Eclipses only occur when the Moon crosses the ecliptice during new or full phase. A lunar eclipse occurs when the moon is full and passes through Earth's shadow. This can only happen when the Sun, Earth and Moon are on a straight line at full Moon. You might suspect that a lunar eclipse should occur every month, however the Moon orbital motion conspires against this phenomena from occuring.

A solar eclipse occurs when the Earth passes through the Moon's shadow. As seen from the Earth, the Moon moves in front of the Sun.

Eclipses occur very infrequently due to the Moon's 5o orbital tilt out of the ecliptic. Because of this tilt, new Moon and full Moon usually occur when the Moon is either above or below the Earth's orbital plane. The Moon's orbital intersects the ecliptic and this forms a line called called the line of nodes. When the Moon, Earth and Sun are all lined up on this line of nodes we have the conditions for an eclipse.

Astronomers have calculated the number of times the a new Moon takes place on the line nodes and have determined that there are at least 2 and no more than 5 solar eclipses each year. The last time there were five solar eclipses in a single year was 1935. Lunar eclipses occur about as frequently as solar eclipses, but the maximum number of eclipses in a possible year is 7.

Even though solar and lunar eclipses occur with comparable frequency, it is still far more commonplace to experience a lunar eclipse. That’s because the darkened full Moon can be seen from anywhere on the nighttime half of the Earth during the eclipse. To see a total solar eclipse, you have to be in the path of totality. This path, sometimes up to 200 miles wide, never covers more than roughly one-half of one percent of the Earth's surface and often traverses open seas or remote regions of the planet. With fewer than 70 total eclipses per century, the chance to see one is for most of us a once-in-a-lifetime event.

It is quite remarkable that total solar eclipses even occur at all. They are possible because the Sun and the Moon appear from Earth to be about the same size in the sky. The Sun and the Moon each appear to be about .5 degrees in angular size. The Sun, whose diameter is 400 times that of the Moon, happens to be about 400 times as far away from the Earth. This condition permits the Moon to just barely cover up the Sun. In fact, if the Moon's diameter (2,160 miles) were just 140 miles less, it would not be large enough to ever completely cover the Sun so a total solar eclipse could never happen anywhere on Earth.