\n<\/p>\n
If you haven\u2019t accomplished as much this summer as you had hoped to, you can blame forces far beyond your control: a few of these dog days, by one measure, are among the shortest you\u2019ve ever lived through.<\/p>\n
For most of humanity\u2019s history, we have measured time by the sun as it rises and sets\u2014essentially, through Earth\u2019s orientation to the cosmos surrounding us. But compare that technique with modern, superprecise timekeeping, and soon you\u2019ll find that each day varies a bit in length at the scale of thousandths of a second. This summer a few factors are adding up to make a handful of Earth\u2019s spins\u2014those occurring on July 10, July 22 and August 5\u2014more than a millisecond faster than the average of the past several decades.<\/p>\n
Yes, there are scientists whose job is to track these things; no, they are not particularly concerned by these developments. \u201cIt\u2019s a very small phenomenon,\u201d says Christian Bizouard, an astronomer at the Paris Observatory and primary scientist at the International Earth Rotation and Reference Systems Service\u2019s Earth Orientation Center. \u201cThere is nothing extraordinary [happening].\u201d<\/p>\n
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Bizouard has a point, of course\u2014no one is going to notice the sun rising a millisecond earlier or later than we might otherwise expect. But tracking Earth\u2019s rotation to this level of precision is vital because countless aspects of modern life rely on our ability to pinpoint locations to within a meter, and high-precision GPS navigation requires that satellites know exactly where they are compared with features on Earth\u2019s surface.<\/p>\n
So Bizouard and his colleagues track Earth\u2019s orientation in space. To do so, they have enlisted astronomers all over the planet to monitor a collection of about 300 objects, he says, primarily the bright, very distant, supermassive-black-hole-powered objects known as quasars. All day, every day, pairs of distant observatories tuned to radio wavelengths of light check in on their specific object. By measuring the timing mismatch between light received at each station, scientists can calculate the precise location of the observatories and, in turn, the planet.<\/p>\n
That\u2019s how scientists know that the amount of time it takes Earth to complete one rotation varies slightly. But why does the planet\u2019s speed vary? Even if you may never notice their loss, the missing milliseconds offer us a glimpse into the intricate oddities of the planet we live on\u2014so let\u2019s track them down.<\/p>\n
Officially, time is defined by nine-billion-some vibrations of a cesium atom per second, 86,400 seconds per day. Inconveniently, Earth\u2019s behavior isn\u2019t governed by cesium atoms. Physics holds that, as a solid object moving in a vacuum, Earth ought to keep spinning at the same rate unless some outside force intervenes, says Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography.<\/p>\n
But Earth isn\u2019t quite a simple solid object, and it has a rather large moon that can provide outside force. That means several different factors can affect Earth\u2019s rotation speed.<\/p>\n
Two of these factors\u2014the core and the atmosphere\u2014each affects Earth\u2019s spin under a similar principle. The overall rotational speed of the Earth system must stay steady, so if a component\u2019s movement changes, then the overall planet has to compensate. \u201cThe sum of all the rotations has to add up to the same thing,\u201d Agnew says. \u201cIf part of the Earth is going slower, another part has to go faster.\u201d<\/p>\n
Take Earth\u2019s core, for example, hiding below what we think of as the solid ground we walk on. Only the inner portion of the core is actually solid; the rest is fluid. \u201cThere\u2019s this giant ball of molten iron about the size of the moon inside the Earth,\u201d Agnew says. All that liquid metal (there\u2019s a little nickel mixed in with the iron) is moving, creating the magnetic field that shields us from some of the many hazards of space.<\/p>\n
The core\u2019s activity is quite a mystery. The region isn\u2019t actually all that far away\u2014less than 2,000 miles from the surface, closer than New York City is from Los Angeles\u2014but it cannot be directly accessed and is therefore very difficult to understand. In recent decades, for whatever reason, the core\u2019s spin has been slowing, forcing the rest of Earth to speed up to compensate.<\/p>\n
\u201cThe core is what changes how fast the Earth rotates on periods of 10 years to hundreds of years,\u201d Agnew says. \u201cThe core has been slowing down for the last 50 years, and as a result, the Earth has been speeding up.\u201d (This speed-up is part of why timekeepers have not implemented an artificial leap second\u2014a tactic used annually during small stretches of the late 20th century\u2014since 2016 and don\u2019t expect to anytime soon.)<\/p>\n
A similar phenomenon plays out in Earth\u2019s atmosphere. Like the core, the atmosphere is a fluid mass\u2014and although it\u2019s a very complex one, scientists have much better insight into it than into the elusive core. The atmosphere changes with the seasons as the sun\u2019s radiation falls disproportionately on different parts of the planet.<\/p>\n
The Northern and Southern Hemispheres each have a primary polar jet stream, a river of strong wind flowing from west to east that wanders north and south as it carries weather around the planet. Because of Earth\u2019s topography and the influence of ocean currents, the Southern Hemisphere\u2019s jet stream is stronger overall than the Northern Hemisphere\u2019s. And each jet stream is fastest during its hemisphere\u2019s winter, slowing somewhat in local summer. Combine those factors and the Northern Hemisphere summer sees a small decrease in total speeds of westerly wind (those flowing west to east), Agnew says\u2014forcing the solid Earth to spin a smidge more rapidly to compensate.<\/p>\n
This atmospheric effect is why the rotation rate changes in an annual cycle, with the days when Earth rotates fastest tending to cluster in the Northern Hemisphere\u2019s summer, particularly July and August.<\/p>\n
To the extent that the core explains decadal changes and the atmosphere explains annual ones, the moon explains millennial and daily differences in Earth\u2019s rotation rate.<\/p>\n
At geologic timescales, Earth\u2019s rotation is slowing down because of the moon\u2019s tidal influences on the water that fills our planet\u2019s oceans. The moon\u2019s gravity sloshes water around, causing an infinitesimal friction between ocean and seafloor. \u201cThat\u2019s been slowing the Earth down since the Earth had oceans,\u201d Agnew says.<\/p>\n
This trend doesn\u2019t register to humans, but over time, the effect is quite noticeable. About 70 million years ago, shortly before the extinction of nonavian dinosaurs, a day was about half an hour shorter than it is today, for example. Wind the clock even further back, to 245 million years ago, when dinosaurs first came on the scene, and a day lasted a bit more than 22 and a half hours, scientists have calculated.<\/p>\n
The moon causes a second phenomenon that affects Earth\u2019s rotation on a human timescale. Beachgoers know full well that the moon\u2019s gravity causes the seas\u2019 daily high and low tides, and the solid Earth rises and falls a little bit in response to the moon as well, albeit not nearly as noticeably.<\/p>\n
But the moon\u2019s orbit doesn\u2019t line up with Earth\u2019s equator: our constant companion\u2019s path is a bit tilted compared with Earth. Because of this, the tidal bulges wander north and south over the course of the moon\u2019s loops around Earth. When the moon is right over the equator, the tidal bulges are, too, and therefore their mass is farther away from the planet\u2019s spin axis; when the moon is the farthest north or south, the bulges move away from the equator, slightly closer to the planet\u2019s spin axis. This taps into the same physics as a spinning ice skater with outstretched arms does when they hug their chest to speed up\u2014Earth\u2019s rotation rate speeds up just a hair when the moon is at the northernmost or southernmost point in its orbit, about every two weeks.<\/p>\n
All these factors combine for the remarkably complicated state of Earth\u2019s rotation rate: it is slowing over geologic time because of ocean friction but has been speeding up over recent decades because of the core, and its spin speed slightly increases every summer from the atmosphere and every two weeks from the moon\u2019s north-south wandering.<\/p>\n
The changes make such good sense in terms of physics that scientists like Bizouard are able to take variations in Earth\u2019s rotation rate for granted. And scientists have some grasp of the annual and weekly changes in Earth\u2019s spin rate, allowing them to expect the speedy summer days. But the mysteries of Earth\u2019s core prevent these experts from confidently charting how Earth\u2019s rotation will change into the future. \u201cWe are not able to predict anything,\u201d Bizouard says.<\/p>\n
Scientists put out predictions anyway, of course. As summer approached, they thought August 5 might be the shortest day of the year, a full 1.5 milliseconds shorter than usual. Current estimates still indicate that this day will be about that much shorter, and that August 18 may be another contender for the year\u2019s fastest rotation. For comparison, the shortest rotation day in recent years was on July 5, 2024, when we lost 1.66 milliseconds.<\/p>\n
Yes, you\u2019ve probably now spent more time wrapping your mind around Earth\u2019s quickest days than you\u2019ve ever lost to the vagaries of our planet\u2019s spin; I know I have. Let\u2019s just call it another reason why we live on the most remarkable planet out there.<\/p>\n","protected":false},"excerpt":{"rendered":"
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