The time of day when the sun is at its highest point above the horizon, due south in the Northern Hemisphere and due north in the Southern Hemisphere.

The center of the sun is between the horizon and 6° below the horizon. The sky is still illuminated, but vehicle headlights need to be on to be seen by others.

The center of the sun is between 6° and 12° below the horizon. Brighter stars used for celestial navigation are visible in a clear sky, and the horizon may still be seen.

The center of the sun is between 12° and 18° below the horizon. A clear sky is dark enough for astronomical work to proceed.