It is sub-zero temperature when about 100 of us excitedly clamber onto the decks. A snowy stream of cloud lights up the dark sky above. We train our smartphones and cameras skywards, but I can see only a faint splash of green on my iPhone screen.

The guy next to me has his camera on a tripod and I can see on his screen a swathe of bright green ribbon, with purplish light on the edges, doing a slow dance across the sky.