Noun
a great circle on the celestial sphere that passes through both celestial poles
Source: WordNetRight ascension (abbreviated RA; symbol α) is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. Source: Internet
Glossary, s.v. hour angle, hour circle, sidereal time. Source: Internet
The hour angle of a point is the angle between two planes: one containing the Earth's axis and the zenith (the meridian plane), and the other containing the Earth's axis and the given point (the hour circle passing through the point). Source: Internet