ...
and the
South Ecliptic Pole in
Dorado are always at right ascension 18
h and 6
h respectively.
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
In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to pert ...
. Prior to J2000.0, astronomers used the successive
Besselian epochs B1875.0, B1900.0, and B1950.0.
[see, for instance, ]
History
The concept of right ascension has been known at least as far back as
Hipparchus
Hipparchus (; el, Ἵππαρχος, ''Hipparkhos''; BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the e ...
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
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...
, 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
External links
MEASURING THE SKY A Quick Guide to the Celestial SphereJames B. Kaler, University of Illinois
University of Nebraska-Lincoln
University of Nebraska-Lincoln
*
(
Torquetum
The ''torquetum'' or turquet is a medieval astronomical instrument designed to take and convert measurements made in three sets of coordinates: Horizon, equatorial, and ecliptic. It is said to be a combination of Ptolemy's astrolabon and the ...
) – to determine RA/
DEC.
{{DEFAULTSORT:Right Ascension
Astronomical coordinate systems
Angle
Technical factors of Western astrology