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.
Related terms
- Solstice
The moment the Sun reaches its greatest declination, +23.4° around 21 June and −23.4° around 21 December. In the northern hemisphere June brings the longest day and December the longest night. Around the June solstice in France and the UK, true darkness is short or missing.
- 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.
- Ecliptic
The Sun’s apparent path among the stars over the year, mirroring the plane of the Earth’s orbit. It is tilted by 23.4° to the celestial equator. The Moon and planets always stay close to it, which helps to recognise them.
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