This week might just my favourite week of the whole year. The reason is because it contains my favourite day of the whole year.
“And which day is that?” I hear you ask. It is not my birthday, although that is also a pretty good one, if only for the excuse to eat and drink whatever I want as I contemplate the widening chasm between my age and my date of birth.
No, my favourite day is the Spring Equinox which falls on Thursday this week. After what seems like an interminably long winter, for the next six months the days are going to get longer and the nights are going to get shorter. That thought makes me very, very happy.
‘Equinox’ comes from the Latin ‘equi’ (equal) and ‘nox’ (night). It refers to the fact that we will have as many daytime hours as nighttime thanks to the sun’s position directly above the Equator as it trundles on its journey northwards towards our summertime. This year the actual moment of ‘equinox’ in the UK is at 9.01am on 20th March. Of course in the Southern Hemisphere, the opposite is happening, and they are welcoming Autumn as their daylight hours reduce in length.
The astronomical boffins among you will know that the date upon which we have true equal day and night does not actually fall upon the equinox, but rather on the lesser-known equilux (‘lux’ is the Latin for ‘light’). Interestingly, my spellchecker proves it is lesser-known because it refuses to recognise ‘equilux’ as a bona-fide word, underlining it with that bossy red squiggle. It wants me to change it to ‘equinox’. Sorry spellchecker, but this time I know better than you!.
The explanation is a bit complicated, but I will give it a go. There are two ways of measuring the times when daylight and nighttime officially start and finish. There’s the way for regular humans who don’t care about the scientific facts but just like to take pictures of pretty sunrises and sunsets, and then there’s the clever people who know what is actually happening from an astronomical perspective. So depending on which one you prefer, the equal 12 hours of daylight and 12 hours of nighttime fall on different dates.
This might help you understand what is happening (or it might not): imagine the sun as a flat circle with a point at its centre. The period of equilux is measured from when the very top of the circle first peeps above the horizon until the last moment just before it dips below it. The Spring and Autumn equinoxes are measured from when the circle’s centrepoint first appears above the horizon and ends when it dips below it at sunset. Therefore, there’s a bit of a discrepancy in timing, and so by the time the equinox occurs on 20th March, the equilux will have already occurred about three days before. For the same reason, the Autumn Equilux is about three days after September’s equinox.
I’m also a bit mind-blown to discover that we don’t ever achieve real night/day equality. The nearest we get is about 12 hours and 10 minutes of daylight, and 11 hours and 50 minutes of nighttime. So the equinox isn’t actually ‘equi’ at all!
But that’s not quite the full picture, because of course the earth is not flat but a sphere encased in its own atmosphere. This means the sun’s rays are refracted from below the horizon before it actually rises, so it tricks our mortal eyes into thinking that it has started its rise before it physically has. The same applies to sunset. In other words, it brings the time of sunrise forwards and puts back the time of the sunset (or what appears to us to be sunrise and sunset). It’s all a bit mind boggling.
Then we have the summer and winter solstices, which occur when the sun is at its furthermost points north and south of the equator. For us in the Northern Hemisphere, the Summer Solstice occurs in mid June, and the winter one in mid December (of course, the opposite is true in the Southern Hemisphere). I think that’s all I need to say about those for now.
Right, after all that, I need a lie down while I watch the sun set.
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This column appeared in the Darlington & Stockton Times on Friday 21st March and the Ryedale Gazette and Herald on Wednesday 19th March 2025
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