On Wednesday, May 16, 2018 at 11:44:31 AM UTC-6, Paul Schlyter wrote:
On Tue, 15 May 2018 21:26:37 -0700 (PDT), Quadibloc
wrote:
Mercury, on the other hand, takes exactly *one* year to make a
complete circle
of the Zodiac, as seen from the Earth - because it never gets very
far from the
Sun as we see it from our vantage point.

The retrograde loops in that one year circle around the Earth...
reflect
Mercury's actual motion around the Sun.

If so, why is the number of retrograde loops of Mercury as seen from
the Earth during one Earth year one less than the number of
revolutions of Mercury around the Sun during the same time period?

Well, that is one of the nuances. The retrograde loops, if the Sun weren't quite
so bright, could be visually appreciated as our seeing Mercury revolving around
the Sun just as we see Jupiter's moons revolving around it.

But our viewpoint of the Sun keeps changing as we orbit the Sun itself. That
causes the Sun's apparent motion in the sky, and it also changes how we view the
Mercury-Sun system. And, yes, that subtracts one loop. (And ignoring this
particular difference, of course, does belong to the family of misconception
that Oriel is famous for.)

So I suppose you could say that Oriel is wrong because the retrograde loops are
not absolutely pure Mercury motion, devoid of influence from the Earth's motion.
But I see his current remarks on this issue merely as saying what is true: in
the case of an inferior planet, the loops reflect the planet's motion, and the
general outer cycle is due to our orbit - and with a superior planet, it's the
other way around.

That basic fact is true, but, yes, this nuance is definitely something we could
expect him to get wrong. I didn't really see him as getting that far yet.

John Savard