In 2023 March 20 is the date of the March equinox and is also the first day of spring (or the first day of autumn if you’re one of my readers in the Southern Hemisphere). In this post I’ll talk about the equinoxes and discuss the commonly held, but not quite correct, view, that they are the two dates in a year when all places on Earth have exactly 12 hours of daylight and 12 hours of darkness.
What is an equinox?
The origin of the word equinox comes from two Latin words aequus (equal) and nox (night). This definition suggests that at an equinox the length of the day and night are equal. However, the precise astronomical definition of an equinox is slightly different.
The diagram above shows the Earth in its orbit around the Sun
- At the December solstice, the North Pole is tilted further away from the Sun than at any other time of the year, and the South Pole is tilted nearest the Sun. In the Northern Hemisphere the period of darkness is longest compared with the period of daylight, and in the Southern Hemisphere the reverse applies.
- At the June solstice, the North Pole is now tilted nearest to the Sun. So, the Northern Hemisphere experiences the longest period of daylight.
- There are two times a year when the neither the North Pole nor the South Pole are tilted towards the Sun and these times are the equinoxes. If we take two places with the same latitude, one of which is north of the equator and the other one south of the equator and roughly the same longitude (for example Tokyo and Adelaide, South Australia) they will both have the same amount of daylight at the equinox.
What dates do the equinoxes occur?
The Earth moves in an elliptical orbit around the Sun. This means that it has further to travel in its orbit between the March equinox and the September equinox than in the return leg of its journey from September to March. The two equinoxes are therefore not exactly half a year apart. The time between the March equinox and the following September equinox is around 186 days, whereas the time between the September equinox and the following March equinox is shorter -only 179 days.
In 2022 and 2023 the dates and time of the equinoxes are
- On 20 March 2022 at 15:33
- On 23 September 2022 at 01:04
- On 20 March 2023 at 21:23
- On 23 September at 06:50
The times are in GMT (which is more correctly known as UTC). So, depending on the time zone you’re in, the equinoxes may occur at a different times. If your time zone is far enough ahead or behind GMT they may even occur on a different date.
Are there 12 hours of daylight at an equinox?
When we answer this question, we need to think about what mean by the term: ‘hours of’daylight’?
Do we consider twilight, the time just after sunrise or just before sunset when it is not completely dark, to be daylight?
Or do we consider daylight as being the time when the Sun is above the horizon?
If we use the definition of ‘daylight’ as being the interval between sunrise and sunset, then there are actually slightly more than 12 hours of daylight at the equinox everywhere in the world. This is because
- The definition of sunrise is when the first light from the Sun’s rays reaches above the horizon, not when the centre of the Sun is above the horizon. The diagram below shows the path of the Sun’s disc around sunrise at the equinox in London. In the early morning, the time when the half of the Sun is above the horizon and half below the horizon is 6:04 am, shown as B in the diagram, but sunrise is about a minute earlier
Similarly, in the early evening, the time when half of the Sun is above the horizon and half below the horizon is 6:14 pm, B in the diagram, but sunset – when the last light from the Sun’s rays is above the horizon – is about a minute later
- Secondly, when the Sun is just below the horizon the Earth’s atmosphere bends the Sun’s rays, causing them to appear just above the horizon. This bending of light is known as refraction and has the effect of slightly extending the hours of daylight.
Taken together, these two effects mean that there are slightly more than 12 hours of daylight at the equinox. The date on which there (almost) exactly 12 hours of daylight and 12 hours of darkness is called the equilux. This term is rarely used and is not in the dictionary of my spell checker 😉 . In London the equilux occurs on 17 March, three days before the equinox At other locations on the Earth the equilux will occur on different dates
The meteorological seasons
Although March 20 is the first day of the Northern Hemisphere spring, just to confuse matters ,in the UK there is an another definition of the seasons used by meteorologists and widely used by the public as well as the ‘correct’ astronomical definitions. Meteorological spring consist of the months of March, April and May and so starts on March 1 (which is the UK is also St David’s day) and finishes on May 31 Similarly,
- meteorological winter is December, January and February,
- meteorological summer June, July and August and
- meteorological autumn September, October and November
I hope you have enjoyed this post and (if you are in the northern hemisphere) you are looking forward to spring with its warmer weather and more daylight
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