Good night
Yesterday the sun set at 4pm (where I am). Official night was therefore at 4.30. Now it was a very clear day but without much of a moon. When I landed at 4.28 it was still surprisingly light. I reckon by about 4.45, just about all the natural light had gone. It got me thinking why we reckon official night starts at half an hour after sunset. Is it just one of those arbitrary numbers?