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Right ascension (abbreviated RA; symbol ''α'') is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question.〔 〕 When combined with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system. An old term, ''right ascension'' (Latin, ''ascensio recta''〔 , 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"〕) 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 with ''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.〔 , at (Google books )〕 ==Explanation== (詳細はlongitude. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an equator. Right ascension is measured from 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 and is located in the constellation Pisces. Right ascension is measured continuously in a full circle from that equinox towards the east.〔 ; at (Google books )〕 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 24h 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 highest point in the sky, called meridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24h of right ascension or 360 degrees of arc°, of a circle is measured as 1h of right ascension or 15°; of a circle is measured as 1m of right ascension or 15 minutes of arc (also written as 15'); and of a circle contains 1s of right ascension or 15 seconds of arc (also written as 15"). A full circle, measured in right ascension units, contains 24×60×60 = 86,400s, or 24×60 = 1,440m, or 24h.〔 Moulton (1916), p. 126.〕 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. Usually measured in degrees ( ° ), it is the complement of right ascension with respect to 24h.〔 ''Explanatory Supplement'' (1992), p. 11〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Right ascension」の詳細全文を読む スポンサード リンク
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