On Thursday, March 8, 2018 at 10:01:02 AM UTC-7, wrote:
By contrast, Venus orbits the Sun in 224 days, but does not return to its
greatest Eastern elongation for 584 days because of the Earth's orbital motion.

That's certainly a good point.

The contrast isn't so striking in the case of Mercury, of course.

If you look at a diagram of the apparent motion of Mars or Jupiter, you see a
circular trail of loops. The circle part - if plotted against the starry
background - has the same period as the actual orbits of those planets.

A diagram of the apparent motion of Venus or Mercury is also a circular trail of
loops. But now the general circle moves through the zodiac... once a year, along
with the Sun. So the *loops*, which lead to retrogrades, are due to Venus and
Mercury orbiting the Sun, and the overall motion is due to the Earth's motion
changing our viewpoint. (How could it be otherwise? Venus and Mercury are always
on the same side of the Earth as the Sun, so their orbital motion *can't* take
them through a full circle of the Zodiac from our point of view by itself.)

That is a fact. I'm not saying that you have to accept that Oriel is right that
this is an important fundamental distinction between retrogrades of inferior and
superior planets. But there is the old saying that one must give even the Devil
his due; and that he has managed to get one thing right, for a change, should be
cause for celebration.

John Savard