The Moon Is Drifting Away from Earth

This slow separation is the result of a gravitational dance between Earth and its satellite. The Moon's gravity pulls on our planet's oceans, creating the familiar tidal bulges. Because Earth rotates much faster than the Moon orbits, these bulges are dragged slightly ahead of the Moon's position. This leading mass of water has its own gravitational pull, which tugs the Moon forward, giving it a tiny but constant boost of energy. This added energy is what forces the Moon into a slightly higher and more distant orbit, centimeter by centimeter.

This exchange is a two-way street. The energy the Moon gains is stolen directly from Earth's rotation, causing our planet's spin to slow and our days to lengthen by about 1.8 milliseconds per century. Billions of years ago, when the Moon was much closer, a day on Earth may have lasted only five or six hours. Looking forward, this drift has a celestial deadline. As the Moon recedes, its apparent size in our sky shrinks. Eventually, it will appear too small to fully obscure the Sun, bringing an end to the breathtaking spectacle of a total solar eclipse.