Thursday June 28th the ‘Red Planet’ Mars begins its retrograde motion. Retrograde motion is the apparent backward, or westward motion that a planet will appear to make at a regular point in its revolution around the Sun. This happens when the faster orbiting Earth catches up and passes by an outer planet. As this is happening the outer planet appears to slow down and then reverse its orbital direction toward the west. After a period of time (days to months) the outer planet again appears to slow down and then return to its regular eastward, or direct motion.
Mars ends its retrograde motion and resumes its regular direct or eastward motion on August 28th as it moves across the constellation of Capricornus the Sea Goat.
An Inner Planet Orbit
We typically think of retrograde motion as being done by an outer planet from the Earth. However the two inner planets Mercury and Venus also undergo retrograde motion. Approximately one-half of their respective orbits is eastward as is with all the other planets. This then brings the inner planets to what is know as eastern elongation in the evening skies. The other half of the orbit for Mercury and Venus is toward the west as they move from eastern elongation through inferior conjunction toward western elongation in the morning skies.
Coordinate Positions for Mars
Date Right Ascension h m June 25 20 51 July 5 20 50 July 15 20 45 July 25 20 35 August 4 20 23 August 14 20 14 August 24 20 08 September 3 20 09