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Hello Richard,
The only time we can have a solar eclipse is during
"new moon" when the moon is between the Earth and the
Sun. And the only time we can have a lunar eclipse
is during "full moon" when the EArth is between the
Sun and the moon. However, as you point out, we
don't have eclipses twice a month. The reason is that
the orbital plane of the Earth-moon system is tipped
slightly (the moon's orbit is inclined about 5 degrees relative
to the the Earth's). That may seem like a small inclination,
but when you consider the geometry together with the
separations of the Sun, Earth and Moon and the fact that
everything has to be perfectly lined up for a total
eclips to occur, it turns out that 5 degrees is enough
to make a big difference.
You asked a question that is much easier to answer in
person - when I can draw a picture - but let me try!
Let your head be the Earth. You are standing on Mt. Nose,
looking out toward the heavens. Now use a hula-hoop encircling
your head to simulate the moons orbital path (an imaginary
hula-hoop is fine!). We know the orbit is tipped - so
tip the hoop up on the side of Mt Nose (then it will be
down on the back side of your head/Earth). Now when you
look toward a bright light source (the sun) you will always
see it - there will never be an eclipse. Indeed if the
Earth-Moon orbit maintained this steady orbit, then eclipses
would never be observed on Mt. Nose - ore anywhere on your
head/earth (which spins inside of the hoop, but the position of
the sun relative to the orbit/hoop is unchanged).
However, the orbital plane doesn't keep this orientation.
Instead, it precesses (the motion a top makes in addition
to spinning) so that sometimes the tilt is a maximum over
one ear and a minimum over the other ear. At this time, the
orbital path crosses down over Mt. Nose and it is possible
to see an eclipse. As you might imagine, lunar eclipses
and solar eclipses are going to be paired. But solar
eclipses are most often partial eclipses with relatively
small projections onto the Earth - so solar eclipses are
generally visible from limited geographic swaths. The
larger shadow of the earth onto the moon means that lunar
eclipses will be total as viewed from a much larger part
of the Earth.
One of my favorite astronomical almanacs is by Guy Ottewell.
I order one every fall (I know they are available from
the Astronomical Society of the Pacific in Fall each year -
they have an online catalog at www.aspsky.org) and you might
enjoy this wealth of very accessible information.
You asked how one can predict when an eclips occurs.
To do this you have to know the orbits of the Earth and
moon precisely. I just look them up in an almanac.
But you can have several lunar eclipses a year when the
orbital plane is aligned favorably. However most of the
corresponding solar eclipses willnot cast a shadow onto
the earth. Total solar eclipses (which are still only
visible from a small swath of the earth) only occur on
average once every one and one half years.
I hope this helps to address your question!
With best regards,
debra Fsicher
(or if I untwist my fingers, Debra Fischer)
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