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Strangeness near the North and South Poles
During the summer, latitudes near the North Pole are in perpetual sunlight
(in varying degrees) and latitudes near the South Pole are in perpetual
darkness. During the winter, conditions are reversed.
Therefore, at these high latitudes some events are listed as
Does not occur. Since the various events (sunset, end of civil
twilight, etc.) are officially defined as when the sun is at various positions
in the sky, it is possible for some of these events to occur and some not to.
(The sun may dip down enough for sunset but not for the end of civil twilight.)
If the sun "just barely" rises on a given day it might have shifted just
enough to "just barely not" set that day, and likewise for other "mirror image"
events. It is also possible for rounding errors to cause one and not the
other to be listed. (You can try a date 15 days before and after to verify
these.)
Definitions of Astronomical events
Sunrise/Sunset is defined as the instant in the morning/evening
under ideal meteorological conditions, with standard refraction of the
sun's rays, when the upper edge of the sun's disk is coincident with an
ideal horizon.
Beginning/Ending of civil twilight is defined as the
instant in the morning/evening, when the center of the sun is at a depression
angle of six degrees (6°) below an ideal horizon. At this time in the
absence of moonlight, artificial lighting or adverse atmospheric conditions,
the illumination is such that large objects may be seen but no detail is
discernible. The brightest stars and planets can be seen and for navigation
purposes at sea, the sea horizon is clearly defined.
The brightest stars and planets can be seen and for navigation purposes at
sea, the sea horizon is clearly defined.
Beginning/Ending of nautical twilight is defined as the
instant in the morning/evening, when the center of the sun is at a
depression angle
of twelve degrees (12°) below an ideal horizon. At this time in the absence
of moonlight, artificial lighting or adverse atmospheric conditions, it is
dark for normal practical purposes. For navigation purposes at sea, the sea
horizon is not normally visible.
Beginning/Ending of astronomical twilight is defined as the
instant in the morning/evening, when the center of the sun is at a depression
angle of eighteen degrees (18°) below an ideal horizon. At this time the
illumination due to scattered light from the sun is less than that from
starlight and other natural light sources in the sky.
An ideal horizon exists when the surface forming the
horizon is at a right angle to the vertical line passing through the
observer's position on the earth. If the terrain surrounding the observer
was flat and all at the same height above sea level, the horizon seen by
the observer standing on the earth would approximate the ideal horizon.
The zenith distance is a vertical angle measured from directly overhead,
down to the required point. An ideal horizon has a zenith distance of 90
degrees.
The vertical angle is the angle measured in a vertical plane, from the
horizon to the required point. Directly overhead would have a vertical angle
of 90 degrees.
The portion of the C code that computes events from a specified Latitude
and Longitude was written by Arlin B. Collins (arlin@tarleton.edu) from
articles in the April 1984 and July 1984 issues of Astronomy Magazine.
URL: http://www.verysecurelinux.com/sunset.html
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Last updated on 09 September 2020.
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