It sounds like science fiction, but it is happening right now: the Moon is slowly moving farther from Earth. Not by much—about 3.8 centimeters a year, roughly the rate your fingernails grow—but over millions of years, that tiny shift adds up.

The reason is gravity, tides, and a little cosmic bookkeeping. The Moon pulls on Earth’s oceans, raising tides. Because Earth rotates faster than the Moon orbits, those tidal bulges are carried slightly ahead of the Moon. Their gravity tugs the Moon forward in its orbit, giving it a small boost of energy. As a result, the Moon’s orbit expands and it drifts away.

But the energy is not free. It comes from Earth’s rotation. As the Moon gains orbital energy, Earth loses a bit of spin. That means our planet turns more slowly, and the length of a day gradually increases. The effect is tiny: scientists estimate days are getting longer by about 1.7 milliseconds per century. You will never notice it while checking the clock, but geological time tells a bigger story.

Hundreds of millions of years ago, Earth’s days were shorter. Fossil coral growth patterns suggest that ancient years contained more days because each day was only about 22 hours long. The Moon would also have appeared larger in the sky, since it was closer to Earth.

This drift will not create sudden danger. The Moon is not about to escape, and the Sun’s future changes will matter long before any extreme tidal outcome. Still, the process is a reminder that even familiar things are not fixed. The Earth-Moon system is dynamic, exchanging energy in slow motion. Each tide, each rotation, and each lunar orbit is part of a long, quiet dance that is stretching our days by fractions, century after century, ever so slowly.