So, do Mars opposition and retrograde motions happen once a year as the Earth returns to its position?
Ah, no. If Mars were in opposition tonight, one year from now the Earth would have returned here but Mars would have moved on. Now Earth has to play catch up and, of course, Mars is moving along too. These two motions cause Mars oppositions to occur every 780 days. Mars underwent retrograde motion in the year 1999 and again in 2001 but Mars did not retrograde in 2000. You will recall from last month’s lesson that a synodic period is the time between successive returns to the same position relative to the Sun as seen from Earth. That’s what really matters in observational astronomy – synodic period. You learned about synodic and sidereal periods last month when I taught you about the Moon’s motion around the Earth and the motion of the Earth-Moon system around the Sun. Let’s recall some ideas from last month’s lesson and apply them to planets. All the planets orbit the Sun at different speeds so they all have different sidereal periods (the time to complete one orbit of the Sun and retu
