Science Fair Project Encyclopedia
- This article describes the time of day. For the work by Elie Wiesel, see Night (book).
Night is the time when a location is facing away from the Sun, and thus dark. On Earth, it is night on just under half the planet at any time. When it is night on one side of the planet, it is day on the other side. Because of the rotation of Earth about its axis, it is alternately day and night, which together form a 24-hour day. The Earth's tilt also means that the nights are shorter in summer and longer in winter.
Two effects contribute to the fact that nights are on average shorter than days. The sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the sun is still below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the sun is still below the horizon by about 50 minutes of arc.
Disregarding these two effects, at the fall and spring equinoxes, the day and night would be the same length (a 1:1 ratio of day to night). In practice around the equinoxes the day is almost 7 minutes longer than the night. The summer and winter solstices mark the shortest night and the longest night, respectively.
The closer a location is to the North or South Pole, the larger the range of variation in the night's length. For example, in locations near the poles, equinoxes still occur with a day and night of equal length. However, after an equinox, the ratio of night to day changes more rapidly than in locations between the Tropic of Cancer and the Tropic of Capricorn. In the Northern Hemisphere, Denmark has shorter nights in June than India has. In the Southern Hemisphere, Antarctica has longer nights in June than Chile has. The Northern and Southern Hemispheres of the world experience the same patterns of night length depending on their latitude, but the cycles are 6 months apart so that one hemisphere experiences long nights (winter) while the other experiences short nights (summer).
Near the two poles, extreme variations in daylight hours result in a continuous day for a portion of the summer, followed by continuous night during the depths of winter.
Throughout most of history, night was primarily a time of rest, because little work can be done in the dark. As artificial lighting has improved, night-time activity has increased and become a significant part of the economy in many places.
Even without artificial light, moonlight sometimes makes it possible to travel or work outdoors at night.
Night is often associated with danger, because bandits and dangerous animals can be concealed by darkness. The belief in magic often includes the idea that magic, or magicians, are more powerful at night. Similarly, mythical and folkloric creatures as vampires and werewolves are thought to be more active at night.
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