Why Eclipses Happen

Ron Hipschman

This coming August 11th, we will be treated to another celestial event in the unending dance of the
planets. On that day, the moon will move exactly between the earth and the sun. The moon's shadow
will fall across the earth, and a few lucky people in the right places will see a total solar eclipse. Those
of us not lucky enough to live in these places will travel thousands of miles to see this event. It's
certainly one of the most awe-inspiring alignments that can happen. Let's take a closer look at the
mechanics of solar eclipses, and see what makes the celestial clock tick.

Alignments

To see a total solar eclipse, you have to be in just the right spot on the earth. When you look up in the
sky at the sun and the moon, you notice a strange coincidence--both look the same size in the sky. Both
the sun and the moon look about one-half degree in diameter. Now, they're not really the same size. The
sun's diameter is actually 400 times the moon's diameter. But, you must also take into account that the
sun is also 400 times further away from the earth, reducing its apparent size to the same as the moon's.
Because of this relationship, when you are standing on the earth, looking up at the two, you must be in a
very limited zone to see the moon cover the entire face of the sun. If you were to move a little north, the
sun would peek out over the top of the moon; a little south, and the sun shines past the southern limb of
the moon. The match is so good that the "path of totality" is never more than 167 miles in diameter, and
is usually less. This means that very few people have seen a total eclipse because the shadow only
covers a very small area on the earth.

The earth and the moon are not fixed objects. The moon is busy orbiting the earth. The earth is busy
orbiting the sun and additionally rotating on its axis. This means that the spot on the earth where the
umbra falls is always in motion and actually traces out a path.

This diagram shows the path of the umbra for the eclipse on August 11th, 1999. Only the central lines
mark out the path of the umbra. The much wider area shows the path of the larger penumbra, where a
partial eclipse can be seen. (Note the percentages of total in the penumbral regions.) The shadow first
touches down near the South Western tip of Great Britain out in the Atlantic Ocean. It travels eastward andfirst sees land at the Scilly Islands, before it brushes past the end of Cornwall and on into continental Europe.

Continental landfall occurs in Northern France and the eclipse then rushes on skimming the Beneluxcountries and into Southern Germany, Austria, Hungary and then Romania before crossing the Black Seaand striking Turkey. The eclipse passes over the major cities of Stuttgart, Munich, Timisoara and Bucharest.After crossing the middle East and Northern India the eclipse finally leaves us in the Indian Ocean to waitfor the next total eclipse in West Africa, 2001.

Seen from the moon, the path of the eclipse will look like this. The large grey
circle is the penumbra from which only a partial eclipse will be seen; the small
circle in the center is the tiny umbra from which the total eclipse will be seen.
Note that the earth is rotating and that the stars seem to move behind the earth
because of the moon's revolution in its orbit.

Orbits

Total eclipses seem to happen infrequently. Why doesn't the moon get between the sun and the earth
every month at new moon and produce an eclipse? Because, I've over-simplified matters. The real
situation is a little more complicated. We need to discuss the orbit of the earth around the sun and the
orbit of the moon around the earth. The orbits of both are not circles, but rather slightly oval-shaped
ellipses. Also, these orbits do not lie parallel to each other in the same plane.

As the earth orbits the sun, taking one year to complete one circuit, it appears to us on earth that the sun
moves around oursky once against the background of stars. If you walk around a campfire (the sun)
looking at your friends on the other side (the stars), to you it would look like the campfire moves past
your friends. Likewise, from earth, it looks like the sun moves against the background of stars, making
one circuit of the sky in one year.

If the sun could draw a line as it moved against the stars, we would see a great
circle called the ecliptic. If we could ask the moon to also draw a line in the sky as
it orbited the earth, we'd notice that the two lines would be close to each other, but
the moon's path is tilted about 5 degrees to the path of the sun.

This is why the moon doesn't eclipse the sun every month. Most of the time, the moon passes over or
under the sun. An eclipse can happen only when both the sun and the moon arrive near one of the
crossing points (these are called nodes). There are two of these nodes on opposite sides of the sky, one
where the moon crosses from south to north, and one where the moon passes from north to south. Since
there are two crossing points in the sky, eclipses happen during two "eclipse seasons" separated by about
six months.

The sun does not have to be exactly on the node when the moon arrives there, only close enough for the
moon to block some portion of the sun. This leaves a "window" of about 18.75 days before and after the
sun gets to the nodes. During this 37.5-day period, the moon can cause an eclipse. Since the moon takes
29.5 days to go from one new moon to the next new moon, this means that an eclipse of some kind is guaranteed about every six months.

The type of eclipse that doesoccur depends on several things. First, if the eclipse happens when the sun
is further from the node, it is more likely that the eclipse will be a partial. In this type of eclipse, the dark
umbra passes above the North Pole or below the South Pole, never touching the earth. All we ever see is
part of the sun covered.

There's another variable, though. Remember that the orbits of the earth and moon are not perfect circles,

but rather ellipses. Note that in the diagram above, the earth is sometimes closer to the sun and
sometimes further. The same is true for the moon--sometimes it's closer to the earth and sometimes it's
further. See the table below:

Close distance
147,101,455 km
356,749 km

Far distance
152,098,155 km
406,282 km

Sun
Moon

As you can see, both the sun and the moon change their distances quite significantly. The moon changes
by about 14%, and we vary our distance to the sun by about 3%. Because of this, the sun and moon look
bigger sometimes and smaller at other times. If we're far from the sun so that it looks smaller, and close
to the moon so it looks bigger, the moon will be able to cover over the entire face of the sun as seen
from earth, and we'll see a total eclipse. If the opposite is true and we're close to the sun and far from the
moon, the moon will appear too small to cover the face of the sun. In this case, it's like trying to cover a
penny with a dime. You would see a ring of copper penny sticking out on all sides of the dime. This
happens with the sun amd moon. You see a ring of the sun shining around the edges of the moon. This is
called an "annular" eclipse (annular comes from the Latin annulus or ring). In an annular eclipse, you
don't get to see any of the "special effects" of a total eclipse, such as the corona, or diamond ring effect.
The thin sliver peeking around the moon is far too bright to allow this. More on this in another section
"What to see..."