A little known, but fascinating, phenomenon on the Mercurian surface is that during certain times of its year, the Sun rises twice near the poles. This happens when Mercury is closest to the Sun at perihelion and the angular speed of the Sun across the sky is about the same as the daily sky rotation speed. From polar latitudes, the Sun can execute a retrograde motion that doubles back on itself, causing a double Sunrise, or even a double Sunset. The next opportunity for this to occur will be in December 2003.
This answer was updated in 2011.
See my books:
The Astronomy Cafe (1998) and
Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011
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