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 summarized 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 22 September when the Sun crosses the celestial equator as it moves southwards from 23.5o North, the northernmost limit of its declination.
The Vernal Equinox occurs on about the 21 March when the Sun crosses the celestial equator as it moves northwards from 23.5o South, the southernmost limit of its declination.
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 23.5o South (the tropic of Capricorn).
Note. The latitude of the tropic of Cancer is currently drifting south at approximately 0.5’’ per year while the latitude of the tropic of Capricorn is drifting north at the same rate.
The dates of the equinoxes and the solstices will vary slightly during the four-year cycle between leap years for the following reason: Each year is approximately 365.25 days in length. However; for the sake of convenience, the Gregorian calendar divides three years of the cycle into 365 days and the fourth (the leap year) into 366. So, the Vernal Equinox sometimes falls on 20 March and sometimes on 21 March. The Autumnal Equinox sometimes falls on 22 September and sometimes on 23 September. Similarly, the Summer Solstice usually falls on 21 June but sometimes falls on 20 June. The Winter Solstice usually falls on 21 December but sometimes falls on 22 December.
Astro Navigation Demystified is available from Amazon.com and Amazon.uk