Most of us have known the old adage that the Sun rises in the east and sets in the west for so long that we can’t remember where we first heard it.
Somewhat like a stopped clock being correct twice a day, however, this old adage is only correct twice a year.
Only on the days that are commonly known as the equinoxes – around March 21 and September 23 – does the Sun actually rise due east and set due west.
The rest of the year the Sun sets and rises north or south of these points depending on your latitude.
This is linked to the length of the day at your given latitude, with the Sun spending more time above the horizon as the days get longer.
From the December solstice onwards, the days become longer in the northern hemisphere and shorter in the south. This continues until the solstice in June, after which days become shorter in the north and lengthen again in the southern hemisphere, until we again reach the December solstice.
The converse of all of this is that as the days get longer, the nights get shorter and vice versa. At some stage the length of the night is equal to the length of the day. But only in passing.
The word equinox derives from the Latin meaning ‘equal night’ (the ‘equi’ part is fairly easy to work out and if you link ‘nocturnal’ to ‘nox’, you are unlikely to forget this either). It is so named because around the time of the equinox the length of the day is around the same length as the night.
You also probably remember, from school or elsewhere, that the lengthening or shortening days (and the seasons) are due to the Earth’s axis being tilted.
The equinox itself happens when the Earth’s axis is tilted neither toward nor away from the Sun. Another way of putting this is that the equator is in the same plane as that in which the Earth orbits (the ecliptic).
Picture a disc centred on the Sun, around the edge of which we orbit. Picture also a disc through the centre of the Earth, cutting it at the equator. When these two discs coincide we have an equinox.
Imagining these two discs also helps you picture that standing on the equator near the time of the equinox means that the Sun rises in the east, is pretty much directly overhead at noon* and sets in the west.
*Local noon is defined as the time at which the Sun is at its highest point above the horizon.
As you travel north on the day of an equinox, the Sun will be a little lower in the sky at noon (to the south) and as you travel south the Sun will be a little lower in the sky at noon (to the north).
On the day of an equinox, wherever you stand – on the equator, or north or south of it, the Sun will rise directly east and set directly west.
Now if you think about it harder, if the Sun is only overhead at noon on the equator at particular points in time, it must be overhead at noon at other neighbouring latitudes on neighbouring days. There is a limit to how far north or south you can go and ever have the Sun overhead at noon – these limits are the Tropics of Cancer and the Tropic of Capricorn. The latitude of these circles (23.4378 degrees north and south respectively) correspond to the tilt of the Earth’s axis away from the plane on which it orbits. The area between them describes a zone where the Sun reaches a point directly overhead at least once per year. This area is known as the Tropics.
North of the Tropic of Cancer, the Sun will always be south of directly overhead, even at noon. South of the Tropic of Capricorn, the Sun will always be north of directly overhead, even at noon.
On the days of the equinox, wherever you are, the height of the Sun at noon will be equal to 90 degrees minus your latitude. So, if you are on the equator, the Sun will be overhead at noon (as discussed above). If you are at one of the poles, the Sun will be on the horizon at noon.
If you are in London (51.6 degrees north), the Sun will be approximately 90 – 51.6 = 38.4 degrees above the horizon at noon. This is only approximate because unless the actual time when the axial tilt is zero relative to the Sun coincides with the local noon in London, the height of the Sun at noon will be a little more or a little less than this.
For example, in 2012, the March equinox actually occurs at 05:14 GMT on March 20th; so by noon, the Sun will be a little higher than our approximate calculation above. Checking the US Naval Observatory website shows that the Sun will be 38.6 degrees above the horizon at local noon (12:06 GMT) in London on this date.
The height of the Sun at noon is not safe to check with the naked eye but you can certainly get your compass out and check sunrise and sunset bearings, particularly when the sun is just below the horizon. In the UK at the moment you won’t have to adjust for the difference between magnetic north and true north but in other parts of the world you might.
If you are reading this near to the time of an equinox, please leave a comment below with where you are in the world, and the compass bearing (in degrees) you obtain for sunrise or sunset, or both.