2
$\begingroup$

This is more a terminology question, since Mercury and Venus certainly "move backwards" in the sky. The question is whether we call this retrograde motion. I'm referring to apparent retrograde motion here, not retrograde rotation.

I'm under the impression we don't, and reserve that name only for planets that are further away from the Sun than us; however conclusive evidence for whether this is (or isn't) the widely-accepted meaning for "retrograde motion" is hard to find.

$\endgroup$
2
  • $\begingroup$ Sure, since the notion of retrograde motion pre-dates the Copernican revolution. $\endgroup$
    – PM 2Ring
    Jan 23, 2019 at 21:12
  • $\begingroup$ Possible duplicate of astronomy.stackexchange.com/questions/9924/… $\endgroup$
    – user21
    Jan 24, 2019 at 14:36

1 Answer 1

2
$\begingroup$

As noted by PM2Ring, the notion of "retrograde motion" predates the "Copernican revolution" So you should think about the motions of the planets from a Ptolemaic view:

The planets have two major motions (in addition to the diurnal motion around the Earth) motion with a period of one year, and a second motion with a period that is specific to each planet. For Mars, Jupiter and Saturn, the main orbit has a long period, and the epicycle has a period of one year. For Mercury and Venus, the main orbit has a period of a year (keeping them aligned with the sun) and the epicycle has a period of less than a year.

When the right ascension of the planet is decreasing, then we say it's motion is retrograde. Retrograde periods occur during inferior conjunctions of the planet with the sun, and so the planet cannot be easily observed while it is retrograde.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .