Lecture 5 C. Retrograde motion & Epicycles

Q. What is retrograde motion?

A: The apparent "backward" motion of a planet, as seen in the Earth's sky.

ignore difference in paths above for now: point is, planets usually move W to E, which we call "prograde" or "direct," then appear to move E to W, or "backward" from normal motion, before resuming W to E motion

  • the key word here is apparent -- in fact, NO actual backward motion occurs

  • this is most easily seen from the Sun -- remember that things really do orbit the Sun, because the Coperinican/Heliocentric picture is the correct one!

  • because planets always rotate W to E around the Sun, and everything orbits the Sun, the view from the Sun doesn't even show apparent retrograde motion!

  • (numbers in this figure are meant to correlate planets' positions in time; points 1-5 would all fall near or within points B-C-D in figure 1.11 in the text, on p. 27)

  • retrograde motion is just a trick of perspective, because we're on an orbiting planet looking at other orbiting planets

    On the other hand, from the WRONG geocentric viewpoint of Ptolemy, with everything orbiting Earth, one would believe any backward (E to W) motion to be actual, not just apparent!

  • so Ptolemy invented epicycles so planets would actually move forward sometimes & backwards others:

    ASIDE: Q: Hey, what's the difference between the two planet paths in this sections first picture?

    A: Only our perspective on them: