Mercury Retrograde Motion

Amidst all the angst over fearsome Friday of June 13, 2014 (full moon on a Friday the 13th), an RASC colleague mentioned on our email list that there was additional concern being expressed around this all happening while Mercury was in retrograde motion.  Well, its harmless, it happens all the time, but it still was fodder for yet another animation.  As the Earth orbits the Sun, the Sun and the planet appear to drift across the stars from West to East, or right to left in the animation.  But, as Mercury races along in orbit passing between the Sun and us, its orbital motion overtakes that movement against the stars, and for a while it travels East to West, or left to right. This reversal in direction of travel against the stars is called retrograde motion.