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.
Related terms
- 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.
- 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.
- Twilight
The period when the sky darkens after sunset. It is civil until the Sun is 6° below the horizon (you can read outside), nautical until 12° (the horizon fades) and astronomical until 18°. Only then is the night truly dark for deep-sky objects.
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