An eclipse of the Sun happens when the Moon moves between the Earth and the Sun, so that the three bodies are in line. Illustration Viewed from the Earth, the Moon masks the Sun. If the Moon covers the Sun completely, it is a total eclipse. If the Sun is only partly obscured, it is a partial eclipse.

The orbits of the Moon around the Earth and the Earth around the Sun are elliptical rather than circular. Because of this, their apparent sizes change. It is only when the apparent size of the Moon is larger than that of the Sun that a total eclipse is possible.

The reason each total eclipse is only visible over a small part of the globe is because the Moon's shadow is relatively small when it falls on the Earth.

An eclipse occurs once every 29 days or so, at the time of the New Moon, when the Sun laps the Moon, usually passing above or below it. When the Sun passes behind it is an eclipse of the Sun. Every year there must be at least two eclipses of the sun, and there maybe as many as five. At any one place on the earth's surface, a total eclipse of the sun will only be visible once in about 360 years.

When the Moon moves through the shadow of the Earth it loses its bright direct illumination by the Sun, although its disk still remains faintly visible. As the shadow of the Earth is directed away from the Sun, a lunar eclipse can occur only at the time of Full Moon--that is, when the Moon is on the side of the Earth opposite to that of the Sun. A lunar eclipse appears much the same at all points of the Earth from which it can be seen.