Quick Answer
Yes, over the long term Earth’s rotation is gradually slowing down, making our days slowly longer — by roughly 1.7 to 2.3 milliseconds per century on average. The main cause is the Moon, which raises tides that act like a brake on Earth’s spin while the Moon slowly drifts away. Billions of years ago a day lasted only about 18–22 hours. Curiously, in recent years Earth has actually sped up slightly, raising the prospect of a first-ever “negative leap second.”
It feels like a day is exactly 24 hours, fixed and unchanging. But Earth’s rotation is not perfectly constant — it has been slowing for billions of years, and it wobbles in surprising ways on shorter timescales. This guide explains whether Earth is really slowing down, how the Moon steals our planet’s spin, how long a day was in the age of the dinosaurs, and the strange recent twist of a possible negative leap second.
Is Earth Actually Slowing Down?
On geological timescales, yes — Earth’s rotation has been steadily slowing for most of its history. The length of a day has been increasing by an average of roughly 1.7 to 2.3 milliseconds per century over the long run. That sounds tiny, and on a human scale it is, but over millions of years it adds up to dramatically longer days.
The picture is more complicated on short timescales. Earth’s spin also fluctuates slightly from year to year due to processes in its molten core, the movement of its atmosphere and oceans, and the redistribution of mass as polar ice melts. These wobbles can briefly speed the planet up or slow it down, on top of the long-term braking trend. So the honest answer is: yes, Earth is slowing over the long haul, but with notable short-term ups and downs.
How the Moon Steals Earth’s Spin (tidal braking)
The primary cause of Earth’s long-term slowdown is the Moon, through a process called tidal braking. The Moon’s gravity raises bulges of water (and a smaller bulge in the solid Earth) on both sides of the planet — the tides. Because Earth rotates faster than the Moon orbits, these tidal bulges are dragged slightly ahead of the Moon’s position.
The friction of this constant tidal motion acts as a brake, very gradually slowing Earth’s rotation. By the law of conservation of angular momentum, the spin Earth loses is transferred to the Moon’s orbit, which causes the Moon to slowly spiral outward — receding from Earth at about 3.8 centimetres per year, roughly the rate your fingernails grow. So every year, our days get a tiny bit longer and the Moon gets a tiny bit farther away. It is a slow, steady exchange that has been going on for billions of years.
How Long Was a Day in the Dinosaur Era?
Because Earth has been slowing for so long, days were noticeably shorter in the deep past. Scientists can actually measure this using “fossil clocks” — growth bands in ancient corals and shells that record the number of days in a year. The total time in a year is fixed by Earth’s orbit, so more days per year means each day was shorter.
- ~1.4 billion years ago: a day lasted only about 18–19 hours.
- ~600 million years ago: roughly 22 hours.
- Age of the dinosaurs (~70 million years ago): about 23.5 hours.
- Today: 24 hours (about 86,400 seconds).
So a dinosaur experienced a slightly faster-spinning planet, with more (and shorter) days packed into each year. The slowdown is gentle but relentless.
Leap Seconds and the “Negative Leap Second” Problem
Because the world’s atomic clocks keep perfectly steady time while Earth’s rotation drifts, the two occasionally need to be reconciled. Since 1972, timekeepers have inserted leap seconds — extra seconds added to official time — to keep our clocks in step with the slowly slowing planet. More than two dozen leap seconds have been added over the decades.
But recently something unexpected happened: Earth started spinning slightly faster. Several of the shortest days ever recorded have occurred in the 2020s, with one in mid-2024 running more than a millisecond short of 24 hours. If this trend continues, timekeepers may, for the first time in history, need to subtract a second — a “negative leap second” — possibly around the year 2029. This would be unprecedented and poses real challenges for computer systems and networks that have never had to handle a second being removed. The short-term speed-up does not reverse the long-term slowing trend; it is a temporary fluctuation layered on top of it.
The Opposite Extreme — Spinning Faster
These tiny millisecond changes hint at a much bigger question: what if Earth’s rotation changed dramatically and the planet spun far faster? A faster spin would shorten the day, intensify the Coriolis effect and our weather, bulge the planet further at the equator, and shift sea levels and surface gravity.
Pushing this to the extreme — a planet rotating twice as fast — is the scenario explored in what if the Earth’s rotation spun twice as fast. It reveals just how much of our world, from the length of the day to the shape of the oceans, depends on the steady rate at which Earth turns.
Q&A
Not in any practical sense. The slowdown is extraordinarily gradual, and the Sun will have died long before Earth’s rotation could grind to a halt. In theory, tidal forces would eventually lock Earth’s spin to the Moon’s orbit, but that lies tens of billions of years in the future — far beyond the Sun’s lifetime.
On average, the length of a day increases by roughly 1.7 to 2.3 milliseconds per century due to tidal braking. This long-term trend is overlaid with short-term fluctuations from the core, oceans, and atmosphere, which can temporarily speed Earth up or slow it down.
It matters for precise timekeeping, satellite navigation, and computer networks, which rely on accurate time. The mismatch between steady atomic clocks and Earth’s drifting rotation is why leap seconds exist — and why a possible future “negative leap second” is a real technical concern.
Both, on different timescales. Over millions of years it is slowing due to the Moon’s tidal braking. But in the short term, Earth has recently sped up slightly, producing some of the shortest days on record in the 2020s — a temporary fluctuation that does not undo the long-term slowdown.
The Bigger Question
Earth’s rotation is changing right now — slowing over eons, yet recently quick enough to threaten a negative leap second. These millisecond shifts already ripple through our technology. So imagine a far more drastic change: a planet spinning twice as fast, with days half as long, fiercer storms, and oceans pulled toward a bulging equator. That dramatic transformation is the subject of what if the Earth’s rotation spun twice as fast.
The spin’s effect on weather is explained in our article on the Coriolis effect. Explore more about our dynamic planet on the Geology hub.
Watch the faster-rotation scenario to see how much depends on the speed of Earth’s spin.