What is a Solar Eclipse?

A solar eclipse occurs when the moon crosses the path between the sun and the earth. If the motions of the celestial bodies were simple, there would be a solar eclipse every month. However, the sun and the moon have complicated motions, which make it difficult for the path of the moon to intersect the path between the sun and the earth. Despite this complication, eclipses are more common than people tend to believe.

The darkest shadow (where the sun is completely covered) is called the umbra. The umbra is narrow at the distance of the Earth, and a total eclipse is observable only within the narrow strip of land or sea over which the umbra passes. The partial shadow is called the penumbra. A partial eclipse may be seen from places within the large area covered by the penumbra. Sometimes the Earth intercepts the penumbra of the Moon but is missed by its umbra in this case a partial eclipse of the Sun is observed. The sun is so bright that even though it is partially covered it can illuminate the Earth.

Nature has created a remarkable coincidence, the sizes and distances of the Sun and Moon are such that they appear as very nearly the same angular size from the Earth, but their apparent sizes depend on their distances from the Earth. The Earth revolves around the Sun in an elliptical orbit, so that the distance of the Sun changes slightly during a year, with a correspondingly small change in the apparent size or angular diameter of the solar disc. Also the apparent size of the Moon's disc changes somewhat during the month because the Moon's orbit is also elliptical.

An annular eclipse occurs when the Sun is nearest to the Earth and the Moon is at its greatest distance, the apparent disc of the Moon is smaller than that of the Sun. When this type of eclipse of the Sun occurs, the Moon's disc passing over the Sun's disc cannot cover it completely but will leave the rim of the Sun visible all around it.

The frequency of solar and lunar eclipses

A solar eclipse, especially a total one, can be seen from only a limited part of the Earth, while a lunar eclipse can be seen wherever the Moon is above the horizon.

In most calendar years there are two lunar eclipses (there can be up to a maximum of three) or none may occur.

Solar eclipses occur two to five times a year, five being exceptional. The average number of total solar eclipses in a century is 66 for the Earth as a whole.

From any point on Earth, on the average you will experience no more than one total solar eclipse in three to four centuries.

The situation is quite different for lunar eclipses. An observer at the same location can see an average of one lunar eclipse per year. A total lunar eclipse can last, as long as an hour and three-quarters, but for a solar total eclipse maximum duration of totality is only 7 1/2 minutes. This difference results from the fact that the Moon is much smaller in cross section than the extension of the Earth's shadow but can be only a little greater in apparent size than the Sun.