Nightfall

why is the night sky dark?

push the edge of a wood until every line of sight ends on a trunk; then night falls, every trunk becomes a star, and light needs time to arrive

In a deep enough wood every line of sight ends on a trunk. Make the trunks stars, and an endless, ageless universe would have a sky as bright as the Sun's disc in every direction: Olbers' paradox. The night is dark mainly because the universe has a finite age. We only see stars whose light has had time to reach us, about 13.8 billion years of travel, while a sight line would have to run about 10^23 light-years to meet a star. The expansion of space dims distant light further, by a smaller factor.

Concepts: stars, light-speed.

An interactive you play with one thumb, on Wundr.