We tend to think of a 24-hour day as fixed and permanent.
It’s not.
Earth’s rotation is slowly slowing down — by about 1.7 milliseconds per century.
That might sound insignificant. But over millions of years, it adds up dramatically.
The main cause? The Moon.
The gravitational interaction between Earth and the Moon creates tidal friction. As ocean tides move across the planet, they generate drag. This drag gradually transfers rotational energy from Earth to the Moon.
As a result:
- Earth spins slightly slower
- The Moon slowly moves farther away (about 3.8 cm per year)
Hundreds of millions of years ago, a day on Earth lasted only about 18 hours.
Scientists confirmed this using fossilized coral growth rings. Ancient corals recorded daily and yearly cycles in their structure. By counting microscopic growth bands, researchers could calculate how many days existed in a year hundreds of millions of years ago.
The conclusion? There were more days per year, meaning days were shorter.
Modern atomic clocks are now so precise that scientists occasionally add a “leap second” to global timekeeping systems to adjust for these tiny irregularities in Earth’s rotation.
It’s a subtle reminder that even something as fundamental as a “day” is not permanent.
Time, in a literal planetary sense, is stretching.