Any units of angular measure can be used for right ascension, 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 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 right ascension, but increases westward rather than eastward. It is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which is how far west an object is from one's local meridian.
Right ascension 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.
For observation purposes, the apparent right ascension is generally used. This gives the position of the body corrected for the delay from the time of light to travel from the body being observed to the earth.
The tilt of the Earth's axis gradually rotates over time. This effect, known as precession, causes the measured right ascension and declination of even a perfectly stationary celestial object to change noticeably over the span of decades. Therefore, equatorial coordinates are inherently relative to the year of their observation. Astronomers always specify equatorial coordinates with reference to a particular epoch. An astronomer comparing coordinates from different epochs must mathematically rotate one of the coordinate systems to match the other, or rotate both to match a common epoch.
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. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, and B1950.0.
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, 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).
Category:Celestial coordinate system Category:Angle Category:Technical factors of astrology
ar:مطلع مستقيم ast:Ascensión reuta bn:বিষুবাংশ bs:Rektascenzija bg:Ректасцензия ca:Ascensió recta cs:Rektascenze da:Rektascension de:Rektaszension et:Otsetõus es:Ascensión recta eo:Rektascensio fa:بعد (ستارهشناسی) fr:Ascension droite ga:Ceartairde gl:Ascensión recta ko:적경 hi:दायां आरोहण hr:Rektascenzija id:Asensio rekta it:Ascensione retta la:Ascensio recta lv:Rektascensija lb:Rektaszensioun lt:Rektascensija hu:Rektaszcenzió mk:Ректасцензија nl:Rechte klimming ja:赤経 no:Rektascensjon nn:Rektasensjon nds:Rektaszension pl:Rektascensja pt:Ascensão reta ro:Ascensie dreaptă ru:Прямое восхождение sk:Rektascenzia sl:Rektascenzija sr:Rektascenzija sh:Rektascenzija fi:Rektaskensio sv:Rektascension th:ไรต์แอสเซนชัน tr:Sağ açıklık uk:Пряме піднесення vi:Xích kinh zh:赤经This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
Coordinates | 9°1′48″N38°44′24″N |
---|---|
name | Fat Jon |
background | non_vocal_instrumentalist |
genre | Hip hop |
associated acts | 3582Five DeezNujabesRebel CliqueStyrofoam |
website | AMPLE SOUL }} |
Fat Jon currently resides in Germany.
Category:1969 births Category:Living people Category:Hip hop record producers Category:People from Cincinnati, Ohio Category:American expatriates in Germany
ja:ファット・ジョン
This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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