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For these of us within the Northern Hemisphere we’re on the time of 12 months when there may be probably the most daylight. On this submit I’ll discuss concerning the lengthy summer time evenings: not solely does the Solar set later right now of 12 months however the quantity of twilight, when the Solar has set however it’s not totally darkish, is best.
What’s twilight?
As mentioned in a previous post twilight is the time period within the night or early morning when, though the Solar is beneath the horizon as a result of it has set or not but risen, its rays can nonetheless hit the higher environment inflicting the sky to glow faintly, so it isn’t utterly darkish. Twilight is split into three levels: civil twilight, nautical twilight and astronomical twilight
The desk beneath reveals the completely different options of the three levels of twilight:
How shortly it will get darkish at completely different occasions of 12 months
The 4 stable traces within the diagram beneath reveals how the elevation of the Solar, measured in levels, adjustments throughout the afternoon and night in Manchester, which has a latitude of 53.5 levels north, on completely different dates within the 12 months.
- The blue line reveals the trail of the Solar on the June solstice
- The brown line reveals the trail of the Solar in the midst of August
- The inexperienced line reveals the trail of the Solar on the September equinox
- The purple line reveals the trail of the Solar on the December solstice
I’ll discuss concerning the mild blue dotted line labelled Shetland later.
In Manchester, on the summer time solstice, the Solar rises at four:39 am within the north east. It ascends within the sky to achieve a most elevation of 60 levels at noon. It then descends, setting within the north west at 9:41 pm. Because it units it’s shifting at a shallower angle to the horizon and thus lowering in elevation extra slowly in comparison with different occasions of 12 months. It’s this which causes the prolonged twilight across the time of the June solstice..
Some precise values for these 4 dates are proven within the desk beneath.
As you possibly can see, in Manchester the sky by no means will get utterly darkish on the times across the June solstice. The Solar by no means drops greater than 13 levels beneath the horizon. So, the darkest it might probably ever get is astronomical twilight.
Additional south, beneath a latitude of 48.5 levels north, the Solar drops greater than 18 levels beneath the horizon, and it will get totally darkish, though the length of twilight remains to be significantly longer across the June solstice than at different occasions of the 12 months. The desk beneath reveals the information for Boulder, Colorado which lies at a latitude of 40 levels north.
Mrs Geek and I’ve spent a lot of this month close to the village of Vidlin within the Shetland Islands midway between mainland Scotland and Norway. At this latitude (60.5 levels north) on the solstice, the Solar rises at three:33 am at units at 10:39 pm giving 19 hours and 6 minutes of daylight and for the rest of the time it will get no darker than civil twilight. In Shetland the interval in the midst of the night time when it doesn’t get darkish is called the Simmer Dim.
Photograph taken by Mrs Geek at round midnight. The Science Geek was quick asleep right now 😉
Yearly on the times across the June solstice there’s a Simmer Dim festival in Shetland the place a whole bunch of bikers come from throughout Europe to have a good time the sunshine nights by consuming, consuming and listening to music. Mr and Mrs Geek desire being tucked up in mattress!
Simmer Dim Pageant 2018
I hope you have got loved this submit and that these of you who dwell effectively north of the equator, like us, are having fun with the lengthy summer time nights.
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Notes
For the extra mathematically inclined readers, the elevation of the Solar at a given time is given by the next formulation:
sin(EL) = sin(DEC)*sin(LAT)+cos(DEC)*cos(LAT)*cos(HA)
the place
- EL is the elevation or peak of the Solar measured in levels
- DEC is the declination of the Solar. This adjustments all year long because the Earth strikes across the Solar in its orbit. On the June solstice the Solar has a declination of +23.5 levels, on the equinoxes it’s zero and on the December solstice it’s -23.5 levels
- LAT is the latitude of the observer
- HA is the hour angle. it the variety of hours because the photo voltaic midday (when the Solar is highest is sky) multiplied by 15. So, 2 hours after the photo voltaic midday is an hour angle of 30 levels, three hours earlier than the photo voltaic midday is an hour angle of -45 levels
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