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The time of dusk is the moment at the very end of astronomical twilight, just before the minimum brightness of the night sky sets in, or may be thought of as the darkest part of evening twilight. [4] However, technically, the three stages of dusk are as follows: At civil dusk, the center of the Sun's disc goes 6° below the horizon in the ...
Twilight occurs according to the solar elevation angle θ s, which is the position of the geometric center of the Sun relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (nearest the horizon), nautical twilight, and astronomical twilight (farthest from the horizon).
Crepuscular rays, sometimes colloquially referred to as god rays, are sunbeams that originate when the Sun appears to be just above or below a layer of clouds, during the twilight period. [1] Crepuscular rays are noticeable when the contrast between light and dark is most obvious. Crepuscular comes from the Latin word crepusculum, meaning ...
The third phase is astronomical twilight, which is the period when the Sun is between 12 and 18 degrees below the horizon. [2] Dusk is at the very end of astronomical twilight, and is the darkest moment of twilight just before night. [3] Finally, night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky. [4]
The following is a list of cities by sunshine duration.Sunshine duration is a climatological indicator, measuring duration of sunshine in given period (usually, a day or a year) for a given location on Earth, typically expressed as an averaged value over several years.
The Belt of Venus as seen from an airliner at an altitude of 42,000 ft (13 km; 8.0 mi) An animated sequence of the Belt of Venus at dusk, showing how its appearance evolves during evening twilight The Belt of Venus – also called Venus's Girdle , the antitwilight arch , or antitwilight [ 1 ] – is an atmospheric phenomenon visible shortly ...
[1] [2] Purple light mainly occurs when the Sun is 2–6° below the horizon, from civil to nautical twilight, while the bright segment lasts until the end of the nautical twilight. [3] [4] Afterglow is often in cases of volcanic eruptions discussed, [5] while its purple light is discussed as a different particular volcanic purple light. [6]
An orbit will be Sun-synchronous when the precession rate ρ = dΩ / dt equals the mean motion of the Earth about the Sun n E, which is 360° per sidereal year (1.990 968 71 × 10 −7 rad/s), so we must set n E = ΔΩ E / T E = ρ = ΔΩ / T , where T E is the earth orbital period while T is the period of the spacecraft ...