Nautical twilight is the stretch while the sun is between 6° and 12° below the horizon: the horizon still shows, and the bright stars are out.

Between 6° and 12° below, enough sky glow remains to show the sea horizon while the navigational stars are visible — the window a sextant needs. On land it is the deep blue after the streetlights come on, when the first-magnitude stars are up and the fainter ones are not yet.

Sailors took star sights in it, needing both a star and a horizon; it is the deep blue giving way to night.