1. An arc of the celestial equator eastward from the vernal equinox.
2. (Astronomy) The angular distance eastward along the celestial equator from the vernal equinox to the intersection of the hour circle that passes through the body; expressed in hours and minutes and second.
In astronomy, the coordinate on the celestial sphere that corresponds to longitude on the surface of the Earth. It is measured in hours, minutes, and seconds eastward from the point where the Sun’s path, the ecliptic, once a year intersects the celestial equator; this point is called the vernal equinox.