7/30/2023 0 Comments Twilight dawn dusk![]() When the solar depression angle is greater than 6° artificial lighting is required for reading and for most outdoor activities. There is enough light during civil twilight for most outdoor activities, and only the brightest planets and stars are visible. (Stars aren't visible in most of the lunar landing photos from the NASA Apollo Program because the lunar surface under direct solar illumination was so bright that camera f-stops had to be stopped down to small apertures, making stars too dim - for further explanation see Why Aren't There Stars in the Moon Landing Photos?).Ĭivil twilight ends after sunset or begins before sunrise when the solar depression angle is 6° = solar zenith angle 96° = solar elevation angle -6°. Thus various stages of twilight are defined in terms of the solar depression angle, in degrees.Ĭlouds can either shorten the duration of twilight or darken its stages, if the clouds are dense and darken the sky, especially if they obstruct sunlight, or they may prolong the duration or brighten its stages, if there is clear sky to the west below the horizon allowing sunlight to reflect from the clouds.Ī planet or moon without an atmosphere has no twilight periods before sunrise or after sunset, and stars are visible at all times, even during the daytime. Generally the sky brightness after sunset or before sunrise correlates with the degree of solar depression, that is the angle between the sea level horizon, the center of Earth, and the center of the solar disk (ignoring the elevation of the locale and the slightly raised apparent level of the local horizon caused by atmospheric refraction). ![]() From sunrise to sunset a horizontally mounted sundial in an open area will display the local apparent time of day, if any portion of the solar disk has a direct path to that sundial. Astronomical sunset is the moment when the upper limb of Sun just disappears below the local westerly horizon. The duration of twilight after sunset or before sunrise depends on atmospheric conditions (clouds, dust, air pressure, temperature, humidity) and on the angle between the path of the setting or rising sun and the local horizon (mainly affects the durations of sunrise, sunset, and civil twilight), and on the diurnal solar path of Sun in the antipodal sky (opposite hemisphere, mainly affects the durations of nautical and astronomical twilight), all of which vary with the seasons (specifically the solar declination) and the terrestrial latitude.Īstronomical sunrise is the moment when the upper limb of Sun first appears at the local easterly horizon. Twilight is that period of dusk after sunset or dawn before sunrise during which the sky is partially lit by atmospheric scattering of sunlight.
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