Coincidence that sun and moon are same size?
Annular eclipse photo by Fred Espenak. The moon is in front of the sun, but it's in a far part of its orbit. So the moon isn't big enough to cover the sun completely. NASA image.
What are the odds that – from our perspective on Earth – the diameter of our moon and the sun appear exactly the same?
At this particular moment in Earth’s history – although the sun is about 400 times larger than the moon – it is also about 400 times farther away. So the sun and moon are nearly the same size as seen from Earth. What are the odds?
No one knows the odds, because no one knows how many planets and moons there are in space. At this writing (late 2007), astronomers have discovered only about 200 planets in distant solar systems so far, and we don’t know much about their moons. Earth & Sky asked Myles Standish of NASA’s Jet Propulsion Laboratory about the similarity in the size of Earth’s moon and sun, as seen from Earth. He said this similarity is unique in our solar system. It doesn’t occur with any other planets and moons nearby. But there may be millions or billions of undiscovered solar systems.
So what are the odds of the moon and sun appearing nearly the same size from Earth? No one knows.
By the way, although it’s fascinating that they are so similar, the sun and moon aren’t always the same size as seen from Earth. In fact, the moon and sun are rarely exactly the same size. 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 moon!
