Why is our moon just the right distance to block the sun in a total lunar eclipse? Great question, Kesh. Since the Earth and moon formed about 4.5 billion years ago, the moon has been moving gradually away from us (about 1.6 inches a year right now). We happen to live during a cosmic period while a solar eclipse is still possible — and just barely. In a billion years or two, the moon will be too far from us to fully cover the sun. If I’m around, I’ll post something about that momentous moment in cosmic time!