I've decided to finish one of yesterday's posts. (sorry, probably not the one you wanted)
Last night the sun set on the District line at 6.20pm in Upminster, at 6.21pm from Hornchurch to Bow Road, at 6.22pm from Mile End to Kew Gardens and at 6.23pm in Richmond. This is because the District line runs from east to west and because the Earth is not as big as you think it is.
These are sunset times along the District line yesterday. The sun set first in Upminster because this is on the eastern side of London. The time was 6.20pm, or more accurately 6.20 and 53 seconds because you can be really accurate about celestial geography. By Upminster Bridge it was 6.20 and 57 seconds, and at Hornchurch it was 6.21 and 1 second. We've only gone 1½ miles west but that's already an 8 second difference. That's how susceptible sunset times are across relatively short distances. (I used the excellent Suncalc website to calculate the times precisely, this simply by clicking on a map)
Heading into my neck of the woods, Bow Road had a sunset at 6.21 and 58 seconds while Mile End's sunset was 6.22 precisely. The switch to 6.23 took place almost at the western end of the line, with Kew Gardens 1 second before and Richmond 3 seconds after. I've focused solely on the Richmond branch here to keep things simple, but on a train heading to Ealing Broadway the divide would come between Acton Town and Ealing Common.
These stripes may look to be of different widths but that's because tube maps aren't geographically accurate. They are in fact each approximately about 11 miles wide, i.e. for every 11 miles you go west, the sun sets a minute later. And there's a jolly good reason for that.
It's all to do with the length of a line of latitude through London. Imagine drawing a line west from the Houses of Parliament all the way round the world until it returned to Westminster. That line would precisely follow 51½°N and would be 15,501 miles long.
Here's how you work that out.
The formula for the length of a line of latitude is q×cos(L°)
where q is the length of the equator and L is the angle of latitude
The length of the equator is 24901 miles.
In London's case L = 51½° and the cosine of 51½° is 0.662.
So our line of latitude is 24901 × 0.622 = 15501 miles long.
There are 1440 minutes in a day and the Earth rotates an equal amount in each of them.
The key calculation is thus 15,501 ÷ 1440 = 10.8
i.e. in London the Earth rotates 10.8 miles every minute.
And that's why sunset in Richmond is a minute after sunset in Bow which is a minute after sunset in Upminster.
And this is always true, it doesn't depend on the season.
Here's how it looks on the Elizabeth line, another east/west railway but considerably longer.
These are sunset times tonight.
Again each of these stripes is 11 miles wide.
But this time there's a five minute difference from one end of the line to the other.
The sun sets five minutes later in Reading than in Shenfield.
And considering just the London bit, three minutes later in West Drayton than in Harold Wood.
This is why "The first 6pm sunset since October will be taking place in London tonight" is a potentially misleading headline. The clickbaiteers published it on 13th March, and it was indeed true for anywhere east of Whitechapel. Crucially it was correct for the Greenwich meridian. But anywhere west of Whitechapel saw its first 6pm sunset on 12th March, and Heathrow Terminal 5 scraped a 6pm sunset on 11th March, so for the majority of Londoners it wasn't actually true.
It's also why the Great Western Railway adopted 'railway time' in 1840, local time in Bristol being 10 minutes later than local time at Paddington. This was the first instance of a standard time replacing local mean time, and would ultimately lead to the official adoption of Greenwich Mean Time in 1884.
Be aware that sunset times also change going north/south, but at this time of year not very much.
Sunset barely changes along the Northern line tonight, being 6.24pm in both High Barnet and Morden.
The big changes are east/west because that's the way the world spins.
Also be aware that the 11 mile thing is only true for those living on a similar latitude to London.
If you live north of London your line of latitude is shorter so your sunset stripes are narrower.
For example Edinburgh is at 56°N and 24901×cos(56°)÷ 1440 = 9.7
So in central Scotland the sun sets a minute later for every 10 miles you go west.
If you live south of London your line of latitude is longer so your sunset stripes are wider.
For example Rome is at 42°N and 24901×cos(42°)÷ 1440 = 12.8
So in the Mediterranean the sun sets a minute later for every 13 miles you go west.
If you need a conclusion after all that, it's that London doesn't have just one sunset time it has several, generally a three minute spread. So try not to be too precise about sunsets if you don't know precisely where you're talking about.
(sorry, you probably wish I'd finished one of the other 39 unfinished posts)
» The book I finished yesterday was The Long Shoe by Bob Mortimer.
» London's most average bus number is the 219.
» The first chocolate biscuit in alphabetical order is probably a Bahlsen.
» The 1-word London place name with the highest Scrabble score is perhaps Bexleyheath (29).
» West Harrow is east of North Harrow.
» Clue 14: What a very tall one-legged clown has
