The Earth and moon have been locked in a gravitational dance for billions of years. Each day, as the Earth turns, the moon tugs upon the oceans of the world, causing the rise and fall of tides. As a result, the Earth's day gets a little bit longer, and the moon gets a little more distant. The effect is small, but over geologic time it adds up. About 620 million years ago, a day on Earth was only 22 hours long, and the moon was at least 10,000 km closer than it is now.
