Post by 4wd on May 11, 2009 6:26:06 GMT
( Interesting piece reproduced from another forum )
Its that time of the year again! In the very far north, it has already begun.
We are now at the time of the year where it never goes completely dark at night and the UK night is actually a lingering twilight. For twilight to end, the sun has to be 18 degrees below the horizon.
Date when Astronomical twilight is not reached for these locations
Lerwick: 21st April
Aberdeen: 1st of May
Belfast: 10th May
Manchester: 15th May
London: 22nd May
Plymouth: 30th May
At the solstice, the sun skims across the horizon at the latitude of the Arctic Circle (66 2/3 degrees north). To calculate the maximum angle that the sun dips below the horizon around the solstice, you simply deduct the latitude of your location from the latitude of the Arctic Circle. eg Manchester's latitude ~ 53 degrees N therefore maximum angle that sun is below the horizon is (latitude of Arctic Circle - latitude of location) = 13 degrees below the horizon. Manchester is within the 18 degrees limit and indeed no location in the UK is outside this limit. The critical latitude is 48 degrees N, so places in the Med do not experience the lingering twilight.
Civil twilight: When the sun is between 0 and 6 degrees below the horizon
Nautical twilight: When the sun is between 6-12 degrees below the horizon.
Astronomical twilight: When the sun is between 12-18 degrees below the horizon.
Only when the sun is 18 degrees and more below the horizon is there true darkness.
The lingering twilight can be seen to the north between midnight and 1 o'clock as a glow near the horizon. The brightness of the glow depends on how far north you are. In the Channel Islands, the glow is very weak but in the Shetlands it is bright.
And the most remarkable example of the lingering twilight of early July 1908 after the Tunguska event
Remember this was before BST, so to gauge it add 1hr to the time stated i.e see it as (11.05pm BST)
Here are reports from the time
30th/1st July
"Twilight all night"
"A peculiar phosphorecent glow in NNE at 11pm caused a mark excess of light."
"Glorious sunset with much cirriform cloud. A watch could be read easily in the open country at 11.45pm and with some strain one could even read a letter."
"Fine spectacle at night, when light was continuous."
"Remarkable twilight, it being possible to read indoors at 9.30pm."
"Remarkably lenghtened twilight."
"The night did not go dark."
"Fine afterglow at night."
July 1st/2nd
"Beautiful afterglow between 10pm and midnight. Remarkable reflected light observed during the night, having the appearance of a prolonged twilight or afterglow."
"A remarkable sunset, the prevailing colours being primrose and green, flecked with wisps of purple cirrus cloud, succeeded by a golden glow in the NW., which lasted until midnight and a fine display of aurora borealis. It was difficult to distinguish the streamers from the cirrus clouds for there was no quivering or darting movement preceptible, but dusky red and rosy clouds, characteristic of the phenomenon were in evidence until past 1am, when a brilliant dawn dawn began to come on. The combination of the afterglow and the aurora produced an effect not unworthy of the land of midnight sun and such as one rarely sees in England. The Malvern Hills were clearly visible at midnight."
"Sunset prolonged until 11.15pm."
"An extraordinary lightness was observed at night. The sky was all cirrus wisps of a greenish hue at 10pm, which might possibly have reflected the sun's rays and caused the prolonged light. From 10.15 to 11.15pm, it scarcely seemed to get any darker and from a bedroom window could be seen the green of the fields and the darker green of the trees beyond."
"Aurora each night. There were no rays visible, but the whole light scintillated."
"Extraordiany prolonged twilight. It was possible to read at 11.pm."
"Remarkably light nights. Newspapers could be read out of doors at 10.30pm on 1st."
From the Daily News
"Our Manchester correspondent wires that, taking advantage of the tropical weather prevailing, and the bright light in the sky at the night, dozens of farmers in South Lancashire and North Cheshire are now working day and night - 24 hours a day- getting in the hay harvest."
Its that time of the year again! In the very far north, it has already begun.
We are now at the time of the year where it never goes completely dark at night and the UK night is actually a lingering twilight. For twilight to end, the sun has to be 18 degrees below the horizon.
Date when Astronomical twilight is not reached for these locations
Lerwick: 21st April
Aberdeen: 1st of May
Belfast: 10th May
Manchester: 15th May
London: 22nd May
Plymouth: 30th May
At the solstice, the sun skims across the horizon at the latitude of the Arctic Circle (66 2/3 degrees north). To calculate the maximum angle that the sun dips below the horizon around the solstice, you simply deduct the latitude of your location from the latitude of the Arctic Circle. eg Manchester's latitude ~ 53 degrees N therefore maximum angle that sun is below the horizon is (latitude of Arctic Circle - latitude of location) = 13 degrees below the horizon. Manchester is within the 18 degrees limit and indeed no location in the UK is outside this limit. The critical latitude is 48 degrees N, so places in the Med do not experience the lingering twilight.
Civil twilight: When the sun is between 0 and 6 degrees below the horizon
Nautical twilight: When the sun is between 6-12 degrees below the horizon.
Astronomical twilight: When the sun is between 12-18 degrees below the horizon.
Only when the sun is 18 degrees and more below the horizon is there true darkness.
The lingering twilight can be seen to the north between midnight and 1 o'clock as a glow near the horizon. The brightness of the glow depends on how far north you are. In the Channel Islands, the glow is very weak but in the Shetlands it is bright.
And the most remarkable example of the lingering twilight of early July 1908 after the Tunguska event
Remember this was before BST, so to gauge it add 1hr to the time stated i.e see it as (11.05pm BST)
Here are reports from the time
30th/1st July
"Twilight all night"
"A peculiar phosphorecent glow in NNE at 11pm caused a mark excess of light."
"Glorious sunset with much cirriform cloud. A watch could be read easily in the open country at 11.45pm and with some strain one could even read a letter."
"Fine spectacle at night, when light was continuous."
"Remarkable twilight, it being possible to read indoors at 9.30pm."
"Remarkably lenghtened twilight."
"The night did not go dark."
"Fine afterglow at night."
July 1st/2nd
"Beautiful afterglow between 10pm and midnight. Remarkable reflected light observed during the night, having the appearance of a prolonged twilight or afterglow."
"A remarkable sunset, the prevailing colours being primrose and green, flecked with wisps of purple cirrus cloud, succeeded by a golden glow in the NW., which lasted until midnight and a fine display of aurora borealis. It was difficult to distinguish the streamers from the cirrus clouds for there was no quivering or darting movement preceptible, but dusky red and rosy clouds, characteristic of the phenomenon were in evidence until past 1am, when a brilliant dawn dawn began to come on. The combination of the afterglow and the aurora produced an effect not unworthy of the land of midnight sun and such as one rarely sees in England. The Malvern Hills were clearly visible at midnight."
"Sunset prolonged until 11.15pm."
"An extraordinary lightness was observed at night. The sky was all cirrus wisps of a greenish hue at 10pm, which might possibly have reflected the sun's rays and caused the prolonged light. From 10.15 to 11.15pm, it scarcely seemed to get any darker and from a bedroom window could be seen the green of the fields and the darker green of the trees beyond."
"Aurora each night. There were no rays visible, but the whole light scintillated."
"Extraordiany prolonged twilight. It was possible to read at 11.pm."
"Remarkably light nights. Newspapers could be read out of doors at 10.30pm on 1st."
From the Daily News
"Our Manchester correspondent wires that, taking advantage of the tropical weather prevailing, and the bright light in the sky at the night, dozens of farmers in South Lancashire and North Cheshire are now working day and night - 24 hours a day- getting in the hay harvest."
but it does get tiring as I then feel I have to work to catch up on all the things that dark days don't encourage.