Twilight

Twilight

Algorithms

Calculates the times of the three twilights times.

procedure Morning_Twilight_Civil(date:TDateTime; latitude, longitude:extended): TDateTime;

procedure Morning_Twilight_Nautical(date:TDateTime; latitude, longitude:extended): TDateTime;

procedure Morning_Twilight_Astronomical(date:TDateTime; latitude, longitude:extended): TDateTime;

procedure Evening_Twilight_Civil(date:TDateTime; latitude, longitude:extended): TDateTime;

procedure Evening_Twilight_Nautical(date:TDateTime; latitude, longitude:extended): TDateTime;

procedure Evening_Twilight_Astronomical(date:TDateTime; latitude, longitude:extended): TDateTime;

Description

Calculates the time of the beginning of the morning twilight (which ends at sun rise) or the end of the evening twilight (which begins at sun set). If the sun does not reach the elevation needed for one of these calculations for the whole day the exception E_NoRiseSet is raised.

Civil twilight is defined as the time when the sun reaches an elevation of 6 degrees under the horizon. When the sun ls deeper than this it is so dark that artifical light would be needed.

Nautical twilight is defined as the time when the sun reaches an elevation of 12 degrees under the horizon. When the sun is deeper than this it is dark enough to have all the bright stars needed for nautical triangulations clearly visible.

Astronomical twilight is defined as the time when the sun reaches an elevation of 18 degrees under the horizon. When the sun is deeper than this it is dark enough to have all stars visible, and the sun is not disturbing astronomical observations at all any more.

The observer's latitude is negative for the southern hemisphere and positive for the northern hemisphere; the longitude is positive for points west of Greenwich, negative for points east, and both are given in degrees.

Reference

These function are based upon chapter 15 (14) of "Astronomical Algorithms".