Why Does the Solar Eclipse Happen?

Cause

• Solar eclipses appear when the moon passes between the sun and earth and casts shadows upon the earth's surface. They only occur during the new moon phase as this is the time of the month when the moon and sun are in line with each other. Between two and five eclipses occur per year, though they can only be viewed in that region of the earth that the moon's shadow is cast.

Types

• There are four general types of solar eclipses, each due to the position of the moon in relation to the sun and the position of an observer on the earth's surface. A partial eclipse occurs when only a portion of the moon passes between the sun and earth. Only a portion of the moon's indirect shadow, the penumbra, blocks out light from the sun. An annular eclipse occurs when the moon completely blocks out the sun at a great distance from the earth, so it appears to only block the center of the sun. A total eclipse occurs when the moon completely blocks out all light from the sun's surface, casting its direct shadow, the umbra, upon a portion of the earth's surface. A hybrid eclipse occurs when both an annular and total eclipse can be viewed from different parts of the earth at the same time.

Why the Different Eclipse Types?

• The moon orbits the earth, and the earth orbits the sun in an elliptical orbit. An ellipse is like a stretched out circle. This changes the distance between the moon, earth and sun during their orbits, thus affecting the apparent sizes of both the sun and moon, depending on the time of year. This causes the varying types of eclipses viewed from earth. The type of solar eclipse that occurs is dependent on the apparent sizes of the sun and moon. For instance, a total eclipse is more likely to occur when the sun is farthest from the earth, while an annular eclipse is more likely when the sun is closest.

Cycles

• The most common cycle used to predict solar eclipses is the Saros cycle. The Saros cycle lasts a little over 18 years and relies on the periodicity of the moon's orbit to make predictions. When a certain type of eclipse occurs, it is known that an identical eclipse will occur at some time in the future when the moon returns to the same position. Once the Saros cycle is complete, the moon, sun and earth will be in approximately the same position, and the moon will have the same phase as the past eclipse.