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Civil Twilight

Civil twilight or beginning and end of daylight is defined when the Sun is 6 degrees below the horizon. It is also known as dusk or dawn in some places and is the time immediately before sunrise and right after sunset. During this period of time, there is still enough light from the Sun at BOD to fly using VFR, however EOD requires using VFR to finish flight 10 minutes before EOD. Civil twilight lasts until the Sun passes 6 degrees below the horizon which starts nautical twilight.

Earth's Seasons

In temperate and polar regions generally four seasons are recognised: spring, summer, autumn, and winter.

The ultimate cause of the seasons is the fact that the Earth's axis is not perpendicular to its orbital plane; it deviates by an angle of approximately 23.5 degrees of arc. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the Sun. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons.

In the temperate and polar regions, seasons are marked by changes in the amount of sunlight, which in turn often cause cycles of dormancy in plants and hibernation in animals. These effects vary with latitude, and with proximity to bodies of water. For example, the South Pole is in the middle of the continent of Antarctica, and therefore a considerable distance from the moderating influence of the southern oceans. The North Pole is in the Arctic Ocean, and thus its temperature extremes are buffered by the presence of all that water. The result is that the South Pole is consistently colder during the southern winter than the North Pole during the northern winter.

As the Earth revolves around the Sun, the seasons in the northern and southern hemispheres are opposite. The cycle of seasons in the polar and temperate zones of one hemisphere is opposite to that in the other. When it is summer in the Northern hemisphere, it is winter in the Southern hemisphere, and vice versa, and when it is spring in the Northern hemisphere it is autumn in the Southern hemisphere, and vice versa.

Polar Day and Night

A common misconception is that, within the Arctic and Antarctic Circles, the sun rises once in the spring and sets once in the fall; thus, the day and night are erroneously thought to last uninterrupted for 183 calendar days each. This is true only in the immediate region of the poles themselves. This provides longer local mean time (LMT) day light hours at the poles during summer and shorter LMT day light hours during winter.

What does happen is that any point north of the Arctic or south of the Antarctic Circle will have one period in the summer when the sun does not set, and one period in the winter when the sun does not rise. At progressively higher latitudes, the periods of "midnight sun" (or "midday dark" for the other side of the globe) are progressively longer. For example, the sun begins to peek above the horizon in mid-February and each day it climbs a bit higher, and stays up a bit longer; by 21 March, the sun is up for 12 hours. However, mid-February is not first light. The sky has been showing twilight, or at least a pre-dawn glow on the horizon, for increasing hours each day, for more than a month before that first sliver of sun appears.

In the weeks surrounding 21 June, the sun is at its highest, and it appears to circle the sky without ever going below the horizon. Eventually, it does go below the horizon, for progressively longer and longer periods each day until, around the middle of October, it disappears for the last time. For a few more weeks, "day" is marked by decreasing periods of twilight. Eventually, for the weeks surrounding 21 December, nothing breaks the darkness. In later winter, the first faint wash of light briefly touches the horizon (for just minutes per day), and then increases in duration and pre-dawn brightness each day until sunrise in February.