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Encyclopedia > Right ascension
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. Image File history File links Equatorial_coordinates. ... Image File history File links Equatorial_coordinates. ... Astronomy, which etymologically means law of the stars, (from Greek: &#945;&#963;&#964;&#961;&#959;&#957;&#959;&#956;&#943;&#945; = &#940;&#963;&#964;&#961;&#959;&#957; + &#957;&#972;&#956;&#959;&#962;) is a science involving the observation and explanation of events occurring outside Earth and its atmosphere. ... See Cartesian coordinate system or Coordinates (elementary mathematics) for a more elementary introduction to this topic. ... The celestial sphere is divided by the celestial equator. ... The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are: declination () right ascension () -also RA-, or hour angle () -also HA- It is the most closely related to the geographic coordinate system, because they use the same fundamental plane, and the same poles. ... In astronomy, declination (abbrev. ...

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 vernal equinox point (also 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. Longitude, sometimes denoted by the Greek letter Î» (lambda),[1][2] describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... The First Point of Aries, also called the vernal equinox point, is one of the two points on the celestial sphere where the celestial equator intersects the ecliptic. ... The March equinox is the equinox which occurs in the month of March. ...

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 ususally used in astronomy, which is how far west an object is from one's local meridian. The March equinox is the equinox which occurs in the month of March. ... Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ... A degree (in full, a degree of arc, arc degree, or arcdegree), usually symbolized Â°, is a measurement of plane angle, representing 1ï¼360 of a full rotation. ... A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60) of one degree. ... A second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 â‰ˆ 7. ... In astronomy, an objects hour angle (HA) is defined as the difference between the current local sidereal time (LST) and the right ascension () of the object: HAobject = LST - object Thus, the objects hour angle indicates how much sidereal time has passed since the object was on the local... This article is about the astronomical concept. ...

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. STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... A typical daytime sky. ... This article is about the astronomical concept. ...

## 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. Hipparchus. ... Hipparchus. ... In astronomy, many stars are referred to simply by catalogue numbers. ... The ecliptic coordinate system is a celestial coordinate system that uses the ecliptic for its fundamental plane. ...

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). A telescope (from the Greek tele = far and skopein = to look or see; teleskopos = far-seeing) is an instrument designed for the observation of remote objects. ... An Equatorial Mount for a telescope. ... The Earths rotation is the rotation of the solid earth around its own axis, which is called Earths axis or rotation axis. ... The equatorial coordinate system is probably the most widely used celestial coordinate system, whose equatorial coordinates are: declination () right ascension () -also RA-, or hour angle () -also HA- It is the most closely related to the geographic coordinate system, because they use the same fundamental plane, and the same poles. ... In astronomy, declination (abbrev. ... Setting Circles are used on telescopes to find astronomical objects in the sky by their coordinates as listed in a star chart or ephemeris using the celestial coordinate system. ... In astronomy, many stars are referred to simply by catalogue numbers. ... John Flamsteed - Wikipedia, the free encyclopedia /**/ @import /skins-1. ...

Results from FactBites:

 Ascension (disambiguation) - Wikipedia, the free encyclopedia (375 words) Generally, an ascension is the act of ascending, usually to a significantly higher personal state, as the ascension of a king to the throne. In astronomy, right ascension is used to describe positions in the sky. Ascension is a process in the Stargate universe where humans become become noncorporeal god-like entities, where they exisit on a higher plane of existence.
 Right ascension - Wikipedia, the free encyclopedia (254 words) Right ascension (RA; symbol α: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. Being closely tied with sidereal time, it is both a unit of time and of angle. An hour of right ascension is equal to 15 degrees of arc, a minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc.
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