diamond geezer

 Tuesday, September 22, 2020

At the autumn equinox, which is precisely now, the northern hemisphere passes from the brightest half of the year to the darkest. But how much darker is it? Let's bash some numbers.

London gets about 4480 hours of daylight a year (the equivalent of 187 days). If that daylight was spread equally then each month would see about 8% of the year's total, but it's not so they don't.

 Sep  Oct  Nov  Dec  Jan  Feb  Mar  Apr  May  Jun  Jul  Aug 

September and March get the average of 8% because they're the months when the year's two equinoxes take place. December has the least daylight, less than half of that enjoyed in June and July. July actually has a smidgeon more daylight than June, solely because it's one day longer.

But what I'm more interested in today is what percentage of the year's daylight falls between the autumn equinox and the spring equinox. How much darker is it about to get?

In London autumn and winter get 39% of the year's daylight while spring and summer get 61%. This may not be as great a variation as you were expecting given how dark it's about to get. But viewed another way it means that spring/summer has about half as much daylight again as autumn/winter, and maybe that chimes better with our imminently disappearing evenings.

The daylight balance varies at other latitudes.

  London (51½°)39%61%
Paris (49°)40%60%
Rome (42°)42%58%
Cairo (30°)45%55%
Manila (15°)48%52%
equator (0°)50%50%

The closer you get to the equator, the smaller the discrepancy between the brighter and darker halves of the year. In the tropics it's almost imperceptible.

But the closer you get to the North Pole, the greater the imbalance.

 London (51½°)39%61%
Newcastle (55°)37%63%
Shetland (60°)34%66%
Reyjkavik (64°)31%69%
Svalbard (78°)13%87%
North Pole (90°)0%100%

The further north you go the smaller the proportion of the year's daylight occurring over the autumn and winter months. Up in Shetland it's only one-third, and by the time you hit the Arctic Circle more like a quarter. The North Pole's weird, so let's not go there.

In short, the further north you live the gloomier the next six months are going to be, possibly quite significantly so. Hang on in there and the good times will return, half a year from now.

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