e.g, where do you get your sunrise number from?
Sunrise and sunset. 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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