What is a solar eclipse?

A solar eclipse can occur several times a year. It is caused by the shadow of the moon falling on the earth. In theory there could be a solar eclipse every new moon, but since the orbit of the moon is at a slight angle to the orbit of the earth around the sun, the shadow, or umbra, often falls above or below the earth. The following diagram (not to scale) shows the effect. Around the umbra is an area where the sun is partially visible. This is known as the penumbra, ("almost-shadow"). There are three types of solar eclipse, depending on the position of the earth in the above diagram:

	(T) Total	When the moon totally obscures the sun. The umbra races across the surface of the earth at around 35 km/minute (22 miles/minute). The longest possible visibility is 7 minutes 31 seconds. A total eclipse occurs around twice every three years.
	(A) Annular	If the moon is in a part of its orbit that is relatively further away from the earth, the sun is visible as a ring around the moon. The umbra ends above the surface of the earth. The longest possible annular eclipse is visible for 12 minutes 24 seconds.
	(P) Partial	This occurs when the penumbra falls on the earth, but the umbra falls outside it. Outside the path of the umbra of a total eclipse, but within the penumbra, the eclipse is also partial.