Eclipses, be they solar or lunar, occur when the Earth, Sun and Moon are in a line. If the Moon is in-between the Earth and the Sun, it blocks the view of the Sun from some parts of the Earth, and this produces a solar eclipse. If, on the contrary, it is the Earth that is in-between the Sun and Moon, then the earth will block the light from the Sun before it can get to the Moon. Since moonlight is just the light the Moon reflects from the Sun, this will darken the Moon, and we get a lunar eclipse.

Since the Moon goes around the Earth every 28 days, shouldn't we expect a solar eclipse about every 28 days (when the Moon is new), and a lunar one in the same period, (when the Moon is full) ?

Well, this would be so if the orbit of the Moon were in the same plane as the orbit of the Earth around the Sun. But we know eclipses are rarer than that; and the Moon's orbit is not in the same plane. Instead, it is tilted with respect to it, and the Moon does not in general pass directly on the Earth-Sun line. Moreover, the Moon's orbit tilt varies slowly. To have an eclipse, then, it is not enough that the three bodies be in the right order; the Moon's orbit should also be at the right tilt.

The behavior of the orbit of the moon (around the solar eclipse of August 11, 1999)

About shadows

Whether it is the Moon between the Earth and Sun, or the other way around, the phenomenon is basically the same: the body in the middle casts a cone of shadow, and if the outer body happens to move into this cone, we have an eclipse. It is important to notice that the shadow is more complicated than just a cone: it actually consists of a darker cone, or umbra, where no sunlight reaches, and a lighter region, the penumbra, where only some of the sunlight is blocked. Whether you will be able to observe a total or partial eclipse will depend on which of the two regions you are located in.

Solar Eclipses

A solar eclipse occurs, as we said, when the Moon is directly between the Earth and the Sun. As noted above, these are not as frequent as one might expect, but there are still at least two each year. That we get total solar eclipses at all is a bit of good luck. The Moon and the Sun appear to have the same size when viewed from the Earth. The Moon is about 400 times smaller than the Sun, but at the same time the Sun is about 400 times farther away from the Earth than the Moon. Because of this, when there is a solar eclipse, the Moon is about the right size to completely cover the disk of the Sun. If the Moon is close enough to the Earth, it will cover it completely, and we get a total solar eclipse. This is the most spectacular kind, where the day changes into darkness, and one can see the stars in plain day. If the Moon is further away from the Earth, then its disk will not be big enough to cover the Sun completely, and we get an annular eclipse, where most of the sun is covered, but an annulus remains, surrounding the dark disk of the Moon.

Total eclipse	Annular eclipse

Were the Moon to be smaller, or the Sun closer to the Earth, there would be no total solar eclipses. On the other hand, were the Moon a bit bigger, the shadow it casts on the Earth during solar eclipses would be larger, and it would be easier to be in the right place to observe them. The dark part of the Moon's shadow is, on the surface of the Earth, a circle of only about 160 miles in diameter. As the Earth moves, this circle traces a path on its surface, called the path of totality. To see a total solar eclipse you have to be inside this rather small region. It is estimated that, on the average, a given spot on the Earth will be on the path of an eclipse only about once every 370 years, so it is better to move to an eclipse than to wait for one to come to you.

Lunar Eclipses

As explained above, lunar eclipses do not occur every month because of the inclination of the Moon's orbit. They do happen at least twice a year, though. And you are far more likely to be able to observe one than a solar one. The reason is that when the Moon gets dark, it is because it does not receive the sunlight, and it then is dark for anyone who can see it. So, instead of having to be in a rather narrow path, as happens for solar eclipses, you only have to be in a part of the world from which the Moon is visible at the time of the eclipse. Pretty much half the world qualifies!

Simulation of a lunar eclipse (April 3, 1996)

As with solar eclipses, there are partial and total lunar eclipses. If the Moon does not enter into the umbra, the darkest part of the Earth's shadow, then it does not darken completely, and we get a partial eclipse. These are hard to notice; the Moon just darkens a bit, but does not disappear completely into the night.