Right ascension (RA)

Right ascension is the celestial-sphere coordinate measured eastward along the celestial equator from the first point of Aries to the hour circle of a body, expressed in hours, minutes, and seconds (0 to 24 hours). It is the astronomer’s equatorial coordinate, paired with declination. Marine practice uses sidereal hour angle instead, which is measured westward and relates to right ascension as SHA equals 360 degrees minus right ascension converted to arc (15 degrees per hour). The Nautical Almanac therefore tabulates SHA, not RA, for its 57 stars, while star catalogs and planetarium data give RA.