Everyone knows that the days are 24 hours long, right? While this is generally true, scientists like to get a little more precise than that, and it is important for keeping the calendars on track and ensuring things like GPS technology remain accurate.

When measuring the actual length of a day, the measurement is exactly how long it takes for the Earth to rotate completely in relation to the sun.

And if it seems like the days have been getting shorter as you get older, you aren’t crazy, they really are (though not in a way that you could actually notice).

For decades now, scientists have been keeping track of the precise length of each day using atomic clocks. This allows them to add in a ‘leap second’ from time to time to ensure everything stays on track. Since 1972, they have added 27 leap seconds to our clocks in order to keep time flowing as expected (on the clocks and calendars anyway. Time itself obviously doesn’t care how we interpret it).

Since 2016, however, there haven’t been any leap seconds added, and nobody is really sure why. What they do know is that it means that the Earth’s rotation is speeding up, which goes against predicted trends. In 2021, Judah Levine, a physicist in the time and frequency division of the National Institute of Standards and Technology, told Discover Magazine:

“This lack of the need for leap seconds was not predicted. The assumption was, in fact, that Earth would continue to slow down and leap seconds would continue to be needed. And so this effect, this result, is very surprising.”

The increase in the speed at which the Earth rotates means that the days are indeed getting shorter, but not by much. For example, July 5nd, 2025, was (at least at the time) the shortest day on record, completing its rotation 1.66 milliseconds faster than the normal 24-hour day. Keep in mind that a millisecond is a millionth of a second, so it is not a noticeable amount for most situations.

July 22nd, 2025, was the second-shortest day on record, at 1.34 milliseconds faster than usual.

So, what is causing this? The short answer is, we don’t know.

Researchers, however, do know the types of things that cause the length of a day to change over time. They generally come down to two main things.

First is the gravity from the moon. As the moon shifts position in the sky, it is pulling on the Earth (and the Earth on it). This can have an effect on how fast the Earth is spinning.

Second, and much closer to home, is the mass of the Earth itself. If you have ever seen figure skaters, you know that when they are spinning around, they can change the speed at which they spin by pulling their arms in closer to their body or moving them out away. The Earth (and everything else that spins) works the same way.

When more mass is compressed in near the axis of a spinning object, it spins faster. Earthquakes, for example, can shift the mass of the Earth in various ways. If the mass shifts toward the axis, the spinning can get faster. An example of this occurred in March of 2011 when a magnitude 9.0 earthquake occurred off the coast of Japan.

This quake actually moved the main island of Japan by 2.4 meters (8 feet) and shifted the axis of the Earth itself by 17 centimeters (6.5 inches).

Then there is the constant shifting of mass deeper within the Earth, closer to the core. This can happen without humans noticing, and may account for the change in the speed of rotation.

In the end, while this change in the precise length of days is important for scientists and researchers, the average person won’t notice it at all, and, at least for now, it is just something that is interesting but not concerning.

If you thought that was interesting, you might like to read about the mysterious “pyramids” discovered in Antarctica. What are they?