Right ascension ( α , RA - from English right ascension ) - the length of the arc of the celestial equator from the vernal equinox to the declination circle of the star. Right ascension is one of the coordinates of the second equatorial system (there is also the first in which the time angle is used). The second coordinate is the declination .
Right ascension is measured eastward from the vernal equinox (that is, in the direction opposite to the diurnal rotation of the sky). Either a degree measure (0 ° to 360 °) or an hourly measure (0 h to 24 h ) is used to measure right ascension. Moreover, 24 h = 360 °.
Right ascension is the astronomical equivalent of Earth longitude in the second equatorial system . Both right ascension and longitude measure the east-west angle along the equator; both measures are counted from the zero point at the equator. The origin of longitude on Earth is the prime meridian ; The origin of the right ascension in the sky is the vernal equinox.
The physical meaning of a right ascension is that if the observer’s local true stellar time is equal to the direct ascension of the star, then it is at the top culmination - the highest, that is, the most convenient point of the celestial sphere that is possible for a given observation point.
History
The concept of right ascension was known even in the time of Hipparchus , which determined the location of stars in equatorial coordinates in the II century BC. e. But Hipparchus and his successors compiled their catalogs of stars in an ecliptic coordinate system .
With the invention of the telescope, it became possible to observe astronomical objects with greater detail. In order to keep the object in the field of view for a long time, it turned out to be convenient to use the equatorial mount , which allows the telescope to rotate around the polar axis. In this regard, the equatorial coordinate system was adopted.
The first catalog of stars in which direct ascension and declination were used to determine the coordinates of objects was John Flemstead's Historia Coelestis Britannica .