Summer twilight

For those of us in the Northern Hemisphere we are at the time of year when there is the most daylight. In this post I’ll talk about the long summer evenings: not only does the Sun set later at this time of year but the amount of twilight, when the Sun has set but it is not fully dark, is greatest.

What is twilight?

As discussed in a previous post twilight is the period of time in the evening or early morning when, although the Sun is below the horizon because it has set or not yet risen, its rays can still hit the upper atmosphere causing the sky to glow faintly, so it isn’t completely dark. Twilight is divided into three stages: civil twilight, nautical twilight and astronomical twilight

 

The table below shows the different features of the three stages of twilight:

 

How quickly it gets dark at different times of year

The four solid lines in the diagram below shows how the elevation of the Sun, measured in degrees, changes during the afternoon and evening in Manchester, which has a latitude of 53.5 degrees north, on different dates in the year.

 

  • The blue line shows the path of the Sun at the June solstice
  • The brown line shows the path of the Sun in the middle of August
  • The green line shows the path of the Sun at the September equinox
  • The purple line shows the path of the Sun at the December solstice

 

I’ll talk about the light blue dotted line labelled Shetland later.

 

In Manchester, at the summer solstice, the Sun rises at 4:39 am in the north east. It ascends in the sky to reach a maximum elevation of 60 degrees at midday. It then descends, setting in the north west at 9:41 pm. As it sets it is moving at a shallower angle to the horizon and thus decreasing in elevation more slowly compared to other times of year.  It is this which causes the lengthy  twilight around the time of the June solstice..

Some actual values for these four dates are shown in the table below.

As you can see, in Manchester the sky never gets completely dark on the days around the June solstice. The Sun never drops more than 13 degrees below the horizon. So, the darkest it can ever get is astronomical twilight.

Further south, below a latitude of 48.5 degrees north, the Sun drops more than 18 degrees below the horizon, and it gets fully dark, although the duration of twilight is still considerably longer around the June solstice than at other times of the year.  The table below shows the data for Boulder, Colorado which lies at a latitude of 40 degrees north.

Mrs Geek and I have spent much of this month near the village of Vidlin in the Shetland Islands halfway between mainland Scotland and Norway. At this latitude (60.5 degrees north) on the solstice, the Sun rises at 3:33 am at sets at 10:39 pm giving 19 hours and 6 minutes of daylight and for the remainder of the time it gets no darker than civil twilight. In Shetland the period in the middle of the night when it doesn’t get dark is known as the Simmer Dim.

 

Photo taken by Mrs Geek at around midnight. The Science Geek was fast asleep at this time 😉

Every year on the days around the June solstice there is a Simmer Dim festival in Shetland where hundreds of bikers come from all over Europe to celebrate the light nights by eating, drinking and listening to music.  Mr and Mrs Geek prefer being tucked up in bed!

Simmer Dim Festival 2018

I hope you have enjoyed this post and that those of you who live  well north of the equator, like us, are enjoying the long summer nights.

The Science Geek


To find out more about the Science Geek’s blog, click here or at the Science Geek Home link at the top of this page.


Notes

For the more mathematically inclined  readers, the elevation of the Sun at a given time is given by the following formula:

sin(EL) = sin(DEC)*sin(LAT)+cos(DEC)*cos(LAT)*cos(HA)

where

  • EL is the elevation or height of the Sun measured in degrees
  • DEC is the declination of the Sun. This changes throughout the year as the Earth moves around the Sun in its orbit. At the June solstice the Sun has a declination of +23.5 degrees, at the equinoxes it is zero and at the December solstice it is -23.5 degrees
  • LAT is the latitude of the observer
  • HA is the hour angle. it the number of hours since the solar noon (when the Sun is highest is sky) multiplied by 15. So, 2 hours after the solar noon is an hour angle of 30 degrees, 3 hours before the solar noon is an hour angle of -45 degrees

 

 

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