NASA webpage Mars retrograde it has resumed prograde motion across the sky reverse its trajectory for a little while move backwards http://mars.nasa.gov/allaboutmars/nightsky/retrograde/ “every two years or so,” retrograde movement http://mars.nasa.gov/allaboutmars/nightsky/retrograde/ reverse its trajectory for a little while Mars Retrograde animation NASA Mars Retrograde Marte Mars retrograde Mars in our night sky retrograde simulation of Mars http://mars.nasa.gov/allaboutmars/nightsky/retrograde/ every two years Mars Retrograde every two years