The apparent north/south movement of the Sun during the course of the year.
Declination. The Declination of a celestial body is its angular distance North or South of the Celestial Equator. The declination of the Sun changes from 23.5o North to 23.5o South and back again during the course of a year. Declination can be summarised as the celestial equivalent of Latitude since it is the angular distance of a celestial body North or South of the Celestial Equator.
The Equinoxes. The Sun crosses the celestial equator on two occasions during the course of a year and these occasions are known as the equinoxes. At the equinoxes, at all places on Earth, the nights and days are of equal duration (i.e. 12 hours) hence the term equinoxes (equal nights). Because the Sun is on the celestial equator at the equinoxes, its declination is of course 0o.
The Autumnal Equinox occurs on about the 21/22 September when the Sun crosses the celestial equator as it moves southwards from 23.5oN , the northernmost limit of its declination.
The Vernal Equinox occurs on about the 20/21 March when the Sun crosses the celestial equator as it moves northwards from 23.5oS, the southernmost limit of its declination.
At the equinoxes, when the declination of the Sun is 0o, days and nights will be of equal length at all places on Earth so that there will be 6 hours of daylight before noon and 6 hours after noon. Because the Sun lies over the equator, the bearing of the Sun at sunrise will be due east (090o) and at sunset it will be due west (270o).
The Solstices. The times when the Sun reaches the limits of its path of declination are known as the solstices. The word solstice is taken from ‘solstitium’, the latin for ‘sun stands still’. This is because the apparent movement of the Sun seems to stop before it changes direction
The Summer Solstice (mid-summer in the northern hemisphere) occurs on about 21 June when the Sun’s declination reaches 23.5o North (the tropic of Cancer).
The Winter Solstice (mid-winter in the northern hemisphere) occurs on about 21 December when the Sun’s declination is 23.5oSouth (the tropic of Capricorn).