Civil, nautical and astronomical twilight

Astronomical darkness begins when the sun is 18° below the horizon. Above that, there is still measurable skyglow, and faint deep-sky work is compromised even though the sky looks black.

The three twilights are defined by the sun's depth below the horizon. Civil twilight runs to 6°, nautical to 12°, astronomical to 18°. Past 18° the sun contributes nothing further and the sky is as dark as your location allows.

The practical consequence is that "after sunset" and "dark" can be two hours apart. In mid-summer at 50° north they never meet at all: the sun does not reach 18° and astronomical darkness simply does not occur. The forecast says so plainly rather than pretending the night is normal.

Not every target cares. The Moon and the bright planets are perfectly workable in nautical twilight, which is why the planetary mode in the window finder barely weights darkness at all. A magnitude 12 galaxy is a different matter — every degree the sun climbs back toward the horizon costs you contrast.

On the hour matrix, twilight is drawn as the shaded ground behind the hours, and true astronomical darkness is the unshaded stretch. The absence of ink is the reading.

REVIEWED 2026-08-12 · SOURCES: MET Norway, sun and moon positions computed on our own servers