Star sights need two things at once: stars bright enough to see in the sextant, and a horizon sharp enough to bring them down to. That only happens in twilight, between about the end of civil twilight (Sun 6° below the horizon) and nautical twilight (12°): often only 20 to 40 minutes, morning and evening.

Predict Where is evening twilight shorter: at the equator or at 50° N?

The Sun's altitude through the evening (local mean time). Shaded: civil twilight (to −6°) and nautical twilight (to −12°). The tall band is the time for star sights: bright stars out, horizon still sharp.

Sunset: 1932
Civil twilight ends: 2005
Nautical twilight ends: 2047
Twilight lasts: 75 min

At higher latitudes the Sun slants down and twilight lasts longer.

Latitude: 40° NDate: June 21

Move the date and latitude: near the equator twilight is short; in high latitudes in summer it never gets dark enough.

Twilight: the window for star sights