Comets at perihelion before expected?

"nitelite" wrote in message
lkaboutscience.com...
| Hello, Everybody:
|
| I'm interested in gravitational anomalies, and thus I found the paper "Is
| the physics within the Solar system really understood?" by Laemmerzahl,
| Preuss and Dittus (2006) most interesting as well (arXiv:gr-qc/0604052
| v1).
|
| I have a question about a point raised in Section 8.2, page 14 of the
| arXiv.org printing. In that paragraph, the authors write: "Comets usually
| come back a few days before they are expected .... The delay usually is
| assigned to the outgassing of these objects .... also leads to the effect
| that comets come back a few days earlier."
|
| They say that comets return earlier than expected, but they also suggest
| (by using the word "delay" twice) that they actually come back later than
| expected. Can anyone here clarify this for me. Do comets come back to
| perihelion earlier or later, or is it unpredictable?

Than "expected"... what does that mean?
a) WHEN you expected to observe it?
b) WHERE you expected to observe it?

Let's make the (generally invalid) assumption that comet perihelion is
closer to the sun than Earth's orbit (obviously not true for Pluto,
recently downgraded to "not a planet").

There is no preferred orbit that all comets must take, and certainly not
in the plane of the planets. Thus if the Earth is on the same side of
the sun as perihelion of the comet's orbit, then closest approach is
only seen in daylight, whereas if perihelion is on the opposite side
of the sun then the comet MAY dissappear (but not necessarily)
behind the sun. Seeing comets in daylight is kinda tough to do,
and Venus is known as the evening or morning star. Mercuy
is never far from the sun either, you won't see that at night.

So let us suppose the comet went behind the sun for (say) three
days. During those three days the Earth itself has moved 3 degrees
in its own orbit, and so a comet will reappear "late" if the orbit
has the same counter-clockwise direction as the Earth and "early"
if it has a clockwise direction. where late an early refer to postion
against the "fixed" backround of the further stars.


| I've contacted the authors of this paper for clarification, but they have
| not yet replied. Can anyone here help me out in the interim?
|
| I've read Page, Dixon and Wallin's paper "Can minor planets be used to
| assess gravity in the outer solar system?" (arXiv:astro-ph/0504367), which
| the authors cite [44] in which they say in Section 2.1, page 6, that
| individual comets sometimes return earlier and sometimes later, due
| primarily to outgassing and other forces discussed by Marsden, Sekanina &
| Yeomans (1973). They write that these are statistical forces that apply
| over many returns and cannot be used to predict the time of a comet's
| perihelion return on any single orbit.
|
| I'd be very grateful if anyone could direct me to sources that confirm
| that specific (or all) comets return "early."

It simply isn't true.


| I would like to see data on
| that subject as I am doing some research on precession.

Egads... Halley's comet takes 76 years to return...
Try this:
http://faculty.ifmo.ru/butikov/Proje...llection1.html
If you want to argue General Relativity, then
http://www.schulphysik.de/physik/perihel/Perihel.htm
where Venus, Earth and Jupiter are simply missing from
the calculation.
Androcles



|
| Many thanks!
|
| Andrew
|