Night
Night, or nighttime, is the period of darkness when the Sun is below the horizon. Daylight illuminates one side of the Earth, leaving the other in darkness. The opposite of nighttime is daytime. Earth's rotation causes the appearance of sunrise and sunset. Moonlight, airglow, starlight, and light pollution dimly illuminate night. The duration of day, night, and twilight varies depending on the time of year and the latitude. Night on other celestial bodies is affected by their rotation and orbital periods. The planets Mercury and Venus have much longer nights than Earth. On Venus, night lasts about 58 Earth days. The Moon's rotation is tidally locked, rotating so that one of the sides of the Moon always faces Earth. Nightfall across portions of the near side of the Moon results in lunar phases visible from Earth.
Organisms respond to the changes brought by nightfall: darkness, increased humidity, and lower temperatures. Their responses include direct reactions and adjustments to circadian rhythms governed by an internal biological clock. These circadian rhythms, regulated by exposure to light and darkness, affect an organism's behavior and physiology. Animals more active at night are called nocturnal and have adaptations for low light, including different forms of night vision and the heightening of other senses. Diurnal animals are active during the day and sleep at night; mammals, birds, and some others dream while asleep. Fungi respond directly to nightfall and increase their biomass. With some exceptions, fungi do not rely on a biological clock. Plants store energy produced through photosynthesis as starch granules to consume at night. Algae engage in a similar process, and cyanobacteria transition from photosynthesis to nitrogen fixation after sunset. In arid environments like deserts, plants evolved to be more active at night, with many gathering carbon dioxide overnight for daytime photosynthesis. Night-blooming cacti rely on nocturnal pollinators such as bats and moths for reproduction. Light pollution disrupts the patterns in ecosystems and is especially harmful to night-flying insects.
Historically, night has been a time of increased danger and insecurity. Many daytime social controls dissipated after sunset. Theft, fights, murders, taboo sexual activities, and accidental deaths all became more frequent due in part to reduced visibility. Despite a reduction in urban dangers, the majority of violent crime is still committed after dark. According to psychologists, the widespread fear of the dark and the night stems from these dangers. The fear remains common to the present day, especially among children.
Cultures have personified night through deities associated with some or all of these aspects of nighttime. The folklore of many cultures contains "creatures of the night", including werewolves, witches, ghosts, and goblins, reflecting societal fears and anxieties. The introduction of artificial lighting extended daytime activities. Major European cities hung lanterns housing candles and oil lamps in the 1600s. Nineteenth-century gas and electric lights created unprecedented illumination. The range of socially acceptable leisure activities expanded, and various industries introduced a night shift. Nightlife, encompassing bars, nightclubs, and cultural venues, has become a significant part of urban culture, contributing to social and political movements.
Etymology
Astronomy
A planet's rotation causes nighttime and daytime. When a place on Earth is pointed away from the Sun, that location experiences night. The Sun appears to set in the West and rise in the East due to Earth's rotation. Many celestial bodies, including the other planets in the solar system, have a form of night.Earth
The length of night on Earth varies depending on the time of year. Longer nights occur in winter, with the winter solstice being the longest. Nights are shorter in the summer, with the summer solstice being the shortest. Earth orbits the Sun on an axis tilted 23.44 degrees. Nights are longer when a hemisphere is tilted away from the Sun and shorter when a hemisphere is tilted toward the Sun. As a result, the longest night of the year for the Northern Hemisphere will be the shortest night of the year for the Southern Hemisphere.Night's duration varies least near the equator. The difference between the shortest and longest night increases approaching the poles. At the equator, night lasts roughly 12 hours throughout the year. The tropics have little difference in the length of day and night. At the 45th parallel, the longest winter night is roughly twice as long as the shortest summer night. Within the polar circles, night will last the full 24 hours of the winter solstice. The length of this polar night increases closer to the poles. Utqiagvik, Alaska, the northernmost point in the United States, experiences 65 days of polar night. At the pole itself, polar night lasts 179 days from September to March.
Over a year, there is more daytime than nighttime because of the Sun's size and atmospheric refraction. The Sun is not a single point. Viewed from Earth, the Sun ranges in angular diameter from 31 to 33 arcminutes. When the center of the Sun falls to the western horizon, half of the Sun will still be visible during sunset. Likewise, by the time the center of the Sun rises to the eastern horizon, half of the Sun will already be visible during sunrise. This shortens night by about three minutes in temperate zones. Atmospheric refraction is a larger factor. Refraction bends sunlight over the horizon. On Earth, the Sun remains briefly visible after it has geometrically fallen below the horizon. This shortens night by about six minutes. Scattered, diffuse sunlight remains in the sky after sunset and into twilight.
File:Nile River Delta at Night.JPG|thumb|alt=space photograph showing the inhabited areas of the Nile glowing against the dark desert|The drainage basin of the Nile River and delta at night
Twilight, the gradual transition to and from darkness when the Sun is below the horizon, has multiple stages. "Civil" twilight occurs when the Sun is between 0° and 6° below the horizon. Nearby planets like Venus and bright stars like Sirius are visible during this period. "Nautical" twilight continues until the Sun is 12° below the horizon. During nautical twilight, the horizon is visible enough for navigation. "Astronomical" twilight continues until the Sun has sunk 18° below the horizon. Beyond 18°, refracted sunlight is no longer visible. The period when the sun is 18° or more below either horizon is called astronomical night.
Similar to the duration of night itself, the duration of twilight varies according to latitude. At the equator, day quickly transitions to night, while the transition can take weeks near the poles. The duration of twilight is longest at the summer solstice and shortest near the equinoxes. Moonlight, starlight, airglow, and light pollution can dimly illuminate the nighttime, with their diffuse aspects being termed skyglow. The amount of skyglow increases each year due to artificial lighting.
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 appears 50 times brighter than a full moon appears from Earth. 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 just over 58 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 by over 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.