If you could venture back in time to the Neoproterozoic era, about 620 million years ago, you’d notice a radically different planet. Most observable life forms would be alien-looking fronds and worms, and, if that didn’t send you running back to your time machine, you’d notice that even the days were different. Hundreds of millions of years ago, a day was only about 22 hours long, the result of a planet spinning about its axis more rapidly than it is now.

For billions of years, Earth’s rotation has been gradually slowing down. It’s a process that continues to this day, and estimates suggest that the length of a day currently increases by about 1.8 milliseconds every century. The day's length varies slightly from year to year, as well; the result of myriad forces both on and off Earth pushing and pulling at its rotation.

Scientists still don’t fully understand all the factors that contribute to changes in the Earth’s rotation. But the advent of extremely sensitive instruments for measuring data and keeping time have allowed them to track variations in our planet’s spin down to the microsecond. The effort has revealed that Earth’s rotation is in constant, microscopic flux. Each day is different than the last.

What’s In a Day?

If you could venture back in time to the Neoproterozoic era, about 620 million years ago, you’d notice a radically different planet. Most observable life forms would be alien-looking fronds and worms, and, if that didn’t send you running back to your time machine, you’d notice that even the days were different. Hundreds of millions of years ago, a day was only about 22 hours long, the result of a planet spinning about its axis more rapidly than it is now.

For billions of years, Earth’s rotation has been gradually slowing down. It’s a process that continues to this day, and estimates suggest that the length of a day currently increases by about 1.8 milliseconds every century. The day's length varies slightly from year to year, as well; the result of myriad forces both on and off Earth pushing and pulling at its rotation.

Scientists still don’t fully understand all the factors that contribute to changes in the Earth’s rotation. But the advent of extremely sensitive instruments for measuring data and keeping time have allowed them to track variations in our planet’s spin down to the microsecond. The effort has revealed that Earth’s rotation is in constant, microscopic flux. Each day is different than the last.

What’s In a Day?

The Earth has been spinning like a top ever since it formed. The way celestial bodies develop, by a gradual gravitational accretion of rocks and dust floating in space, results in a natural rotational movement. But, once set in motion, that rotation is never constant. Forces within the planet itself, like the movement of its core and winds on its surface, affect how Earth spins, as well as external processes like the gravitational pull of other bodies.

Scientists today can measure even the smallest changes in the Earth’s rotation thanks to a technique known as Very Long Baseline Interferometry. It relies on space telescopes located far apart on Earth’s surface attuned to signals from outer space. As the Earth spins, these signals come in and out of view. By comparing the time it takes for a signal to disappear and reappear, scientists can calculate with great precision how long it’s taken for the Earth to make a full rotation.

Though many factors affect the planet’s rotation, by far the most important and long-lasting is our natural satellite, the moon. For billions of years, the moon has been ever so gently tugging at the Earth and slowing down its rotation. The moon’s gravity is the reason those Neoproterozoic days were shorter than ours today; it is also why the days millions of years from now will be longer still.

The mechanism boils down to an exchange of energy between the Earth and moon. The moon’s gravitational pull creates a slight bump in the solid surface of the Earth, near to, but not exactly underneath where the moon is. The disparity between the bump’s position and the moon’s pull creates a torque on both the Earth and moon with the end result that the Earth slows down gradually. That rotational energy is transferred to the moon, which is moving away from the Earth ever so slowly, at a rate of about an inch and a half every year.

It’s a process that has been going on ever since the moon began circling the Earth. Some studies have attempted to look even further back in time, and one group of researchers estimates that 1.4 billion years ago a day was just 18.7 hours. At that time, the moon was likely some 27,000 miles closer to Earth than it is now, they say.

Days of Our Lives

On shorter time scales, there are many different things that influence how quickly the Earth is rotating. Among the most significant of these is the motion of the Earth’s molten core. Turbulence within our planet’s liquid interior impacts the rotation of the planet as a whole, though it’s difficult to say by how much. Observations of the Earth’s core are difficult to make, and scientists still aren’t able to quantify exactly how much it influences the planet’s rotation.

On the surface of the planet, the motions of wind and waves also change how fast the Earth spins. The tides that slosh the oceans back and forth affect rotation speeds, and so does the wind. As air currents push against mountains and pull on the Earth’s surface through friction, they change the Earth’s rotation rate ever so slightly. During El Niño years, for example, the Earth spins slightly slower, due to the way the winds shift.

Seismic activity, too can affect how quickly Earth rotates. Following the 2004 earthquake that devastated Indonesia and other counties along the coast of the Indian Ocean, the Earth spun some three microseconds faster, conclude scientists from NASA’s Jet Propulsion Laboratory. The effect was caused by a slight change in the balance of Earth’s mass as continental plates shifted. Just a figure skater rotates faster when they pull their arms in, when mass on Earth moves closer to its center, the planet will spin more quickly, and vice versa.

This effect, the logical result of Newton’s laws dictating conservation of energy, leads to ongoing changes on Earth’s rotation rate today. As glaciers melt and sea levels rise, relatively more mass is flowing (in the form of meltwater) from near the poles to closer to Earth’s equator. That’s slowing the Earth down and gradually lengthening our days.

Earth’s rotation also varies seasonally, speeding up in the summer months of the northern hemisphere and slowing down in winter. That’s because the Earth’s orbit takes it slightly farther from the sun in summer and slightly closer in winter. When the Earth is closer in to the sun it moves slightly faster, and that causes a corresponding slowdown in its own rotation rate, again because of conservation of momentum.

You can see those seasonal swings in this chart tracking changes to Earth’s rotation rate since the year 2000. As it shows, Earth has been picking up speed in the last few years, after slowing down for much of the previous two decades. But don’t get too worried about your days slipping away from you — the change is minuscule, and in line with previous changes to the Earth’s rotational speed. So, while the days may never quite be the same length, changes to our planet’s rotation should be pretty far down the list of things to worry about.