To envision haw a solar eclipse works it is
best to first have an idea about the motions of
the earth and moon in the solar system.
First off, the earth orbits the sun once every
year. From above the North Pole of the earth, the
earth moves on a counter clockwise orbit about the
sun. The earth’s orbit around the sun is confined
to a plane. This plane is called the
ecliptic. If you envision a basketball
(diameter =10 inches) as the sun, the earth is 0.1
inch and the earth travels in the plane defined
by the center of the sun and Earth.
Now, the moon travels along with the earth in
an orbit around the sun but while doing that, the
moon also orbits the earth itself. So if you draw
a simple picture you will see that there will be
numerous times when the moon gets between the sun
and the earth. That is we might expect each MONTH
to have the moon come in front of the earth and
block our view of the sun!
But wait, there is something I did not mention.
The orbit of the moon around the earth is NOT
in the same plane as the orbit of the earth around
the sun. The moon is about 5 degrees out of
the plane of the ecliptic. This means that instead
of having a solar eclipse every month (how boring)
we have them more infrequently only at those times
when the moon is in front of the sun AND when
the two orbital planes - the earth around the sun
and the moon around the earth intersect.
In order to visualize this take two pieces of
paper… partially tear one and insert the torn
piece into the other non-torn piece. You will see
that if one body, say Earth is confined to its
orbit (the ecliptic) whereas the other is the
orbit of the moon INCLINED with respect to the
ecliptic, then there are restricted number of
times when both the SUN, Earth and Moon are co
linear!!! It is precisely these times when a
solar eclipse can occur.
I hope this picture helps you visualize the