Why Eclipses Happen Continued

 

Image: Moon's shadow on Earth
Seen from the moon, the moon's shadow looked like this when we were in total eclipse in Zambia, June 21, 2001. The large gray circle is the penumbra from which only a partial eclipse was seen; the small circle in the center is the tiny umbra from which the total eclipse was seen.

See a Quicktime movie of the moon's shadow on the earth as it traces out this eclipse:
View of the whole globe 1.9MB
Close up of Africa 2.6 MB

(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.) This is seen as if viewed from the moon. Just by coincidencen the beautiful Trifid Nebula is in our view!

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 oversimplified 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 our sky once against the background of stars. Here's an analogy--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 seeImage: Sun & Moon cross paths 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.

Diagram: Ecliptic

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 new moon to new moon, this means that an eclipse of some kind is guaranteed about every six months.

 The type of eclipse that does occur 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 one. 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.

Diagram: Earth & Moon orbits

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 (again wildly out of scale), 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

Far Distance

 Sun

147,101,455 km 152,098,155 km

 Moon

356,749 km 406,282 km

As you can see, both the sun and the moon change their distances quite significantly. The moon changes by about 14 per cent, and we vary our distance to the sun by about 3 per cent. 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 and 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..."

Solar Eclipse Why Eclipses Happen