What is really happening in space during the periods when we see Mars going through apparent retrograde motion?

Answer and explanation:

Most times, Mars appears to be moving left relative to the stars. However, in periods of "retrograde motion", Mars moves right relative to the stars. Truth is, that is just an illusion that happens every two years or so. What is really happening is Earth is passing by Mars in its orbit around the Sun. Earth is closer to the Sun and faster than Mars, overlapping the red planet about every 26 months. We usually see Mars moving in the sky from the west to the east but, when Earth is passing by it, we see it moving from the east to the west, giving the impression that its moving backwards.