Author:  Joseph Lazio <jlazio@patriot.net>

A solar eclipse occurs when the Moon passes between the Earth and Sun
and the Moon's shadow crosses the Earth, viz. (not to scale!)

       Sun                            Moon  Earth

Solar eclipses can be total, partial, or annular.  A total eclipse is
when the Moon obscures the Sun entirely.  A partial eclipse is when
the Moon only covers a portion of the Sun.  Because the Moon's orbit
about the Earth is not perfectly circular, sometimes it is slightly
farther away from the Earth.  If a solar eclipse occurs when the Moon
is at the far point in its orbit, the Moon will not cover the Sun
entirely.  A thin ring, or annulus, of sunlight will be visible around
the Moon.  This kind of eclipse is called an annular eclipse.

**Solar eclipses can be damaging to one's eyesight, unless proper
precautions are taken!** See FAQ Question B.11 and the Eclipse Home
Page, <URL:http://sunearth.gsfc.nasa.gov/eclipse/>.

A lunar eclipse occurs when the Earth passes between the Moon and Sun,
viz. (again, not to scale)

       Sun                            Earth  Moon

Lunar eclipses are either total or partial, depending upon whether the
Moon moves completely into the Earth's shadow or not.  Lunar eclipses
are always safe to view.

Eclipses do not happen once a month because the Earth's orbit about
the Sun and the Moon's orbit about the Earth are not in the same
plane.  The above "pictures" are if one is looking "down" on the Earth
from the North Pole (or "up" on the South Pole).  If we look at the
system from the side (looking at the Earth's equator), the typical
situation is 

       Sun                            Earth  
                                             Moon

(with the angle shown exaggerated greatly, the actual angle is about 5
degrees).  Only when the three bodies are in the same plane can an
eclipse occur.  The total number of eclipses, both lunar and solar,
never exceeds seven in a year.  Because the Moon is so much smaller
than the Earth, and casts a smaller shadow, solar eclipses are more
infrequent than lunar eclipses; in a year, between 2 to 4 lunar
eclipses will occur and at least 2 solar eclipses will occur.  *Total*
solar eclipses happen only every 1.5 years or so.

For additional information see the Eclipse Home Page,
<URL:http://sunearth.gsfc.nasa.gov/eclipse/>.