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In article <1993Jan21.230333.46626@kuhub.cc.ukans.edu> christos@kuhub.cc.ukans.edu writes:
>Hello there,
>
>	I have a question concerning the solar eclipse.
>1. Is the moon rotating around the earth in a single plane sothat every 29 days
>it comes in between the sun and the earth? If that is the case different places
>of the earth should have solar eclipse every 29 days(period of the ratatioon of
>the moon around the earth).
>
>	I would appreciate any relevant response..
>
>Feel free to send mail to 
>Internet Address: RSL11@kuhub.cc.ukans.edu

Since the moon is not in the Earth's orbital plane, we don't get eclipses
every 29 days.  I don't even think the moon is in the Earth's equatorial
plane, otherwise we would get eclipses only at or very near equinoxes.
Perhaps someone will find for us the moon's inclination to the equatorial
plane in the Astro. Almanac?.
-- 
Stephen Leroy                                | Disclaimer:  Caltech knows
Division of Geological and Planetary Science | better than to take
California Institute of Technology           | responsibility for what I
leroy@(cluster,satur1).gps.caltech.edu       | may claim.

In article <1993Jan21.230333.46626@kuhub.cc.ukans.edu> christos@kuhub.cc.ukans.edu writes:
>
>1. Is the moon rotating around the earth in a single plane sothat every 29 days
>it comes in between the sun and the earth? If that is the case different places
>of the earth should have solar eclipse every 29 days(period of the ratatioon of
>the moon around the earth).
>

The plane of the Moon's orbit about the Earth is inclined by about 5 degrees
to the plane of the Earth's orbit around the Sun. (The latter is called the
plane of the ecliptic.)  The intersection of these two planes is called
the "line of nodes".  An eclipse of the Sun or of the Moon can only occur
when the line between the Sun and the Earth coincides with the line of nodes.
This occurs twice a year and those times are called "eclipse seasons".  
One season is for lunar eclipses and the other for solar eclipses.
The eclipse seasons are not exactly six months apart, as you would expect,
because the orbit of the Moon is slowly precessing with respect to the
fixed stars.  The inclination of the Moon's orbit stays constant, but
the direction of the line of nodes in space is rotating with a period of
18.6 years.  Because of this, the eclipse seasons occur about 173 days
apart rather than 182 days apart (365/2).  This is called "regression of
nodes".  But eclipses do not occur twice a year because the Moon is not
always at, or even near to, one of the nodes during the eclipse seasons.
The 29.5 day period of the Moon in the Earth's sky is independent of 
the periods of the eclipse seasons, so an eclipse can only happen when
they coincide.  Actually, because the Earth and Moon are not point
objects, and their shadows have decent cross sections, some slop is
allowed in how close the line of nodes, the Earth-Sun line, and the
position of the moon in its orbit in order for an eclipse (partial or
total, solar or lunar) to occur.

					Frank Eparvier
					University of Colorado-Boulder
					Laboratory for Atmospheric
					  and Space Physics
					eparvier@sertan.colorado.edu

In article <1993Jan24.184850.7369@ucsu.Colorado.EDU> eparvier@rintintin.Colorado.EDU (EPARVIER FRANCIS G) writes:
>
>[...]  The inclination of the Moon's orbit stays constant...
>
Very Minor Nit:  For purposes of the eclipse discussion, this statement is 
fine.  However, FYI the Moon's inclination isn't truly constant (hah, 
nothing ever is).  The component of the gravitational force due to the Sun 
perpendicular to the Moon's orbital plane causes the Moon's inclination to 
vary.  The largest term has a period of half an anomalistic year (perihelion 
to perihelion) and an amplitude of 8.7 arc minutes.  
---------------------------------------------------------------------
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In article <1jp9fbINN9nj@gap.caltech.edu> leroy@cluster.gps.caltech.edu (Stephen Leroy) writes:
>In article <1993Jan21.230333.46626@kuhub.cc.ukans.edu> christos@kuhub.cc.ukans.edu writes:

>>1. Is the moon rotating around the earth in a single plane sothat every 29 days
>>it comes in between the sun and the earth?

>Since the moon is not in the Earth's orbital plane, we don't get eclipses
>every 29 days.

<much deleted above.>

This brings up a point that is mentioned in Wagner's "Introduction to
the Solar System" (p. 208) that has been nagging me. He states 

	"Unlike most planetary satellites, the Moon does not orbit in
its planet's equatorial plane. Instead, it orbits nearly in the
ecliptic plane, with an orbital inclination of 5.15 deg. Various
pertubations cause this value to vary between 4.95 and 5.33 deg. The
fact that the Moon does not orbit in the Earth's equatorial plane may
be a clue to its origin, indicating that it is a captured object."

	I have not seen any such statement elsewhere, though it is the
kind of thing that might be buried in Hartmann and I missed. My
questions are:

1. Is the geometry described true?

2. Has anyone speculated on the relation to the origin of the
Moon?

3. Are there other satellites orbits similarly oriented?


Joseph Cain		cain@geomag.gly.fsu.edu   
cain@fsu.bitnet		scri::cain
(904) 644-4014		FAX (904) 644-4214 or -0098