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Old May 16th 18, 05:26 AM posted to sci.astro.amateur
Quadibloc
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Default Illusory loops vs actual loops.

On Tuesday, May 15, 2018 at 2:45:07 PM UTC-6, palsing wrote:
On Tuesday, May 15, 2018 at 11:18:57 AM UTC-7, Gerald Kelleher wrote:


The faster moving Venus and Mercury and their direct/retrograde motions are entirely different as they process a real and actual loop around the parent
Sun hence their loops are not illusory -


This claim of yours, of course, is incorrect. Sure, Mercury and Venus make
loops in the evening and/or morning sky "with respect to the Sun" over the
course of weeks (Mercury) or months (Venus), but nevertheless, they ALSO make
'apparent' loops and zig-zags with respect to the background stars, just like
the superior planets! See this page...


As I've been trying to explain, even if Oriel isn't getting the terminology or
the nuances quite right, what he is expressing here is the actual astronomical
truth of the matter.

Jupiter takes 12 years to make a complete circle of the Zodiac, as seen from the
Earth - and, as seen from the Earth, it loops around due to retrograde motion
multiple times during that period.

Jupiter's orbit around the Sun has a 12 year period - so the overall trend of
its apparent motion is "real", due to its orbit around the Sun - and the
retrograde loops are "illusions", a result of the Earth's orbit around the Sun
changing our point of view.

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.

So in this case, Mercury's real motion - its orbit around the Sun - creates the
loops in its apparent path from the Earth... and the illusory motion caused by
the change in our point of view by the Earth's motion causes the big overall
circle in which the loops are situated.

So indeed the apparent motion of the inferior planets is, in a sense, "inside-
out" compared to that of the superior planets. Pretty much exactly as Oriel is
trying to say.

He may not be getting some nuances right, or some of the terminology right, but
here he is just expressing a basic truth. As if it were a new discovery on his
part, rather than something so well known as to be utterly trite... although,
given the reaction to what he is saying here, maybe that impression is less
wrong than I think.

John Savard