Roughly speaking, how often does Jupiter, which is about 5 A.U. from the Sun, undergo retrograde motion?
A) about once every 12 years.
B) about once a year.
C) about once a month.
D) about once a week
Retrograde means that a planet appears to be moving "backward" in the sky, that is, backwards relative to its normal apparent motion around the sun (all planets orbit in the same direction).
Because all the planets orbit in the same direction, and because planets closer to the sun orbit faster than the outer ones, something unusual happens when an inner planet, like the Earth, "laps" an outer planet. Due to the motion of both planets, and the changing angle between them, the outer planet appears to move backwards for a short period of several days to months.
Thus, the correct answer is B - about once a year. Each time the Earth completes a year, it "laps" Jupiter, and produces the apparent retrograde effect.