Civil twilight is the light after sunset and before sunrise while the sun is under 6° below the horizon — bright enough to work outdoors without lamps.

The sun is below the horizon but the air high above you is still in sunlight, and it scatters that light down. At 6° the illumination falls to about that of a bright moonlit night, which is where almanacs, aviation and the law draw the line. Its length depends on how steeply the sun crosses the horizon: about twenty-five minutes near the equator, over half an hour in London, and at midsummer far enough north it never ends.

It ends at civil dusk and begins at civil dawn; the brightest planets show, and the horizon stays sharp.