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Sunrise and Sunset
Daylength
Daylength is total amount of daylight hours in a day, and is typically calculated as the hours between sunrise and sunset. As such, daylength gets longer during the summer months, and shorter during the winter months.
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Dusk and Dawn
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Dusk occurs at the very end of astronomical twilight after sunset and just before night. In other words, at the darkest stage of twilight.
Dawn is derived from Old English, and means to become day. Dawn begins with the first sight of lightness (Astronomical twilight) in the morning, and continues until the sun peaks above the horizon.
Twilight
Twilight can best be defined as the illumination of the lower atmosphere when the sun is below the horizon and no longer directly visible. It is produced when sunlight scatters in the upper atmosphere, thereby illuminating the lower atmosphere, and results in the Earth’s surface not being completely lit, or completely dark. Evening twilight starts when the Sun dips below the horizon and lasts until the Sun reaches 18° below the horizon. At that point, is it officially night. The reverse is true for morning twilight. Twilight has 3 sub categories, Civil, Nautical, and Astronomical.
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Civil twilight is the period when enough natural light remains that artificial light is not needed. Civil twilight typically begins when the sun is 6° below the horizon, and ends at sunrise. The reverse is true for evening civil twilight.
Nautical twilight begins when there is enough illumination for sailors to distinguish the horizon at sea. Morning Nautical twilight, or nautical dawn begins when the sun is 12° below the horizon, and ends when it reaches 6° below the eastern horizon. Evening nautical twilight occurs in the western horizon, when the sun reaches 6° below the horizon and ends when it reaches 12°
Astronomical twilight is when the sun reaches 18° below the horizon. Astronomical dawn marks the beginning of astronomical twilight and is also defined as “first light”. This continues until the sun reaches 12° below the horizon, before transitioning to nautical twilight. In the evening, the reverse is true. Astronomical twilight begins at 12°, and ends at 18° below the horizon, when it officially becomes night.
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