Night
Other celestial bodies

Night exists on the other planets and moons in the solar system. The length of night is affected by the rotation period and orbital period of the celestial object. The lunar phases visible from Earth result from nightfall on the Moon. The Moon has longer nights than Earth, lasting about two weeks. This is half of the synodic lunar month, the time it takes the Moon to cycle through its phases. The Moon is tidally locked to Earth; it rotates so that one side of the Moon always faces the Earth. The side of the Moon facing away from Earth is called the far side of the Moon and the side facing Earth is called the near side of the Moon. During lunar night on the near side, Earth is 50 times brighter than a full moon. Because the Moon has no atmosphere, there is an abrupt transition from day to night without twilight.

Night varies from planet to planet within the Solar System. Mars 's dusty atmosphere causes a lengthy twilight period. The refracted light ranges from purple to blue, often resulting in glowing noctilucent clouds. Venus and Mercury have long nights because of their slow rotational periods. The planet Venus rotates once every 243 Earth days. Because of its unusual retrograde rotation, nights last 116.75 Earth days. The dense greenhouse atmosphere on Venus keeps its surface hot enough to melt lead throughout the night. Its planetary wind system, driven by solar heat, reverses direction from day to night. Venus's winds flow from the equator to the poles on the day side and from the poles to the equator on the night side. On Mercury, the planet closest to the Sun, the temperature drops over 1,000 °F (538 °C) after nightfall.

The day-night cycle is one consideration for planetary habitability or the possibility of extraterrestrial life on distant exoplanets. In general, shorter nights result in a higher equilibrium temperature for the planet. On an Earth-like planet, longer day-night cycles may increase habitability up to a point. Computer models show that longer nights would affect Hadley circulation, resulting in a cooler, less cloudy planet. Once the rotation speed of a planet slows beyond 1/16 that of Earth, the difference in day to night temperature shifts increases dramatically. Some exoplanets, like those of TRAPPIST-1, are tidally locked. Tidally locked planets have equal rotation and orbital periods, so one side experiences constant day, and the other side constant night. In these situations, astrophysicists believe that life would most likely develop in the twilight zone between the day and night hemispheres.
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