Here are the television transmission masts on Sydenham Hill settling down for the night. The red warning lights on the Crystal Palace mast, seen on the left, always seem to come on later than the one on the right. This may be because it is slightly further away and does not show up quite so clearly until the light sky has begun to grow darker.

This time of day, shortly after sunset, is known to photographers as the blue hour - because the sky grows deep blue before the dark of night sets in. You also get a blue hour before sunrise, but that involves getting up at very painful o'clock or staying up all night... either way, the only thing I can think of in these situations is bed...