Eclipses are possibly the most spectacular sights of the skies. Solar eclipses have been known to stop wars. Mark Twain's Connecticut Yankee used a solar eclipse to stop his own execution. Lunar eclipses are a spectacular vision enjoyed by half the world on the evenings they occur. Do you know when and where the next eclipse will be?

An eclipse occurs because the sun, earth, and moon align in a special manner. With this alignment comes the shadow of one object falling upon another. The alignment is more exacting in the case of an eclipse. The alignments discussed in phases were shown in two dimensions. This was all that was required in this case. For eclipses, the alignment must occur in all three dimensions. Your textbook shows the importance of this three dimensional alignment.

For a lunar eclipse to occur, the moon must be full, with the moon on the opposite side of the earth than the sun. When this occurs, the shadow of the earth falls over the moon, darkening it. Why does this not occur every month at full moon? Because the moon may be slightly above the earth or below the earth in its alignment with the sun.

The drawing above illustrates the 3-D importance. In the top figure we are viewing the alignment from above the north pole. In this view the three objects are aligned. However, in the bottom portion, we are viewing from the side (note the poles on the earth). In this view, the shadow of the earth misses the moon and hence no eclipse. A similar alignment problem can occur for solar eclipses.

The possibility of the alignment not occurring is due to the fact that the moon orbits the earth in a plane which does not coincide with the plane of the earth's orbit around the sun. When two planes cross, they cross at a line. The line created by the crossing of the moon's orbital plane and the earth's orbital plane is called the line of nodes. In order for any eclipse to occur, the moon must be at or near the line of nodes. So an eclipse requires the moon to be on the line of nodes AND in the proper phase. Both of these occurring together is rare, which is why eclipses are rare.

This page was last updated on 06/06/01.