The celestial “longitude”, counted eastwards along the celestial equator from the vernal equinox point, in hours from 0h to 24h (1h = 15°). With declination it fixes a star’s position on the celestial sphere. Abbreviated RA.
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Related terms
- Declination
The celestial “latitude”: an object’s angle north (+) or south (−) of the celestial equator, from +90° to −90°. Vega is at +38.8°, Sirius at −16.7°. From a given latitude, declination tells you whether an object is always up, never up, or rises and sets.
- Hour angle
The time since an object crossed the meridian, in hours: hour angle = local sidereal time − right ascension. It is 0h on the meridian and grows towards the west; an object at 2h crossed the meridian two hours ago. Polar scopes and equatorial mounts rely on it.
- Equinox
The moment the Sun crosses the celestial equator, around 20 March and 22 September; day and night then last about 12 hours everywhere. The point the Sun crosses in March, the vernal equinox point, is the zero of right ascension. On that day the Sun rises due east.
- Sidereal time
Star time: local sidereal time is the right ascension currently on the meridian. A sidereal day lasts 23 h 56 min 4 s, about 4 minutes less than a solar day. That is why stars rise 4 minutes earlier each night, or 2 hours per month.
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