What Causes an Eclipse of The Sun?

To get a picture of what happens during an eclipse, think of these three bodies: the earth, the sun and the moon. The moon revolves around the earth. The sun shines out there beyond the moon. But sometimes, as the moon goes around the earth, it passes directly between the earth and the sun.

An eclipse of the sun occurs only when the moon is new, for then the moon is on that side of the earth facing toward the sun. If its path lay directly in line with the orbit of the earth about the sun, an eclipse would take place at every new moon.

But in its trip around the earth, which takes about 29 1/2 days, the moon passes sometimes above and sometimes below the path of the earth. Astronomers can predict very accurately the exact time that an eclipse will take place and how many hours and minutes it will last.

They are also able to tell in advance whether an eclipse will be "total", "annular", or "partial". If the moon hides the sun completely, then when an eclipse takes place, the moon is seen as a dark disc which covers the whole sun except a narrow ring around its edge.

This thin circle of light is called "the annulus" meaning ring. This is an annular eclipse. An eclipse is partial whenever only part of the disc of the moon comes between  the sun and the earth.  Every Year there must be at least two eclipses of the sun, and there may be as many as five.

At any one place on the earth’s surface, a total eclipse of the sun will be visible only once in about 360 years. This is why astronomers have to travel great distance  to "catch" a total eclipse of the sun. 

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