Expert: Tom Whiting - 1/13/2010

Question
Its known to us that the moon takes 29.5 days to go around the earth which denotes that we experience both the eclipses during the synodic month when its a new moon and a full moon. Then how different is the solar and the lunar eclipse, please explain?? ( Also refer to the degree of moon's revolution around the earth)

Answer
Lunar eclipses can only occur at exact full moon, the other condition being that the moon
must be passing up through, or down through the ecliptic... to hit our Earth's shadow.
(The ecliptic is defined as the plane of the Solar System... a plane through the center
of the sun and the center of the Earth, which also includes the Earth's shadow. Since the moon's orbit around the Earth is tilted 5 degrees, eclipses can only occur when the moon is passing up through, or down through, the ecliptic plane. This occurs roughly twice during our calendar
year, so right now we get eclipses in June and December, called "eclipse seasons".
These seasons migrate very slowly. During the other months, the moon passes either above our
shadow or below our shadow... thus no eclipse.   

In most months, the full moon or new moon passes either above or below (north or south) our shadow, or in the case of a solar eclipse, the moon passes above or below the sun's disk... remember, space is 3 -dimensional.) So passing through the ecliptic plane at full moon and new moon is all important.  Actually, eclipses are simply celestial 'hide and seek'.

For solar eclipses, the moon must be exactly at new moon, and again, has to be crossing thru
the ecliptic, to hide the sun's disk, either partially, totally, or annular (ringed eclipse).
For a total solar eclipse, one other additional requirement is added... to have a large enough
moon to hide the entire solar disk, the moon must be at, or near, perigee (closest to Earth in
it's orbit).   Otherwise, you get an annular (ringed) eclipse, a bright ring of sunlight all
around the dark silhouetted new moon.  So that's why total solar eclipses only happen, on average, once every 18 months or so... somewhere on the Earth's surface.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA

FOLLOW UP:
For current eclipses, see
http://eclipse.gsfc.nasa.gov/eclipse.html
Notice how the eclipse seasons have migrated... in 2008 they were in February and August.
This year they are in January/December, and the June/July time frame.
By 2013, they will migrate to the April/May and October/November time frame.
tom