The Earth's moon is unique for several reasons. For one thing, it's much bigger in comparison to its parent planet than is any other satellite in the solar system. In addition, compared to other moons, ours is located considerably farther away. Furthermore, by sheer coincidence, when you look at the moon and the Sun they appear to be about the same size in the sky. No other moon in our solar system shares this amazing feature with the Sun. The ratio of the diameter of the moon to that of the Sun is about 400. If the moon and the Sun were at the same distance from us, the Sun would look 400 times broader. The ratio of the distance of the Sun to that of the moon as measured from Earth is about 390. As a result, the size and distance just about cancel each other, and the Sun and moon cover about the same amout of our sky. This is why solar eclipses are such remarkable events.

Also, almost all moons revolve around their parent planet in the plane of the equator (directly over its equator). The moon's orbit about Earth is tilted somewhat so that it's in the plane of the ecliptic (the appparent path the Sun moves across the sky). The moon completes one revolution of the Earth in about 29.5 days. So there is a full moon or a new moon about every two weeks.

Since this is the case, why are eclipses relatively uncommon? In other words, why isn't there one lunar eclipse and one solar eclipse each month? The reason is, the moon's orbit is offset slightly from the plane of the Earth's equator. This means that only occasionally will the shadow of the Earth be in line with the moon or will the moon's shadow cross the Earth's surface. For any given year there are no more than seven total eclipses (four or five solar and two or three lunar), and there are at a minimum two (both solar) total eclipses each year.

Thus, lunar eclipses occur less frequently than do solar eclipses. This is because for a total lunar eclipse, the darkest part of the Earth's shadow (umbra) must completely cover the entire surface of the moon. However, for a total solar eclipse the umbra of the moon need only cover a sliver of the Earth's surface. Interestingly enough, lunar eclipses are visible from about half the Earth (the night side of the Earth), whereas the area covered by a solar eclipse is small. On average, the width of a total solar eclipse (the width of the moon's umbra on the surface of the Earth) is only about 100 km. Therefore, many more people see lunar eclipses.

For those of you who were able to see the lunar eclipse this past Wednesday, you perhaps noticed that the disk of the moon appeared to be a burnt orange or ruddy color. I mentioned in last week's question that there wouldn't be much to see at totality. I was wrong. For the two or three previous lunar eclipses that I have watched, the disk of the moon was hard to make out. But the appearance of the lunar disk during a total eclipse can vary in brightness and color, depending on how close the eclipse occurs to sunset or sunrise and on the Earth's atmospheric conditions (clouds, haze and volcanic dust). This eclipse was very bright.

The moon's color is reddish for the same reason the Sun looks red when it's rising or setting. We are essentially looking through more layers of the atmosphere when we look towards the horizon than when we look overhead. When we observe a sunset it appears red because the seemingly thicker atmosphere is able to scatter the blue and yellow light from our view, leaving only red and orange light. However, the moon appears a darker red, since light from the Sun must penetrate an atmosphere that's much thicker than the one we look through at sunset or sunrise, and so most of the orange light is scattered out, too. Try to picture the Sun's rays going over your head at sunset and toward the darkening eastern sky. The path the sunlight takes through the Earth's entire atmosphere is about twice as long as the path of sunlight that your eyes intercept on the surface.

While total solar eclipses are spectacular events, total lunar eclipses are more subtle or modest. However, lunar eclipses like solar eclipses are awe-inspiring and have provoked fear in cultures which don't really understand why they occur. For example, in ancient Greece, the city of Athens was defeated and 7,000 of its citizens were enslaved by Syracusans as a result of their superstitions about the total lunar eclipse of August 27 in 413 BC. In 1504, Columbus used his foreknowledge about a lunar eclipse to convince the native Indians of Jamaica to give him provisions he needed. Even today some people are superstitious about a dark moon rising.

For those of you planning to take a little side trip to Antarctica in the next couple of weeks, you'll be able to see a partial solar eclipse on April 17. The next total lunar eclipse will be on September 26 of this year. You won't have to take a special trip to see it - just look out your window, and hope for clear skies.