Sunday, June 18, 2023
When's the brightest hour of the year?
By which I mean, during which hour is the sun highest in the sky?
spoiler: it's not a single hour, it's several minutes over several days.
spoiler: it starts today.
Here in London the highest the sun gets in the sky is 61.94° above the horizon.
geography: I'm taking London to be at 51½°N, 0°E (i.e. approximately the Millennium Dome)
formula: maximum solar elevation = 90 - Latitude + Earth's tilt = 90 - 51.5 + 23.44 = 61.94
Maximum solar elevation occurs at solar noon on the day of the summer solstice.
This year that's 1:02pm on 21st June.
That makes 13:02 BST on 21st June the brightest minute of the year.
To make the brightest hour we need to find the next 59 brightest minutes.
To two decimal places, the sun reaches 61.94° for just 15 minutes a year.
20th June: 13:01-13:04
21st June: 13:00-13:05
22nd June: 13:01-13:05
4 bright minutes + 6 bright minutes + 5 bright minutes = quarter of an hour
The elevation of the sun reaches 61.93° for 38 minutes a year.
19th June: 13:01-13:04
20th June: 12:58-13:06
21st June: 12:58-13:07
22nd June: 12:58-13:07
23rd June: 13:01-13:05
That's the brightest 38 minutes of the year.
That starts tomorrow.
caveat: These are also the times if you live due north or due south of London.
caveat: If you live east of London this happens earlier.
caveat: If you live west of London this happens later.
The sun reaches 61.92° for 54 minutes a year.
The sun reaches 61.91° for 78 minutes a year.
So to find the best hour we need to hit somewhere inbetween.
For tedious rounding reasons, the threshold is about 61.913°.
Which means the brightest hour of London's year is this.
18th June: 13:01-13:02 (2 mins)
19th June: 12:58-13:06 (9 mins)
20th June: 12:57-13:08 (12 mins)
21st June: 12:56-13:08 (13 mins)
22nd June: 12:57-13:08 (12 mins)
23rd June: 12:58-13:07 (10 mins)
24th June: 13:02-13:03 (2 mins)
It all kicks off just after one o'clock today.
And it'll all be over in a week.
Best make the most of it.
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