At what speed is the Earth moving?
Earth is spinning too fast — the consequences for timekeeping may be unprecedented
Our home planet is in a hurry. On June 29, 2022, Earth completed the shortest day since scientists started keeping records in the 1960s, pulling off a full rotation 1.59 milliseconds faster than usual.
Terrestrial haste is a trend. In 2020, the planet recorded the 28 shortest days on record, and it kept spinning rapidly into 2021 and 2022. Before scientists could even verify that record-setting day time of June 29, our world almost outdid itself: It blazed through July 26, 2022, 1.50 milliseconds ahead of schedule.
We will likely see more record-short days as Earth continues to accelerate, says Judah Levine, a professor at the University of Colorado-Boulder and longtime time expert at the National Institute of Standards and Technology (NIST). That Earth’s days are getting shorter is no cause for alarm, he says, because the actual time difference amounts to fractions of a second over the course of a year. But what is weird is that, while scientists know that changes to the Earth’s inner and outer layers, oceans, tides, and climate can affect how fast it spins, they don’t know what’s driving the current haste.
Nobody’s perfect — not even our planet. On average, Earth rotates on its axis every 24 hours, or every 86,400 seconds. But for various reasons, from the planet’s imperfect shape to its complicated interior, every day is not exactly the same length as the day before.
What’s more, a day lasting exactly 24 hours is merely a standard we’ve come to expect right now. Earth’s rotation is slowing down over the long term thanks to the Moon’s pull on our world. Just a few hundred million years ago, for example, an Earth day was only 22 hours long. In millennia to come, an Earth day will last far longer.
So what gives with the shorter days of late, which buck the long-term trend? One hypothesis that’s been floated so far has to do with the “Chandler wobble.” Discovered in the 1800s, the phenomenon explains how the not-quite-perfectly-round Earth wobbles ever so slightly, like a spinning top as it slows down. Leonid Zotov told timeanddate.com that the wobble had mysteriously disappeared between 2017 and 2020, which could have helped the Earth finish the day a bit faster.
Another idea is that climate change might affect the rotational speed of the planet. When glaciers melt into the ocean, the shape of the Earth changes slightly, becoming flatter at the poles and bulging at the equator. But Levine says this effect can’t explain why the planet suddenly would spin faster because melting glaciers should have the opposite effect: The planet’s moment of inertia would increase, which would slow us down.
For Levine, the likely culprit is more mundane.
“One of the possibilities is the exchange of momentum between the Earth and the atmosphere,” he says. “The sum of those two is a constant, which means, for example, if the atmosphere slows down, then the Earth speeds up. Or conversely, if the atmosphere speeds up, then the Earth slows down.”
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The same thing can happen deep inside our world: It’s possible for the deep core and the mantle — the large layer that exists between the core and the surface — to move at slightly different speeds. There could be an exchange of angular momentum between the Earth’s deep core and the mantle, he speculates.
“Both of those effects … can either pump speed into the surface of the Earth, or take speed out of the surface of the earth,” Levine says. But the dynamics of the atmosphere and the Earth’s interior are so complex that it’s impossible, at least right now, to point to one of these factors as the sure cause of the planet’s brisk pace.
Nature does not always adhere to the rigidity of a clock or a calendar, and planetary timekeepers are accustomed to making a few tweaks. A leap year, for example, exists because we need an extra day every four years to keep the 365-day calendar in sync with the Earth’s revolution around the sun. Because the day is getting longer over time as Earth’s rotational speed slows down, timekeepers toss in a leap second every now and then to keep human time in step with the solar system.
With Earth accelerating, we face an unprecedented possibility: adding a “negative leap second.” In other words, Levine says, if the planet continues to spin too fast, then by the end of the decade clockmasters may need to delete a full second. For example, they might have the clocks skip from 23:59:58 on Dec. 31, 2029 to 00:00:00 on Jan. 1, 2030.
“If you had asked me about the negative [leap second] five years ago,” Levine says, “I would have said, ‘Never.’ But over the last year or two, the Earth is definitely speeding up. And now, if that speed-up were to continue — and there’s a big if there — then we could need a negative leap second in about seven years, maybe eight.”
This has never been done before. Some scientists wonder whether doing so could introduce a troubling hiccup into computer systems. Given the way our world keeps surprising us, though, Levine is not yet convinced that time will come to pass.
“You should remember, that requires an extrapolation over six years — and we’ve been burned before about extrapolations. So, I wouldn’t be ready to bet the farm.”
How fast is the earth moving?
Questions about how fast the earth—or anything, for that matter—is moving are incomplete unless they also ask, «Compared to what?» Without a frame of reference, questions about motion cannot be completely answered.
Consider the movement of the earth’s surface with respect to the planet’s center. The earth rotates once every 23 hours, 56 minutes and 4.09053 seconds, called the sidereal period, and its circumference is roughly 40,075 kilometers. Thus, the surface of the earth at the equator moves at a speed of 460 meters per second—or roughly 1,000 miles per hour.
As schoolchildren, we learn that the earth is moving about our sun in a very nearly circular orbit. It covers this route at a speed of nearly 30 kilometers per second, or 67,000 miles per hour. In addition, our solar system—Earth and all—whirls around the center of our galaxy at some 220 kilometers per second, or 490,000 miles per hour. As we consider increasingly large size scales, the speeds involved become absolutely huge!
The galaxies in our neighborhood are also rushing at a speed of nearly 1,000 kilometers per second towards a structure called the Great Attractor, a region of space roughly 150 million light-years (one light year is about six trillion miles) away from us. This Great Attractor, having a mass 100 quadrillion times greater than our sun and span of 500 million light-years, is made of both the visible matter that we can see along with the so-called dark matter that we cannot see.
Each of the motions described above were given relative to some structure. Our motion about our sun was described relative to our sun, while the motion of our local group of galaxies was described as toward the Great Attractor. The question arises: Is there some universal frame of reference relative to which we can define the motions of all things? The answer may have been provided by the Cosmic Background Explorer (COBE) satellite.
In 1989, the COBE satellite was placed in orbit about the earth (again, the earth is the frame of reference!) to measure the long-diluted radiation echo of the birth of our universe. This radiation, which remains from the immensely hot and dense primordial fireball that was our early universe, is known as the cosmic microwave background radiation (CBR). The CBR presently pervades all of space. It is the equivalent of the entire universe «glowing with heat.»
One of COBE’s discoveries was that the earth was moving with respect to this CBR with a well-defined speed and direction. Because the CBR permeates all space, we can finally answer the original question fully, using the CBR as the frame of reference.
The earth is moving with respect to the CBR at a speed of 390 kilometers per second. We can also specify the direction relative to the CBR. It is more fun, though, to look up into the night sky and find the constellation known as Leo (the Lion). The earth is moving toward Leo at the dizzying speed of 390 kilometers per second. It is fortunate that we won’t hit anything out there during any of our lifetimes!
Here’s Why We Don’t Feel Earth’s Rotation, According to Science
It should not come as a surprise to you that our planet, with its atmosphere and everything on it, is constantly spinning. At the equator the speed of rotation is about 1,675 kilometres per hour (1,040 mph), which means that right this very moment, you’re travelling at something like 465 metres per second, or a little less if you’re located closer to one of the poles.
So why can’t we all feel it? The answer lies in the nature of Earth’s movement. Think of being on an aeroplane when it’s smoothly travelling at a constant speed and constant altitude. You’ve unbuckled your seatbelt to go on a walk down the aisle, but you can’t feel the movement of the plane. The reason is simple: you, the plane, and everything else inside it is travelling at the same speed. In order to perceive the movement of the plane, you have to glance at the clouds outside.
It’s the same with Earth’s rotation — our planet completes a full turn around its axis every 23 hours and 56 minutes, spinning incessantly at an almost entirely constant rate. One way to feel motion is to feel wind on your face — but remember that Earth’s atmosphere is travelling with us at the same speed.
If Earth were to change acceleration, we’d certainly feel that, and it wouldn’t be pleasant, like a sudden slam on the brakes at a planetary scale (while the atmosphere would keep moving at the same 465 metres per second and wipe the surface of the planet). But just like we can’t feel the constant movement of a plane, the spin of our gigantic space ride is normally imperceptible, too.
So why does Earth spin so constantly? Because there’s nothing stopping it. When our Solar System formed out of a collapsing dust cloud and spun out into a flattened accretion disk with a bulge in the middle, all the planets inherited that rotation. The Sun, all our neighbouring planets, their moons, and everything else scattered in our system is still spinning after billions of years because of inertia.
To interfere with that, an external unbalanced force would have to be applied — in simple terms, the whole shindig would have to collide with some other object, and throw the rotation into disarray.
Now, as I mentioned earlier, the spin of our planet is happening at an almost constant rate. If we’re being precise, Earth is slowing down ever so slightly thanks to the Moon being a bit of a gravitational drag. It pulls on the tidal bulge of our planet, which causes tidal friction, putting energy into the Moon’s orbit.
As a result, sometimes we need to add an extra second to our clocks, because Earth’s rotation is slowing down by two-thousandths of a second every day. However, because this change in speed is so infinitesimally small, for our purposes, it still feels like Earth is rotating at a constant rate. In other words, it feels like nothing at all.