On Tuesday, November 13, 2018 at 4:52:55 PM UTC-7, palsing wrote:
He still denies that the inferior planets perform apparent retrograde loops,
despite overwhelming evidence otherwise.

You are right that the apparent path of an interior planet also includes
retrogade loops, just like the apparent path of a superior planet.

I don't argue with that, and your image certainly proves that.

Where my dispute is that from the way I understand what he is saying, he isn't
denying that loops are a feature of the apparent path of an inferior planet as
seen in the sky.

It is true that he is not applying the term "apparent" to the loops, he is
calling them "real". But conventionally, when the apparent path of a superior
planet is described, it is often the retrogade loops specifically that are
called "apparent", not the planet's overall progress through the Zodiac.

If you go back to Ptolemy, the distinction that I think is being made becomes
explicit.

In the case of Jupiter, the _cycle_ has a period of about 12 years, while the
_epicycle_ has a period of 365 1/4 days.

In the case of the inferior planet Mercury, though, the _cycle_ has a period of
365 1/4 days, and the _epicycle_ has a period of 88 days.

That's what I mean when I say the "overall orbit" of an inferior planet is the
illusion - like the retrogade loops in the apparent path of a superior planet -
while the "retrogade loops" in the apparent path of an inferior planet are *not*
an illusion caused by the Earth's motion around the Sun, they reflect the
planet's real motion arount the Sun.

This is what he is trying to say. And since he isn't right that often, I think
giving him credit when he does get something right for once is important.

John Savard