Scientists say that the odds of this happening else where in the universe is about 1 In 1 Billionth chance! And what’s more, there is no apparent physical reason why this has to be the case. Is it only an interesting coincidence that the Moon should so nearly perfectly blot out the Sun? Did you know that Moon happens to be about 400 times smaller than the Sun, but the Sun happens to be about 400 times further from the Earth than the Moon?
At this particular moment in Earth’s history, the sun and moon appear nearly the same size as seen from Earth. And that’s why we on Earth can sometimes witness that most amazing of spectacles, a total eclipse of the sun. Scientists say that rather simple geometry explains why the apparent disk of the Moon is almost exactly the size of the apparent disk of the Sun. When the Sun is eclipsed by the Moon on August 21st, many observers in the path of totality across much of the world it will be temporarily replaced by a beautiful ring of fire – a brilliant annulus of stellar plasma just peeking out around the dark lunar disk!
Although it’s fascinating that they are so similar, the sun and the moon aren’t always the same size as seen from Earth. The moon’s distance from Earth varies slightly over the course of a single month. So the moon’s apparent size in our sky is always changing. 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. It’s essentially a partial eclipse. The sky doesn’t darken. You can’t look at the eclipse without special filters. Still, it’s very beautiful and fascinating to witness any eclipse – and stand in line with the sun and the moon!
However, this match is not always quite the same, the Earth orbits the Sun in a modestly non-circular, elliptical, path and so our nearest and furthest distances (perihelion and aphelion) differ by about 3.3%. And the Moon’s orbit has a roughly 10% difference between its near and far point to us, so the precise degree of the total solar eclipse will vary a little as the apparent sizes of Sun and Moon vary. This Sunday the distance variations conspire to make the Moon appear 94.4% the size of the Sun.
Every year the Moon’s orbit grows by some 3.8 centimeters and our day lengthens by about 0.000015 seconds. At this present rate, in about 50 million years the Moon will never completely eclipse the Sun, it will simply appear too small in the sky.