Techniques of Observational Astronomy
AST3722C

Calculating Rise and Set Times

Calculations of rise and set times usually involve a transformation for the HA/Declination system (which locates an object in equatorial coordinates) to the AltAzimuth system and solving for a specific altitude.

solving for altitude a (which is 90°-zenith angle):

For a given latitude, declination, and desired a, solve this equation for H. This is the hour angle at altitude a and hence the number of hours before or after transit the object reaches that altitude. The time of transit can easily be calculated from the object's Right Ascension which equals the sidereal time at transit.

For twilight we calculate the time when the sun reaches altitudes of -6° (civil), -12° (nautical), and -18° (astronomical).

Tables of Hour Angle for Rise/Set and SecZ=2

This page was last edited September 25, 2002 7:18 PM