While summer isn't the best time for astronomy - hindered by short nights and a lack of full astronomical darkness – there is at least one compensation. For observers at high latitudes in the northern hemisphere the season of noctilucent (night-shining) clouds is almost upon us. These tenuous ultra high-altitude clouds are only visible during deep twilight, being lit directly by the Sun while the observer stands in darkness. They are too insubstantial to be seen when standing directly underneath.
Last Summer I was lucky enough to observe and photograph this rare phenomena, only a week or two after learning of their existence. It took me some time to realise what I was seeing, at first I assumed it was a low and near cloud lit-up by light pollution, but eventually I realised it didn't appear to be moving.
This image, taken at about 1:30AM, was featured on the BBC News website. It’s an 8 second exposure taken with a Canon 1100D DSLR fitted with a 50mm lens.
I was curious as to how far away the clouds were, so I plate-solved the image to identify the visible stars and used a software planetarium to find their angles above the horizon. Noctilucent clouds form at an altitude of about 50 miles, so with a quick bit of trigonometry I was able to work out that they were several hundred miles away. From a location a few miles north of London I was looking at clouds somewhere off the west coast of Norway!
Plate-solved image shows the constellation Auriga on the horizon, with the bright star Canopus at upper right.
So if you find yourself at a dark site between June and August it’s well worth taking a moment to scan your northern horizon. A bright display like the one pictured above is clearly visible even from a moderately light-polluted site, while fainter ones may only show up on long exposure photographs.