diamond geezer

 Sunday, November 04, 2018

Today's the day that sunset in London slips over from "nearer 5pm" to "nearer 4pm". Yesterday the sun set at 4.31pm, and today it's 4.29pm. Sunset then remains "nearer 4pm" through the winter solstice and out the other side, until 21st January. Sorry, that's 11 weeks.

I can do similar calculations for "nearer 6pm", "nearer 7pm", "nearer 8pm" and "nearer 9pm", again all for London. Dates are for the next twelve months. Greenwich Mean Time is in blue, and British Summer Time is in red.

Nearer 4pm4 Nov - 21 Jan
Nearer 5pm22 Jan - 23 Feb27 Oct - 3 Nov
Nearer 6pm24 Feb - 30 Mar5 Oct - 26 Oct
Nearer 7pm 9 Sep - 4 Oct
Nearer 8pm31 Mar - 5 May12 Aug - 8 Sep
Nearer 9pm6 May - 11 Aug

"Nearer 6pm" is a GMT thing in the spring, but a BST thing in the autumn, because the dates we change the clocks aren't symmetrical. More intriguingly, "nearer 7pm" only happens in the autumn, and gets skipped over in the spring. In southeast England, 7pm sunsets are a September thing. Meanwhile we only get one week of 5pm sunsets in the autumn (it's just finished), but a full month of them later in the winter.

Let's do weeks.

Nearer 4pm11 weeks
Nearer 5pm6 weeks
Nearer 6pm8 weeks
Nearer 7pm4 weeks
Nearer 8pm9 weeks
Nearer 9pm 14 weeks

The time of sunset changes fastest at the equinoxes, and slows down at the solstices, which is why the largest numbers appear at the top and bottom of the table. British Summer Time very much messes up the pattern in the middle. But if you need something to hang onto during the next 11 dark weeks, remember that "nearer 9pm" lasts a good bit longer than "nearer 4pm".

Update: In response to a comment from a reader in the Orkney Islands, where the time of sunset ranges from 3.14pm to 10.28pm, here's the Orkney data.

Nearer 3pm6 weeks
Nearer 4pm8 weeks
Nearer 5pm4 weeks
Nearer 6pm6 weeks
Nearer 7pm4 weeks
Nearer 8pm6 weeks
Nearer 9pm7 weeks
Nearer 10pm 12 weeks

Update: A reader points out that Orkney is further west than London as well as a lot further north, making sunset a little later even before taking latitude into account. So here's the data for Newport, Wales, which has the same latitude as London but the same longitude as Orkney.

51½°N 0°W
51½°N 3°W
59°N 3°W
Nearer 3pm  6 weeks
Nearer 4pm11 weeks9 weeks8 weeks
Nearer 5pm6 weeks7 weeks4 weeks
Nearer 6pm8 weeks7 weeks6 weeks
Nearer 7pm4 weeks5 weeks4 weeks
Nearer 8pm9 weeks8 weeks6 weeks
Nearer 9pm 14 weeks13 weeks7 weeks
Nearer 10pm 3 weeks12 weeks

Update: A reader asks what the underlying pattern is, i.e. what the data would look like if we didn't put the clocks forward and stuck to GMT all year round. Here's London, GMT.

Nearer 4pm11 weeks
Nearer 5pm9 weeks
Nearer 6pm9 weeks
Nearer 7pm9 weeks
Nearer 8pm 14 weeks

Update: I was expecting sunrise times to look similar but, because we yank them in the opposite direction when the clocks change, the spread looks very different. Here's sunrise in London.

Nearer 5am14 weeks
Nearer 6am13 weeks
Nearer 7am14 weeks
Nearer 8am 11 weeks

Update: A reader has opened up the can of worms relating to what would happen if we no longer put the clocks back, i.e sticking to GMT+1 throughout the year. This is what'd happen to sunrise and sunset in London.

Nearer 5am14 weeks
Nearer 6am9 weeks
Nearer 7am9 weeks
Nearer 8am9 weeks
Nearer 9am 11 weeks
Nearer 5pm11 weeks
Nearer 6pm9 weeks
Nearer 7pm9 weeks
Nearer 8pm9 weeks
Nearer 9pm 14 weeks

Update: Another reader mentions Singapore, which lies one degree north of the equator. Here the maximum difference in sunset times is only thirty minutes, the earliest being in November (6.50pm, today!) and the latest in February (7.20pm). Reasons for this are complicated.

Nearer 7pm 52 weeks

n.b. Geography dictates that Singapore's time zone should be GMT+7, whereas in fact it adheres to GMT+8 to match Malaysia, China and the Philippines, which is why sunset is always nearer 7pm than 6pm.

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