An extract on #night
Complete darkness or astronomical night is the period of time between astronomical dusk and astronomical dawn when the sun is between 18 and 90 degrees below the horizon and the sky is not illuminated by the sun. At latitudes between 48.5 and 66.5 degrees north or south of the equator, complete darkness does not occur near the summer solstice because even though the sun sets, it is never more than 18 degrees below the horizon at its lower culmination. The opposite of night is day (or "daytime", to distinguish it from "day" as used for a 24-hour period). The start and end points of time for a night vary, based on factors such as season, latitude, longitude and timezone. Twilight is the time of night when the sky is illuminated by the sun but the sun is below the horizon.
At any given time, one side of the planet Earth is bathed in light from the Sun (the daytime) and the other side of the Earth is in the shadow caused by the Earth blocking the light of the sun. This shadow is called the umbra. Natural illumination is still provided by a combination of moonlight, planetary light, starlight, diffuse zodiacal light, gegenschein, and airglow. In some circumstances, bioluminescence, aurorae, and lightning can provide some illumination. The glow provided by artificial illumination is sometimes referred to as light pollution because it can interfere with observational astronomy and ecosystems.
Nights are shorter than days on average due to two factors. Firstly, the sun is not a point, but has an apparent size of about 32 arc minutes. Secondly, the atmosphere refracts sunlight so that some of it reaches the ground when the sun is below the horizon by about 34 arc minutes. The combination of these two factors means that light reaches the ground when the center of the sun is below the horizon by about 50 arc minutes. Without these effects, day and night would be the same length at the autumnal (autumn/fall) and vernal (spring) equinoxes, the moments when the sun passes over the equator. In reality, around the equinoxes the day is almost 14 minutes longer than the night at the equator, and even more towards the poles. The summer and winter solstices mark the shortest and the longest night, respectively. The closer a location is to either the North Pole or the South Pole, the larger the range of variation in the night's length. Although equinoxes occur with a day and night close to equal length, before and after an equinox the ratio of night to day changes more rapidly in high latitude locations than in low latitude locations. In the Northern Hemisphere, Denmark has shorter nights in June than India. In the Southern Hemisphere, Antarctica has longer nights in June than Chile. The Northern and Southern Hemispheres of the world experience the same patterns of night length at the same latitudes, but the cycles are 6 months apart so that one hemisphere experiences long nights (winter) while the other is experiencing short nights (summer).
Between the pole and the polar circle, the variation in daylight hours is so extreme that for a portion of the summer, there is no longer an intervening night between consecutive days and in the winter there is a period that there is no intervening day between consecutive nights.