The dates in the year (20 or 21 June, or 21 or 22 December) when the Sun is at its greatest northern (summer solstice) or southern (winter solstice) declination. These dates correspond respectively to the beginning of summer and of winter, and conversely in the southern hemisphere.
There are four notable events during the year in the journey around the Sun:
- the winter solstice (around 21 December);
- the spring equinox (around 20 March), when the days start to get longer than 12 hours;
- the summer solstice (around 21 June), the longest day;
- and the autumn equinox (around 23 September), when the days start to get shorter than 12 hours.