Friday, December 12, 2014
For years I've been aware that the earliest sunset of the year isn't on the day of the winter solstice, it's a few days earlier. More specifically for London, the earliest sunset of the year isn't on December 21st (3:53pm), it's today December 12th (3.51pm). [data] [full data] [interactive map]
| Date | Length of day | Sunset |
| Nov 1 | 9h 39 | 16:34 |
| Nov 11 | 9h 05 | 16:17 |
| Nov 21 | 8h 35 | 16:04 |
| Dec 1 | 8h 11 | 15:55 |
| Dec 11 | 7h 55 | 15:51 |
| Dec 21 | 7h 49 | 15:53 |
| Dec 31 | 7h 54 | 16:01 |
But I'd never quite got my head around why this discrepancy should arise. It seems entirely counter-intuitive that the shortest day and earliest sunset don't coincide, so some other factor must be at play to cause the difference. And so I dug around a bit, and I think I finally understand what's going on. Bear with me on this one.
First of all you need to understand this next table, based on data for central London. The amount of daylight each day is fixed, decreasing from almost 10 hours at the start of November to less than 8 hours throughout December. Half of that daylight occurs before noon and half after, so if we halve the total amount of daylight we get the length of the afternoon. And if the afternoon starts at noon, then it's possible to work out at what time the sun sets. Noon + afternoon = sunset. Like so...
| Date | Length of day | Noon | Length of afternoon* | Sunset |
| Nov 1 | 9h 39 | 12:00 | 4h 49 | 16:49 |
| Nov 11 | 9h 05 | 12:00 | 4h 32 | 16:32 |
| Nov 21 | 8h 35 | 12:00 | 4h 17 | 16:17 |
| Dec 1 | 8h 11 | 12:00 | 4h 05 | 16:05 |
| Dec 11 | 7h 55 | 12:00 | 3h 57 | 15:57 |
| Dec 21 | 7h 49 | 12:00 | 3h 55 | 15:55 |
| Dec 31 | 7h 54 | 12:00 | 3h 57 | 15:57 |
* half the length of the day
But those aren't the actual sunset times, as you can see if you compare them with the first table. And that's because the middle of the day, the time the sun is highest in the sky, isn't twelve o'clock. This so-called 'solar noon' changes throughout the year, wobbling over fifteen minutes away from 12:00 at different times of the year. For example at the start of November the sun is highest in the sky at quarter to twelve, but by the end of January it's quarter past. You'd never spot this, day to day, but it's true.
| Date | Solar noon* |
| Nov 1 | 11:44 |
| Nov 11 | 11:44 |
| Nov 21 | 11:46 |
| Dec 1 | 11:49 |
| Dec 11 | 11:54 |
| Dec 21 | 11:58 |
| Dec 31 | 12:03 |
* the time the sun is highest in the sky
The reasons for solar noon changing are quite complicated. It's to do with the Earth's orbit not being a circle, but an ellipse. It's to do with the Earth travelling at different speeds at different points in its orbit. It's to do with the Earth being tilted on its axis at an angle of 23½°. It's to do with all three of these things combining to create something called The Equation Of Time, which you can read all about here - I'm not going to go into the details. But The Equation Of Time is why the sun isn't always at its highest at noon. It's why sundials only tell the right time four days a year. And it's why the middle of the day today is at 11.54, but the middle of the day on December 21st will be at 11.58.
To find out what's really happening to sunset times, you have to start counting the afternoon from solar noon, not from twelve o'clock. Solar noon + afternoon = sunset. Like so...
| Date | Length of day | Solar noon | Length of afternoon | Sunset |
| Nov 1 | 9h 39 | 11:44 | 4h 49 | 16:34 |
| Nov 11 | 9h 05 | 11:44 | 4h 32 | 16:17 |
| Nov 21 | 8h 35 | 11:46 | 4h 17 | 16:04 |
| Dec 1 | 8h 11 | 11:49 | 4h 05 | 15:55 |
| Dec 11 | 7h 55 | 11:54 | 3h 57 | 15:51 |
| Dec 21 | 7h 49 | 11:58 | 3h 55 | 15:53 |
| Dec 31 | 7h 54 | 12:03 | 3h 57 | 16:01 |
At the moment solar noon is at 11.54 and the afternoon lasts 3h 57, which means sunset is at 3.51pm. By the winter solstice the afternoon is two minutes shorter at 3h 55, but solar noon is four minutes later at 11.58, making sunset two minutes later at 3.53pm. And that's why the earliest sunset is now rather than next week... because the sun is highest in the sky earlier in the day.
Here's how the next ten days pan out, sunset-wise.
| Date | Length of day | Solar noon | Length of afternoon | Sunset |
| Dec 11 | 7h 55 | 11:54 | 3h 57 | 15:51:49 |
| Dec 12 | 7h 54 | 11:54 | 3h 57 | 15:51:43 |
| Dec 13 | 7h 53 | 11:54 | 3h 56 | 15:51:44 |
| Dec 14 | 7h 52 | 11:55 | 3h 56 | 15:51:46 |
| Dec 15 | 7h 52 | 11:55 | 3h 56 | 15:51:53 |
| Dec 16 | 7h 51 | 11:56 | 3h 56 | 15:52 |
| Dec 17 | 7h 50 | 11:56 | 3h 55 | 15:52 |
| Dec 18 | 7h 50 | 11:57 | 3h 55 | 15:52 |
| Dec 19 | 7h 50 | 11:57 | 3h 55 | 15:52 |
| Dec 20 | 7h 49 | 11:58 | 3h 55 | 15:53 |
| Dec 21 | 7h 49 | 11:58 | 3h 55 | 15:53 |
A few caveats. Times have been rounded and may not be 100% accurate. All times are for central London. Locations further north and east see sunset earlier, and locations south and west see sunset later [see map]. Locations well to the south of London have already had their earliest sunset (New York's was on December 6th), while locations north of London haven't had theirs yet.
And if all of this has gone completely over your head, don't worry. All that's important is that the earliest sunset of the year is today, at around eight minutes to four. And from tomorrow, second by second, then minute by minute, the evenings start getting longer and longer.
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