Each day is getting slightly longer due to the Moon's gravitational pull on Earth's oceans. Tidal friction slows Earth's rotation by about 2.3 milliseconds per century - meaning days are lengthening by roughly one second every 50,000 years.
Earth's Days Are Getting Longer Due to the Moon
You might not notice it in your lifetime, but Earth's days are getting longer. Thanks to the Moon's gravitational tug on our planet, each day is now about 2.3 milliseconds longer per century than it was 100 years ago.
That might sound impossibly small, but it adds up: days lengthen by approximately one second every 50,000 years. Over Earth's 4.5-billion-year history, this has had dramatic effects.
The Moon Is Slowing Us Down
The culprit is tidal friction. As the Moon orbits Earth, its gravity creates bulges in our oceans—the tides. Earth rotates faster than the Moon orbits, so these tidal bulges try to "drag" slightly ahead of the Moon.
The Moon's gravity pulls back on these bulges, acting like a brake on Earth's rotation. At the same time, the tidal bulge pulls the Moon forward in its orbit, causing it to gradually drift away from Earth at about 3.8 centimeters per year.
When Days Were Just 19 Hours Long
Billions of years ago, Earth spun much faster. Research published in the Proceedings of the National Academy of Sciences found that a day on Earth was only about 19 hours long roughly 1.4 billion years ago.
Going back even further, when the Moon first formed about 4.5 billion years ago, it was much closer to Earth and days may have lasted only 5-6 hours. The Moon has been gradually slowing us down ever since.
It's Not Perfectly Steady
While the long-term trend is clear, day length doesn't increase perfectly smoothly. Earth experiences short-term fluctuations in rotation speed due to:
- Atmospheric changes and wind patterns
- Ocean currents redistributing mass
- Movements in Earth's molten core
- Post-glacial rebound (continents rising after ice sheets melted)
In fact, post-glacial rebound actually speeds up Earth's rotation slightly, shortening days by about 0.6 milliseconds per century and partially offsetting the Moon's effects.
So while you won't get any extra sleep from this phenomenon, Earth's slowing rotation is a reminder that even our planet's most fundamental features—like the length of a day—are constantly evolving.