Astronomy
Gradual Slowing of Earth's Rotation and Day Length Increase
Quick fact
Fossil corals show that 370 million years ago, Earth had about 400 days per year, meaning each day was only about 22 hours long. The Moon is currently slowing Earth's rotation by about 1.8 milliseconds per century.
Why this is interesting
Earth takes 24 hours to spin once—but it hasn't always been that way. Billions of years ago, a day was only about 6 hours long. What has been putting the brakes on our planet?
Read the full explanation
Understanding Gradual Slowing of Earth's Rotation and Day Length Increase
Earth's rotation is not a perfect, unchanging spin. It is gradually slowing down because of a subtle but constant tug between Earth and the Moon. Imagine a figure skater spinning with arms out: if they pull in their arms, they spin faster; if they extend them, they spin slower. In the Earth-Moon system, the Moon's gravity pulls on Earth's oceans and solid crust, raising tidal bulges. As Earth rotates, these bulges don't line up perfectly with the Moon’s pull; friction drags the bulges slightly ahead of the Moon. This friction acts like a giant cosmic brake on Earth’s spin. The lost rotational energy isn’t destroyed—it’s transferred to the Moon, nudging it into a higher, slower orbit. Over millions of years, these small daily effects accumulate, lengthening the day noticeably on geological timescales.
A deeper explanation
The mechanism behind this slowing is tidal friction, a process driven by the conservation of angular momentum. The Moon’s gravity creates a tidal bulge on Earth, both in the oceans and in the solid rock. Because Earth rotates faster than the Moon orbits, the bulge is carried ahead of the Moon's orbital position. The Moon’s gravity pulls back on this off-center bulge, creating a torque that opposes Earth's rotation. This torque gradually removes angular momentum from Earth’s spin. To conserve total angular momentum in the Earth-Moon system, the Moon must gain that angular momentum. This gain pushes the Moon to a larger orbit, making it recede from Earth at about 3.8 cm per year. As Earth loses spin, the length of a day increases—about 1.8 milliseconds per century. Over hundreds of millions of years, this adds up: days have lengthened by several hours since the early Solar System. This process matters not only for understanding our planet's history but also for predicting future day lengths and for precise timekeeping, as leap seconds must be added to atomic clocks to keep them aligned with the slowing astronomical day.