Summer nights close to the solstice are all night twilight affairs at 54 degrees north.

This panorama taken at 02:00am local time reveals a band of electric blue noctilucent clouds drifting somewhere high above the Shetland Islands and possibly even further north towards the Faroe Islands. In the background on the left are faint stars of Gemini and Lynx, in the middle is northern Auriga with Capella and the Kids and on the right is Perseus.

These blue shining high clouds occupy a thin region about 82 km above the surface of the Earth and form when meteor smoke nucleates the miniscule amounts of water present in summer to form thin clouds of tiny ice crystals high enough to be still lit by the sun.

An hour earlier at local midnight the northern sky looked like this:



Science:

Some basic trigonometry will help work out how far north the clouds right on the horizon are and how far away this is by line of sight. It is also possible to work out where the top of the cloud display is by using known stars.

NLCs are about 80 to 82 km above the Earth. Earth's radius is 6371 km.

The increase in latitude from your location to the clouds right down on the horizon is given by:

cos-1 (6371 / (6371 + 80)) = cos-1 (0.9876) = 9 degrees.

The picture was taken close to 54 North so the clouds on the horizon are at latitude 63 North which is 1 degree north of the Faroe Islands.



The direct line of sight distance to the furthest visible clouds is:

tan (9) = distance / 6371

distance = 0.1582 * 6371 = 1009 km

The bright star in the picture Cappella is very very close to 10 degrees above the northern horizon (measured via Stellarium). In the panorama the top of the clouds are 9 degrees above the horizon. This gives (via the sine rule and angle sum rule ):

@ 5 degrees above horizon (1/2 way to Cappella) - cloud latitude = 59.3 degrees; line of sight distance = 600 km (between Orkney and Shetland Islands)

@ 9 degrees above horizon (top of clouds) - cloud latitude = 57.7 degrees; line of sight distance = 424 km (over Northern Scottish Highlands)

The visible portion of these clouds cover a vast area. From 5 degrees down to the horizon you are looking at a 400 km distance (hugely foreshortened)

Processing:

The image is a 6 panel mosaic stitched with AutoPano Pro 2.5. Each frame was pre-treated in Photoshop CS2 with a) despeckle filter b) Astrotools colour blotch removal (low frequency chrominance noise) c) Neat image noise reduction with noise profile sampled from dark sky area.