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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 ...
Crepuscular rays usually appear orange because the path through the atmosphere at dawn and dusk passes through up to 40 times as much air as rays from a high Sun at noon. Particles in the air scatter short-wavelength light (blue and green) through Rayleigh scattering much more strongly than longer-wavelength yellow and red light.
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).
Anticrepuscular rays, or antisolar rays, [1] are meteorological optical phenomena similar to crepuscular rays, but appear opposite the Sun in the sky. Anticrepuscular rays are essentially parallel , but appear to converge toward the antisolar point , the vanishing point , due to a visual illusion from linear perspective .
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 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 ...
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 ...
[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]