Thanks Pete, that's helpful. Though regarding #4, our Moon technically
orbits the Earth/Moon barycenter which is just inside the Earth's crust. A
good definition would be it would have to orbit a barycenter internal to
the host planet. An object can be a natural satellite for a brief period
of time. So where in our definition of 'moon' is a small rock that is part
of Saturn's ring system fail the definition of 'moon'?
Mark
On Tue, May 9, 2017 at 8:37 AM, Peter Turner <peteturner@xxxxxxxxxxx> wrote:
Thoughtco.com has a pretty good definition:
1. A moon orbits a body that orbits a star. (Otherwise planets would
be considered moons)
2. A moon is a distinct whole object and is solid.
3. A moon is a natural satellite of a body that orbits a star.
(Eliminating briefly captured objects and man-made objects)
4. Charon is not a moon of Pluto as it orbits a point somewhere
between Pluto and Charon. Otherwise Pluto could be said to be a moon of
Pluto. Instead it is propsed that they be defined as dwarf binary
companions.
That still leaves many objects open to question, but narrows the field
somewhat.
*Pete Turner*
*From:* pasmembers-bounce@xxxxxxxxxxxxx [mailto:pasmembers-bounce@
freelists.org] *On Behalf Of *Mark Johnston
*Sent:* Tuesday, May 9, 2017 8:01 AM
*To:* pasmembers@xxxxxxxxxxxxx
*Subject:* [pasmembers] moon definition
Hi gang,
I'm reaching out to you for help answering what should be a very basic
question: what is a moon?
The textbook answer is a moon is a natural satellite orbiting another
body. Ok...so that means every pebble and dust particle orbiting the Earth
is a moon? There does not seem to be any lower limit on the size of a
moon, nor does there appear to be any stipulation that the orbit be stable
or sustainable. A rock that is captured and does one lap before falling to
the Earth is, during that one lap, a moon apparently.
Does anyone have any reliable sources that limit 'moon' any further than
'natural satellite'??
Otherwise, I guess the Earth has at any given moment, several hundred
thousand moons.
Thanks
Mark
