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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.)
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 (firstname.lastname@example.org) from articles in the April 1984 and July 1984 issues of Astronomy Magazine.
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Last updated on 21 July 2006
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