How fast our planet turns—and, in turn, how long our day is—is not a stagnant thing. In fact, it changes every 5.9 years, according to a new study published in Nature. The Earth spins slightly faster or slower on a regular cycle, adding or subtracting milliseconds to our 24-hour day; LiveScience uses the analogy of a figure skater opening or closing her arms to explain the phenomenon. While it might seem like a miniscule change—especially when earthquakes can basically do the same thing—scientists are stumped as to the cause.
The study looked at decades' worth of data to find the changes occurring in regular intervals, which rules out the sun's energy flux as the source. While the true cause is still a mystery, it must come from inside the Earth's core, reports Science News. Researchers also found a correlation between the changes and abrupt alterations in our planet's magnetic field known as "geomagnetic jerks," which could help explain that mysterious behavior, too. For now, the findings will help improve the geomagnetic forecasts for mining and drilling.