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 how far the centre of the sun sits below the horizon. The US Naval Observatory puts the boundaries at 6°, 12° and 18°. Astronomical twilight ends in the evening when the sun's centre is geometrically 18° down, and past that point, in the USNO's own terms, scattered sunlight is less than the light arriving from starlight and other natural sources. There is no further darkening to wait for.

The practical consequence is that "after sunset" and "dark" can be two hours apart, and the gap widens with latitude and season. Far enough north or south in summer the sun never reaches 18° at all and astronomical darkness simply does not occur. The forecast says so plainly rather than scoring the night as though it were normal.

The USNO also notes that under astronomical twilight most casual observers would call the sky fully dark, particularly anywhere with light pollution. That is precisely why it is worth stating numerically rather than by eye: the residual skyglow that you cannot see is still sitting underneath a faint galaxy you are trying to.

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 applies only a light darkness penalty. A faint galaxy is another matter.

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.