Why Does The Spinning Earth Speed Up If The Tides Are Slowing Us Down?
In 2020, we experienced the shortest ‘day’ in 50 years. Here’s the reason why.
On the timescales that humans experience, there are lots of things that we consider to be extremely regular. Every day lasts 24 hours, on average, a number that never differs significantly from that value, with only the occasional leap second inserted to preserve our timekeeping. Each year last 365 days, with an occasional leap year designed to keep the calendar in line. And the Earth orbits the Sun in an ellipse, making almost exactly the same orbit year after year. As far as we’re concerned, the differences that occur from the day we’re born until the day we die need never cross our mind.
Over long periods of time, however, none of these things are constant. There are many different forces and phenomena at play that change these properties of our planet. The Sun and Moon, for example, not only create tides on Earth, but tidal forces that change our orbit. The other planets in the Solar System tug on Earth, causing long-term changes as well. And there are internal factors as well, from earthquakes to global warming, that can alter our planet’s behavior. According to atomic clocks, Earth experienced its quickest day in 50 years on July 19, 2020, which was 1.4602 milliseconds shorter than our normal 24 hours. Here’s the science of why.
On long timescales, the biggest continuous change that occurs is from the combined effects of the Moon and the Sun. We typically visualize this as the Sun’s gravitational force determining the elliptical orbit of the Earth, while the Earth’s gravitational force determines the elliptical orbit of the Moon. But beyond that, we have to consider that the Sun, the Earth and the Moon aren’t exactly single points in space, but rather are spheres.
Consider what this means, for example, for the force that the Moon exerts on the Earth. A any moment in time, the Earth gets pulled on a little bit more strongly on one side and a little less strongly on the other, causing it to get stretched out. At the same time, if you draw an imaginary line connecting the Earth to the Moon, the part of the Earth “above” that line gets pulled down relative to the center, while the part “below” gets pulled up. This is how tidal forces work, and how the Moon is the primary driver of Earth’s tides.
Simultaneously, the Earth is spinning on its axis as it revolves around the Sun. One side of the spinning Earth — the side closer to the Moon — feels a slightly larger force than the center of the Earth. Meanwhile, the other side of the spinning Earth — the side farther away from the Moon — feels a slightly smaller force than the center of the Earth. The difference between these two forces on the Earth not only causes our planet to have tides, but does something else as well: it acts a little bit like a braking mechanism, similar to how, if you had a spinning top, you touched it ever so slightly on one side with a piece of paper.
The braking effect may be small, but on extremely long timescales, even small effects can add up. This slows down the Earth’s rotation, but there’s a little bit of a cost. Because angular momentum is conserved, the slowdown of the spinning Earth, which loses angular momentum, means something else needs to gain angular momentum. When you look at the combination of all the changes happening to the Earth, the Sun, and the Moon, you find that Earth’s rotation slows down, and the Earth-Moon system spirals slowly away from the Sun while the Moon spirals slowly away from the Earth.
With each year that goes by, on average, these tidal forces cause our day to lengthen by just a tiny amount. If it took Earth exactly 24 hours to complete one full 360° rotation precisely one year ago, then today, one year later, it will take Earth 24 hours and an extra 14 microseconds to complete that same 360° rotation. This extra 14 microseconds per day adds up over time, so much so that, on average, we have to add a leap second to our clocks approximately every 18 months.
Slow but consistent processes, like this one, can really add up over time. Geologically, we can look at the daily patterns that get left in the soil due to the tides: tidal rhythmites. They aren’t only produced regularly on Earth’s coasts today, but were produced on a daily basis for the entire history of Earth. There are a few cases of ancient tidal rhythmites that have been preserved in the sedimentary rock of Earth’s geological layers, and we can use these rhythmites to determine how long it took Earth to make a complete rotation back when these rhythmites were created.
The most ancient one we know of on Earth was formed a whopping 620 million years ago, and indicates that a day back then was a little bit under 22 hours long, consistent with our predictions from physics.
This means, if we extrapolate tidal breaking backwards to when the Earth-Moon system first formed some 4.5 billion years ago, the length of a day — the amount of time it takes Earth to rotate 360° on its axis — was originally somewhere between 6 and 8 hours long. If you had been around back then, a year on Earth would have consisted of more than 1000 days each; the Moon would have been much closer and therefore larger in the sky than the Sun; there would not have been any such thing as an annular eclipse back then, as every time the Moon passed directly between the Sun and the Earth, the Moon’s shadow would have fallen on Earth’s surface, never ending before it reached our planet.
But although this effect dominates on extremely long timescales, lengthening our day, there are internal effects that occur on our planet, many of which work to shorten the day instead. If you’ve ever spun yourself around as fast as you can — in a swiveling chair, on a rotating platform, on a pair of ice skates, etc. — and held your arms out (possibly while holding weights as well) while spinning and then brought them in towards your center, you’ll find that your rate of rotation speeds up tremendously. If you can understand this, you can understand how the Earth speeds up.
We mentioned this law of physics earlier: the conservation of angular momentum. Angular momentum is a measure of two things simultaneously combined.
- Your moment of inertia: how large your mass is and how it’s distributed, either close to or far away from the axis you’re going to rotate around.
- You angular velocity: how quickly, in terms like revolutions-per-second, you’re actually spinning about your axis.
If you watch an Olympic figure skater, for example, holding her arms outstretched while she spins, you’ll always find that her rotation speeds up to faster and faster speeds when she brings her arms in. This is because her mass remains constant, but she’s bringing more of it closer in to her rotational axis; her moment of inertia goes down. In order to keep angular momentum conserved, her angular velocity has to go up, and hence she spins more rapidly.
Although these are easy surface changes to see, most of Earth’s mass resides beneath the surface.
Inside of our planet, we have multiple layers. The outermost layer, which is all we’ve ever accessed directly, is the crust, with Earth’s liquid water sitting atop that crust and the atmosphere “floating” atop that. Underneath the crust, moving in towards the center, we have the (solid) mantle, the (liquid) outer core, and the (solid) inner core.
To the best of our understanding, the solid inner core wasn’t always present! Early on, there was more internal heat trapped inside the Earth, which has radiated away and escaped over time. Even though Earth generates additional heat through two main processes — the radioactive decay of heavy, unstable elements and the slow, gravitational contraction of the planet itself — the interior of Earth is still cooler than it used to be.
The solid core formed only between 1 and 1.5 billion years ago, and continues to grow, as the portion of the outer core that touches the inner core slowly solidifies over time.
Inside the Earth, then, the center is becoming more dense and stable, as more and more of the mass gets concentrated towards Earth’s center. This is a big deal for the rotation of the Earth, because if there’s more of Earth’s mass moving from the outskirts towards the center, then its moment of inertia, just like our spinning figure skater, is decreasing. And again, just like the figure skater, its rotational speed has to increase in order to compensate for that.
This doesn’t always happen in a smooth and gradual fashion, either. Sure, the formation of the inner core is likely a gradual process, but you have to realize that any rearrangement of how Earth’s mass is distributed is going to change the rate of rotation. Overwhelmingly, these rearrangements fall into two categories.
- Subterranean rearrangements, which usually occur during events like earthquakes, and tend to speed up Earth’s rotation slightly but noticeably.
- Surface rearrangements, where material that was typically at one point at a higher elevation on Earth falls down to be at a lower point.
In 2011, a catastrophic earthquake occurred off the coast off Japan, triggering a tremendous and devastating tsunami in the process. This magnitude 8.9 quake, one of the most powerful in recorded history, changed the distribution of Earth’s mass so significantly that the length of a day shortened, all at once, by 1.8 microseconds. In that same event, Japan’s main island, Honshu, shifted by about 8 feet. These rearrangements have occurred before and will occur again, with earthquakes always shortening, not lengthening, the day.
But surface rearrangements are happening right now as well: as glaciers and icecaps melt, particularly over land masses. The Antarctic ice sheet, for example, is the largest single mass of ice on Earth, which contains 30 million cubic kilometers of ice: about 30 quadrillion tons of material. It’s located, on average, at an elevation of between 8000 and 9000 feet (2400–2700 meters) above sea level. Every time some of that ice melts or calves into the ocean, it not only causes sea levels to rise, it redistributes Earth’s mass so that it’s closer to the central rotational axis. Changes in ice and water storage on Earth may be responsible for both the current speed-up in Earth’s day, as well as newly observed wobbles in Earth’s rotation.
The changes that are occurring in Earth’s rotational rate right now aren’t driven by astronomical phenomena, but rather by factors occurring both on and within planet Earth itself. If our days continue to shorten, rather than lengthen as astronomical concerns alone would indicate, we may not only need to revise how we insert leap seconds into our calendar (none was inserted in December of 2020, despite one being previously scheduled), but to consider subtracting leap seconds as the changing day would necessitate. They would be the first “negative leap seconds” ever introduced, and would be required to keep our recorded time in line with Earth’s rotation, a vital concern for keeping satellites and other communications infrastructure synchronized with our planet.
The advances that have occurred in recent years and decades from atomic clocks have enabled precise timekeeping as never before, with atomic clocks being an inseparable part of both the 1989 and 2012 Nobel Prizes in Physics. As a perhaps unintended consequence, we’re observing a wholly unforeseen consequence of how humans are affecting the planet we live on: causing minuscule but very real, measurable changes to the length of a day.
Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.
