diamond geezer

 Wednesday, June 21, 2006

Postcard from the Hebrides: midsummer twilight

It doesn't get dark in the Hebrides in June. Not properly dark, anyway. I was quite taken aback on my first night to discover it was still broad daylight at 10pm. By eleven the local streetlamp had come on, but it really wasn't necessary. The sky had dimmed noticeably by half eleven, but you still couldn't call it dark. It was even possible to read outdoors at midnight (I checked by studying the Stannah stairlift advert at the back of my Radio Times). Half an hour later night had definitely arrived, but the horizon was still distinctly bright in places. I didn't stay up to watch the sun rise, but I'm guessing it didn't stay hidden for long. There's barely time enough to dream on a Hebridean midsummer night.

Stornoway lies at 58° north, just six degrees off the Arctic Circle, so all this extended daylight shouldn't have been too surprising. The late arrival of summer darkness is related not just to the high latitude but also to the time of (cue official astronomical term) "civil twilight". This is defined as the time after sunset when the centre of the Sun lies six degrees below the horizon (before which time outdoor activities are still possible without the need for artificial light). The midsummer sun may set in the Hebrides around 10:30pm, but 'civil twilight' doesn't officially arrive until much later, around midnight. Then it's a mere three hours before the sky starts getting light again, making the nights seem much shorter than they should. You can check out civil twilight times wherever you live here (be patient), or just scan down my solstice summary table below...

Duration of the shortest night, June 21 2006 (all times BST)
Locationlatitudesunsetcivil twilight beginsdurationcivil twilight endssunrise
Arctic Circle66½°N no sunset---no sunrise
Shetland60°N22:3300:251h 22m01:4703:38
Stornoway58°N22:3323:523h 9m03:0104:20
Glasgow56°N22:0623:094h 20m03:2904:31
London51½°N21:2222:105h 46m03:5604:44
Madrid40½°N20:4721:207h 50m05:1005:43
Timbuktu17°N19:4720:1110h 4m06:1506:39

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