Night is defined as being half an hour after sunset in the UK.....
A rigid bureacratic definition for a condition in these latitudes that is variable, to say the least! In December, the sun sneaks around the sky at very low level. In June it hardly disappears at all. If the weather is crap, night comes quickly. Several times, when retrieving a glider from an outlanding, while slowly increasing height to keep the glider stable on the long aerotow, it comes as rather a shock to realise that it is getting awfully dark down there! Did anyone happen to find out what time was sunset today? errr.....umm... seem to have overlooked that detail.
Gliders that are pushing the boundaries of night flying from their local airfield will stay local....thermals are scarce after dark. On one occasion the neighbouring helicopter operation, not receiving any answer from their radio call (training gliders seldom carry radios), proceeded boldly straight across the gliding field and caused consternation to the gliders in the circuit. Apologies all around.
Also possible that a glider could be sliding in late and low from a field retrieve. Best to keep well clear.