From: Amara Graps (amara@amara.com)
Date: Mon Dec 16 2002 - 03:22:42 MST
chris hibbert:
>A good friend who knows of my interest in Cruithne is Chief Scientist
>at the Exploratorium. (http://www.exo.net/~pauld/) He was attending
>the American Geophysical Union meeting last week
>(http://agu.org/meetings/fm02/ Apparently Cruithne counts as Geophysics.)
No, it counts as a planetary object for a planetary science meeting.
The AGU is a good planetary science meeting as well as a good
meeting for solar physicists. The AGU and the DPS (division of
the planetary sciences of the American Astronomical Society)
are normal meetings for planetary scientists to attend (this Spring
it will be in Nice, France, co-sponsored with the European counterpart:
the European Geophysical Society: EGS). It's an extremely
large meeting- not my favorite because there are about 10,000 people
there, and it's sometimes hard to meet the people that you want
to talk with. The advantage is that you can mingle in fields outside
of your own- I usually attend a lot of time-frequency methods
talks when I'm at the AGU, because the geophysicists are the most
expert with those techniques.
>{My friend said he talked to the people who are into Cruithne, and
>the "plausible" justification for calling it a moon is that Earth
>isn't in the gap of the horseshoe orbit that Cruithne follows. (!)
This object is an asteroid. (Yes I know about how horseshoe
orbits look)
Cruithne moves about the Sun, the minimum distance to the Earth is
40 lunar orbital radii, *well outside the Hill's sphere*. This
distinguishing characteristic is well known in celestial mechanics
and astronomy, it is the surface of zero velocity, based on gravity
and used to distinguish between satellites and independent solar
system bodies.
I am not keen on your friend's explanation. It is attempting to
explain this object with how its orbit "looks" (say, epicycles?),
rather than the physics that govern the objects actions (gravity).
I couldn't find alot on the Web about Hill spheres (starting
with the three body problem might be better)- you need Danby or
some other standard Celestial Mechanics text for a good explanation
of Hill spheres. But here's something:
Some Formula
http://www.dausha.net/sciences/orbits.shtml
Stability Zones
http://apollo.cnuce.cnr.it/~rossi/publications/napoli/node2.html
Amara
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