Dusk is the light after sunset — the twilight that ends when the sun is 18° below the horizon and the night is complete.

The same passage as dawn, reversed. How long it takes depends on the angle the sun crosses the horizon at: steep near the equator, so night comes fast, and shallow in the north, so the light lingers. The drawing shows that angle for where you stand — it is ninety degrees minus your latitude at the equinox.

It passes through civil, nautical and astronomical twilight in turn, each a deeper blue than the last.