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Right ascension is measured eastward up to 24 h along the celestial equator from the primary direction. Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the Earth. [1]
α - RA, right ascension, if the Greek letter does not appear, á letter will appear. δ - Dec, declination, if the Greek letter does not appear, ä letter will appear. P or P orb or T - orbital period; a - semi-major axis; b - semi-minor axis; q - periapsis, the minimum distance; Q - apoapsis, the maximum distance; e - eccentricity; i ...
Alternatively to right ascension, hour angle (abbreviated HA or LHA, local hour angle), a left-handed system, measures the angular distance of an object westward along the celestial equator from the observer's meridian to the hour circle passing through the object. Unlike right ascension, hour angle is always increasing with the rotation of Earth.
The change μ α, which must be multiplied by cosδ to become a component of the proper motion, is sometimes called the "proper motion in right ascension", and μ δ the "proper motion in declination". [11] If the proper motion in right ascension has been converted by cosδ, the result is designated μ α*.
The International Astronomical Union defines it as the angle measured relative to the north celestial pole (NCP), turning positive into the direction of the right ascension. In the standard (non-flipped) images, this is a counterclockwise measure relative to the axis into the direction of positive declination.
Star position is the apparent angular position of any given star in the sky, which seems fixed onto an arbitrary sphere centered on Earth. The location is defined by a pair of angular coordinates relative to the celestial equator: right ascension (α) and declination (δ). This pair based the equatorial coordinate system.
An object's right ascension and the local sidereal time can be used to determine the time of its culmination (see hour angle). The term meridian comes from the Latin meridies , which means both "midday" and "south", as the celestial equator appears to tilt southward from the Northern Hemisphere .
The International Celestial Reference System (ICRS) is the current standard celestial reference system adopted by the International Astronomical Union (IAU). Its origin is at the barycenter of the Solar System, with axes that are intended to "show no global rotation with respect to a set of distant extragalactic objects".