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Jared Sherman wrote-

>
>
>It might help some of us if this was more clearly phrased. Parallax is a
>change in apparent position caused by a change in the observer's position.
>So "changing Moon parallax" means that "John changed the parallax of the
>moon by doing xyz" or perhaps it means "there are changes in the change of
>apparent position" which in turn is nearly meaningless gibberish.
>
>Surely I am not the only one to be lost by the inclarity of jargon here.

=======================

George replies-

That quotation appears to have been extracted from a posting of mine,
though Jared doesn't say so. I am sorry to have caused him problems in
understanding and will try to express myself more clearly. The concepts are
difficult enough to take in, without them being encumbered by sloppy
writing. But there's a conflict all the time between brevity and clarity.

I will try to clarify the matter.

Parallax is, as Jared says, a change in the apparent position caused by a
change in the observer's position. It could indeed be the consequence of
John changing it: "John changed the parallax of the Moon by travelling from
A to B", but in the circumstances I was referring to it was the consequence
of John travelling round with the Earth's surface as the Earth rotated,
affecting the apparent position of the Moon. So the "changing" could be
active changing (by John) or passive changing (in which he played no part),
which perhaps Jared was trying to distinguish between.

Stellar parallax, by the way, is the result of the observer's position
changing by riding on the Earth in its annual orbit around the Sun;
similar, but different..

On the other hand, says Jared, "perhaps it means "there are changes in the
change of apparent position" which in turn is nearly meaningless
gibberish."

Well, that phrase was Jared's invention, so if it's "meaningless gibberish"
he only has himself to blame. It's not far off, though. If he had said
"there are changes in the change of true position", he would have been
closer.

Think about it. Parallax displaces the apparent position from the true
position (of the Moon). Parallax changes throughout the day, as a result of
the observer riding round the Earth's surface. So the displacement of the
apparent Moon from the true Moon also changes throughout the day. This is
the effect of changing Moon parallax. Is that clearer, Jared? And if so,
please suggest how you would express it.

George.




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