The following icons and colors are in used:
Twilight is the period before sunrise and after sunset when the sky is partially illuminated by the scattered sunlight. The calendar will display three degrees off twilight: civil, nautical and astronomical twilight.
Civil twilight, the Sun has set but is less than 6° below the horizon. There is still enough light to read and only the brightest stars are visible.
Nautical twilight, the Sun is between 6° and 12° below the horizon. Many stars are visible and the horizon is barely visible.
Astronomical twilight, the Sun is between 12° and 18° below the horizon. When the Sun is 18° below the horizon the sky it is completely dark.
The duration of twilight is a result of the Suns path with respect to the horizon. Near the equator the twilight is always of short as the Sun disappears almost vertically straight down below the horizon. The farther North or South you go from the equator, the shallower is the angel the Sun travels as it disappears below the horizon and twilight lasts longer. North of 49°N it never gets completely dark during Summer Solstice, and north of 67°N the Sun is circumpolar at this time.