Right ascension
In Astronomy, the right ascension ( has or '' α '') is one of the two terms associated with the equatorial Frame of reference with the variation. It is the equivalent on the celestial Sphère of the terrestrial Longitude.
Just like the longitude of a place measures the angle between the meridian line of this place and a meridian line of reference also called the meridian main thing , the right ascension of a star measures the angle between the Horary circle of this star and a horary circle of reference. Just as the intersection between the meridian line of Greenwich and the equator is used as point of origin for terrestrial longitude, there exists a point of origin for the right ascension. This point, called Not vernal and noted G or '' γ '', is one of the two points where the celestial equator and the ecliptic cross and the horary circle passing by this point is the horary circle of reference. The passage of the Sun by these two points of crossing defines the two equinoxes. The vernal Point '' γ '' corresponds to the equinox of March (beginning of spring in the northern hemisphere, of the autumn in the southern hemisphere).
The right ascension is always measured in the form of a angle expressed in hours (H), minutes (M), seconds in sidereal Temps; one hour being equivalent to 15 degree S.
The right ascension also makes it possible to determine easily how long it will be necessary for a star to reach a certain point in the sky. For example, if a star of right ascension 01:30: 00 is with the meridian , one will need 18:30 for another star of right ascension 20:00: 00 to be with the meridian line (20h00 - 1:30).
Units of the right ascension
The hour of right ascension is a measuring unit of plane angle and is worth 360°/24 = 15°.
The minute of right ascension is its subdivision and is worth, in all logic, 1/60 hour of right ascension, that is to say 1/4 degree or 15 minutes of arc.
In the same way the second of right ascension is worth 1/60 minute of right ascension = 1/3600 hour of right ascension = 1/240 degree = 1/4 minute of arc = 15 seconds of arc.
In the same way the third of right ascension is worth 1/60 second of right ascension = 1/216000 hour of right ascension = 1/14400 degree = 1/4 second of arc = 15 thirds of arc. This last unit is used rather little.
Important remarks on these units
These units are employed only in the case of celestial longitude. The Parsec, unit of length employed in astronomy, is defined according to the second of arc. It is in seconds of arc that one expresses the orbital anomaly of Mercure finally explained by Einstein (43 seconds of arc).
In general, the evocation of seconds (respectively of minutes) when one speaks about angles returns to the seconds of arc (respectively at the minutes of arc), except contrary precision. For a student, one can consider that a subject of having referring to the second of right ascension must obligatorily recall the definition of it.
See too
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