🚀 go-pugleaf

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