Astronomical twilight
Astronomy Astronomical dawn Astronomical dusk
Astronomical twilight occurs with the sun between 12° and 18° below the horizon. Astronomical night begins beyond that point, provided the sun sinks far enough.
Night to the eye, but not quite to the telescope
During this final evening twilight phase, the sky already looks dark. Remaining scattered sunlight can still hide faint objects. Astronomical twilight ends at 18° below the horizon; the same angle marks its beginning in the morning.
Latitude and season
The darkness of a night depends on the date and latitude. At northern latitudes, the sun does not reach 18 degrees below the horizon every summer night. Astronomical twilight can then last throughout the night.
Astronomical night does not guarantee a dark sky
Moonlight, the city’s glow and natural sky brightness remain. Clouds and aerosols can scatter artificial light too. The 18° boundary describes only the sun’s contribution.
Which observations benefit?
Faint nebulae and galaxies benefit from darkness and good transparency. The Moon and bright planets can often be observed during twilight. Steady air, or good seeing, also matters for them.