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.
ToolsPolar alignment, Celestial fix
Related terms
- 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.
- Meridian
The imaginary line from north to south through the zenith. When an object crosses it, it reaches its highest point: its culmination, the best time to observe it. The Sun crosses it at solar noon, around 1 pm in winter and 2 pm in summer in France, close to noon or 1 pm in the UK.
- Right ascension
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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