What is a "Solar Eclipse"?

19 June 2014

The Short Answer (TSA)

            “Eclipse” is an ancient Greek word meaning "the darkening of a heavenly body."  Two things must happen to produce a solar eclipse.  First, the Moon must move in between the Earth and the Sun.  Second, the Earth must move exactly into the shadow cast by the Moon.

            Solar eclipses don’t happen that often, but the Moon moves in between the Earth and Sun every month.  In fact, the monthly “New Moon” is the exact moment when the Moon is, more or less, in between the Earth and the Sun.  But if there’s a “New Moon” every month, why isn’t there a solar eclipse every month?

            There are two reasons.  First, the Moon casts a very small shadow and the Earth is a small target.  Second, the orbit of the Earth around the Sun and the orbit of the Moon around the Earth are not exactly “level.” 

            Before things get technical, let’s put it this way.  The Sun is too bright to give you a clear view.  But let’s say you could watch the Moon passing by the Sun every month at the “New Moon.”  Most often, you’d see the Moon pass over or under the Sun.  Once in a great while, you’d see the Moon hit the “target” exactly and pass in front of the Sun.  And, when the Moon did hit the target, you’d see a solar eclipse. 

            So, with most “New Moons,” the Earth, the Moon, or both are a bit above or below the “level” of the Sun.   Because the Moon casts such a small shadow and Earth is such a small target, just the slightest difference in angle will cause the shadow of the Moon to miss the Earth.

 Why Isn't There a Solar Eclipse Every Month?

            Again, once in a while, everything will line up, and you’ll see a solar eclipse.  And what will you see?   You’ll see part of the bright light of the Sun suddenly dim as a shadow moves across the Sun’s face.  The Moon isn’t big enough to block all the Sun’s light.  So, even when there is a full solar eclipse, the sky doesn’t become completely dark.  You can still see the bright outline of the Sun around the edges of the shadow of the dark Moon as it moves across the bright Sun.  

            In fact, because the Moon is so small, its shadow doesn’t cover the whole Earth.  That’s why each solar eclipse can “be seen” from some places on Earth, but not from others.   From some locations, you will see a full or “total eclipse” with the round shadow of the Moon visible against the Sun.  But, from other places, you’ll see a “partial eclipse.”    The Moon, in a partial eclipse, doesn’t pass directly over the Sun.  Instead the Moon’s shadow will “clip” the bright face of the Sun and cover only part of the top or bottom of the solar disk.  In other places, you might not see “any” eclipse at all.    

 Solar Eclipse

M Grossmann of Hazelwood, Missouri
(& Belleville, Illinois)
About the Author

19 June 2014