You might think the Moon is a timeless constant in our sky, but it’s actually moving farther from Earth—ever so slowly. This tiny shift is gradually lengthening our days.
How Fast Is the Moon Moving Away?
Thanks to laser reflectors left on the Moon by Apollo astronauts, NASA has precisely measured its motion. The Moon is drifting away from Earth at about 1.5 inches (3.8 cm) per year, roughly the same pace as your fingernails grow.
Tides, Torque & a Slower Spin
This recession is caused by tidal forces. The Moon pulls on Earth’s oceans, creating tidal bulges. As Earth spins, these bulges are slightly ahead of the Moon, dragging it forward and transferring rotational energy from Earth to the Moon. This makes Earth spin more slowly—lengthening the day—and propels the Moon into a higher orbit.
Days Stretched in the Distant Past… and Future
Use of astrochronology—where astronomers study ancient rock layers—reveals that billions of years ago, a day on Earth may have lasted only 18 hours.
Today, days get longer by about 1 millisecond every century —a tiny amount, but one that builds up over cosmic time.
What Lies Ahead for Earth and Moon?
This process is known as tidal locking, a fate where the rotation period of each body matches the other’s orbital period. The Moon already shows only one face to Earth (tidally locked), but Earth might eventually do the same—always presenting the same face to the Moon. However, this would take some 50 billion years, far longer than Earth’s future under our evolving Sun.
Summary: Why It’s Happening & Why It Matters
| Mechanism | Effect |
|---|---|
| Moon receding | Earth’s rotation slows |
| Slower rotation | Longer days (by ~1 ms per century) |
| Long-term trend | Could lead to tidal locking, billions of years out |
In short: Earth and Moon are locked in a slow gravitational chair dance, one that’s lengthening our days while nudging our lunar companion gradually farther away.
