Re: North Pole

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

John Clement wrote:

> The moon's orbital plane being out of the ecliptic does not say that the
> earth-moon's orbital plane is out of the ecliptic.  It implies that the
> earth's orbit bobs up and down with respect to the ecliptic because the
> moon's period is shorter than the earth-moon's period.  The CM will not show
> such bobbing, and the statement does not tell one whether or not the earth's
> orbit is completely in the ecliptic.
>
> However once could deduce from this that the moon's orbit precesses.

An even easier way to deduce that the moon's orbit precesses is
simply to notice that eclipses don't always occur in March and
September (or some other pair of months separated by a half year.) If
the moon's orbit did not precess we *would* have two very well known
"eclipse months" each year.  In fact the moon's orbit has a
precession period of just 18.6 years so that "eclipse seasons" are
separated by somewhat less than 6 months.

--
John Mallinckrodt        mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.