http://www.newsweek.com/moon-earth-distance-orbit-958404

More than a billion years ago, a day on Earth lasted just 18 hours, a new study reports. The distance from our planet to the moon, scientists say, is one major reason for the extra six hours we have today.

Our faithful rocky companion used to lie far closer to our planet—close enough to alter the way it moves, researchers reported Monday in Proceedings of the National Academy of Sciences.