Re: [tuning] Digest Number 1058

Hi Paul,

> >However, astronomers get a much more accurate value for a planet's
> >orbit once it completes a full cycle.

> No, I don't know why you're assuming that. Nothing special happens when the
> cycle is completed.

Sorry, don't have anything to back that up. Thought it was true, but on reflection,
don't know for sure. Feel that I've read / heard it - I've read a fair number of
books and articles on this subject, but one doesn't recall what exactly one read where.

I'll let you know if I come across it again, whatever it was, or find out more.

You could be right, certainly don't know anything special.

In fact, also, if one observes a number of orbits, one can refine the
calculations even further. Galileo made an early pre-discovery observation
of Neptune when it was passing really close to Jupiter, in the same field.

Of course, he didn't know what it was, but noted it down, and observed that
it seemed to change position relative to the fixed stars next time he saw
it. But then he lost it because the only way he had to locate it was in
relation to Jupiter, and it was no longer in the same field of view, so he
never knew what he had seen.

If we knew how to interpret his drawing, it would give a more
accurate figure for Neptune's orbit (or would have when the article about
it was published, not sure if still true). Though his observations were
comparatively crude, they were still rather good because of a special method
he used to find the relative separation of the satellites from jupiter
compared to jupiter's diameter. Good enough to be usefule because of
the longn time-line.

However it has a kind of dashed line or something on it, and nobody knows
for sure what he meant exactly. I think there was an article about this observation
in Scientific American a number of years back, and his drawings were published.
(can't see anything about it on the web except a mention of it with dates here:
http://www.h2g2.com/A386598)

Maybe it is just a kind of rule of thumb that once a planet or asteroid has
completed an orbit, one has a large enough arc to have a pretty good idea of
its period.

> >Would make sense for the asteroid belt, where asteroids are cleared out of
> >(some) of the resonances - the Kirkwood gaps,

> Exactly, and also the material in the rings of Saturn.

Yes, and these are also kept into very narrow rings as well, by "shepherd satellites"
to either side of the ringlet.

> Thanks for the links, they're very informative.

Good!

N.b. here is a site I just found which is rather fun: vanished planets, ones that were
thought
to exist, but then found to be a mistake:
http://www.solarviews.com/eng/hypothet.htm

Robert.