Night or nighttime (sp. night-time or night time) is the period of time between the sunset and the sunrise when the Sun is below the horizon. Complete darkness or astronomical night is the period of time between astronomical dawn and astronomical dusk when the sun is between 18 and 90 degrees below the horizon and the sky is totally dark. 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.