Why does the moon perfectly fit the sun in a solar eclipse?

For part of every month, the moon is in a far part of its orbit from Earth. At such times it isn't big enough to cover the sun completely. If an eclipse happens then, the outer part of the sun's surface will appear as a ring around the moon. This type of event is called an annular or ring eclipse. read more

As you can see, the range of earth:moon and earth:sun distances actually encompasses 16.8% of the sun:moon size ratio, making the coincidence that there happens to be overlap unusual, but not particularly mind-blowing. read more

However the Moon does not always almost perfectly fit over the Sun. Many eclipses (not including partial ones) are in fact called"annular", where the Moon does not cover the entire discof the Sun, and the Sun shows itsef as a (very) bright circle (annulus) aroubd the Moon, weather permitting of course. read more