Sunday, October 31, 2021
When the clocks go back our mornings get lighter and our evenings get darker, but the overall amount of daylight barely changes. Yesterday London had 9 hours 46 minutes, today 9 hours 42 minutes, and yet suddenly it feels much gloomier.
In each case the yellow bar is 40% of the overall length, i.e. the amount of daylight at this time of year is 40% of a day. The percentage is a lot higher in the summer and a lot lower in the winter, but around now we experience 40% day and 60% night.
n.b. Daylight is a measure of the time between sunrise and sunset. It is not the same as sunlight, which is a meteorological variable.
n.b. Daylight hits exactly 40% on Tuesday when a day lasts 9 hours and 36 minutes.
At the start of October the proportion of daylight was 48% (because we were just past the equinox). If it's now 40%, it won't surprise you to hear that the overall proportion for the month is 44%.
The maths: There are 744 hours in October and 328½ hours of daylight, which is 44%.
October is 44% daylight and 56% dark.
Unfortunately where we're going next is November and that's a 37%/63% split.
I thought it'd be interesting to do the calculations for every month of the year.
How much of each month is daylight and how much is dark?
Overall a year is 50% day and 50% night, because that's how living on a spherical planet works. But the only months with a 50/50 split are March and September, near enough, because that's where the equinoxes are.
The two solstice months of June and December are very much the outliers. In June we get twice as much day as night, and in December twice as much night as day.
Given it's the end of October we're now set for three months with less than 40% daylight, sorry, and it's a long wait until we get three months over 60% again next summer.
My calculations work for London and anywhere else around 51½°N. But head closer to the equator and the difference between summer and winter becomes a lot smaller. Here are the proportions of daylight for June and December.
| Daylight | Jun | Dec |
| London (51½°) | 67% | 33% |
| Rome (42°) | 62% | 38% |
| Cairo (30°) | 57% | 43% |
| Manila (15°) | 53% | 47% |
| equator (0°) | 50% | 50% |
Meanwhile the closer you get to the North Pole, the greater the imbalance.
| Daylight | Jun | Dec |
| London (51½°) | 67% | 33% |
| Newcastle (55°) | 71% | 29% |
| Shetland (60°) | 77% | 23% |
| Reyjkavik (64°) | 86% | 14% |
| Svalbard (78°) | 100% | 0% |
Those living in the north of Scotland enjoy three times as much day as night in high summer, but the payback is three times as much night as day in midwinter. Daylight is always a zero-sum game.
It's hard to say where the sweet spot is, where the benefits of long summer evenings aren't outweighed by the gloom of premature winter sunset. I guess we get used to what we have... which for the next few months is mostly dark.
n.b. My usual apologies to those of you in the southern hemisphere for whom this is all backwards. You win... for now.
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