Right ascension

Right ascension (abbreviated RA; symbol α)

Pre Astro Earth specific Astronomy Term
is the angular distance measured only eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question. When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.

Latin: ascensio recta
An old term, right ascension (Latin: ascensio recta) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle. It contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle.

Term: right ascension
The stars on the celestial sphere are like cities on the globe. Cities are located on the globe using latitude and longitude. Longitude says how far the city is east or west along the Earth's equator; latitude says how far a city is north or south of the Earth's equator. Right ascension (RA) is like longitude. It locates where a star is along the celestial equator. The zero point of longitude has been chosen to be where the line straight down from the Greenwich Observatory in England meets the equator. The zero point for right ascension is the vernal equinox. To find the right ascension of a star follow an hour circle "straight down" from the star to the celestial equator. The angle from the vernal equinox eastward to the foot of that hour circle is the star's right ascension.

There is one oddity in right ascension: the unit used to report the angle. Right ascensions are always recorded in terms of hours, minutes, and seconds. One hour of right ascension (1h) is 15°. Since 24x15°=360°, there are 24h of right ascension around the celestial equator. The reason for this oddity is that the celestial sphere makes one full rotation (24h of RA) in one day (24 hours of time). Thus the celestial sphere advances about 1h of RA in an hour of time.