One of the things I never really thought about, but learned recently. Perhaps this is obvious to many. The relative sizes of the sun and the moon, and their relative distances from earth make it possible for the moon to "almost" completely block out the sun during a total eclipse, allowing us to study the sun's corona. Of course if either the ratio of the sun's diameter to the moon's, or the ratio of their distances were different, then this would not happen. I find this a remarkable coincidence!