Does Mercury rotate?

It is one of the hardest planets to spot in the sky because it never strays more than a few dozen degrees from the Sun. It often gets lost in the clutter of the eastern or western horizon. In 1889, astronomer Giovanni Schiaparelli's (1835-1910) telescopic observations of dark features on the planet's surface seemed to prove that its day was 88 Earth-days long, and that is exactly what textbooks continued to say until around 1960. By then, radar pulses reflected from the surface were successfully detected, which led Cornell University astronomers Gordon Pettengill and Rolf Dyce to conclude in 1965 that Mercury rotated once every 59.3 give or take two days. This has been steadily improved to the current value of 58 days and 15.6 hours, give or take 15 minutes.

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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