![]() The Astronomical Almanac for the Year 2010. "Time Scales and Coordinate Systems, 2010". ![]() Naval Observatory Nautical Almanac Office, Nautical Almanac Office U.K. "A Compendious Treatise on the Use of Globes and Maps". , 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" ![]() University Science Books, Mill Valley, CA. Explanatory Supplement to the Astronomical Almanac. Naval Observatory Nautical Almanac Office (1992). The first star catalog to use right ascension and declination was John Flamsteed's Historia Coelestis Britannica (1712, 1725). Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of setting circles. 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. A motorized clock drive often is used with an equatorial mount to cancel out the Earth's rotation. 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. 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. But Hipparchus and his successors made their star catalogs in ecliptic coordinates, and the use of RA was limited to special cases. 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. An object's right ascension was its ascension on a right sphere. At the poles, objects did not rise or set (right, sphaera parallela or parallel sphere). From almost anywhere else, they were not (center, sphaera obliqua or oblique sphere). As seen from the equator, both were on a great circle from pole to pole (left, sphaera recta or right sphere). 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. Ancient astronomy was very concerned with the rise and set of celestial objects. Since a complete circle has 360°, 1 h of right ascension is equal to 1⁄ 24 of this, or 15 degrees of arc, 1 m of right ascension is equal to 15 minutes of arc, and 1 s of right ascension is equal to 15 seconds of arc. This is a result of the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates. Īny units of angular measure can be used 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. Right ascension is measured continuously in a full circle towards the east. 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. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an equator. Right ascension is the celestial equivalent of terrestrial longitude. Main article: Equatorial coordinate system
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