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Summary
DescriptionSunrise-Sunset angle.svg
English: A diagram of the sun at sunrise or sunset, showing the effects of atmospheric refraction. The sun's angular radius is 16 arcminutes, and the average atmospheric refraction is 34 arcminutes. These combine to cause the sun to remain visible until it is 50 arcminutes below the horizon.
Date
Source
Own work;
US Navy Twilight time calculator (new link): "For computational purposes, sunrise or sunset is defined to occur when the geometric zenith distance of center of the Sun is 90.8333 degrees. That is, the center of the Sun is geometrically 50 arcminutes below a horizontal plane. For an observer at sea level with a level, unobstructed horizon, under average atmospheric conditions, the upper limb of the Sun will then appear to be tangent to the horizon. The 50-arcminute geometric depression of the Sun's center used for the computations is obtained by adding the average apparent radius of the Sun (16 arcminutes) to the average amount of atmospheric refraction at the horizon (34 arcminutes)."
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