APOD: 2015 March 22 - A "Double Eclipse" of the Sun

APOD: 2015 March 22 -"A Double Eclipse of the Sun"

One of the most marvelous coincidences in nature is the fact that the apparent sizes of our sun and moon are almost identical, which is what makes total eclipses appear so dramatic. The sun is about 400 times larger than the moon, but also about 400 times farther away on average. (Since the orbits are elliptical rather than round, the apparent sizes of the two objects vary somewhat.)

And it is a coincidence. There is no principle of celestial mechanics which dictates this ratio, and there is no other planet in our solar system with a moon which appears to be the same size as the sun when viewed from the planet's surface. In fact, it will not be true on Earth forever, because the moon gets slightly farther from Earth every year. OK, it only recedes by four centimeters per year, but astronomical times are measured in billions of years, and any number gets pretty large when you start multiplying it times a billion. In 3.5 billion years, the moon will have moved nearly 90,000 miles farther away. Unfortunately, there won't be any humans around to witness the shrunken moon from Earth's surface because the sun will be about 40% brighter then, the oceans will have boiled away, and the Earth will no longer be able to sustain life.

So plan your vacations accordingly.

How rare is the coincidence that the images of the moon and sun seem to be the same size? We don't really know, and won't be able to make any sensible guess until we know more about the distant solar systems of the universe.