Tuesday, January 12, 2010

Definition of Sunrise/Sunset Times

The times of sunrise and sunset are by definition the times at which the upper edge of the sun appears on the horizon.  This assumes an observer on level ground with an unobstructed view to the horizon.  Official times take into account refraction (yes, light bends around the edge of the earth) under average atmospheric conditions.  When the top edge of the sun appears on the horizon, it is actually already about 34 minutes of arc below the horizon - that is, we can still see it, but it is actually below the horizon.

It is important to remember that rise/set times vary from place to place - they are very location-specific and it is not sufficient to look up the rise and set times for a place other than your own, and then to assume it applies for you.  If your location is not one of the samples posted on our web site, let us know and we will send you the rise/set times for the current month for your location for free. 

See http://www.sunposition.com/tables.html for details.

Cheers, Ralph

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