Scientists don't understand why Earth's days have been lengthening mysteriously

Scientists are unsure what is causing the length of a day to suddenly lengthen, but atomic clocks and precise astronomical measurements have revealed it.

The impact of this extends beyond our timekeeping to other technologies that govern our lives in the modern age, including GPS.

A day's length is determined by how fast the Earth rotates around its axis over the past few decades. In June 2022, the shortest day in the past half a century was recorded; this trend has been making our days shorter.

The steady speedup has, however, since 2020 switched to a slower pace - the days are getting longer again, and the reasons remain a mystery.

Our phones display a 24-hour clock, but the actual amount of time it takes for Earth to complete one rotation of the planet varies slightly. Even earthquakes and storm events can cause these changes, which can take millions of years to occur.

It turns out that 86,400 seconds in a day are very rarely exactly accurate. As the tides driven by the Moon slow down Earth's rotation over millions of years, the rotation has slowed down. As a result of that process, the length of each day is added by about 2.3 milliseconds every century. Earth days used to last roughly 19 hours billions of years ago.

In the past 20,000 years, a different process has sped up Earth's rotation in the opposite direction. Melting polar ice sheets caused the Earth's mantle to move steadily toward the poles after the last ice age ended.

Just as a ballet dancer spins faster as their arms approach their body - the axis around which they revolve - so our planet spins faster as its mantle moves closer to Earth's axis. As a result, each day is shortened by about 0.6 milliseconds every century.

It takes decades and longer for the connection between Earth's interior and surface to take effect. There are times when major earthquakes can alter the length of the day, although usually by a small amount. A magnitude 8.9 earthquake in Japan in 2011 is thought to have sped up Earth's rotation by 1.8 microseconds.

Aside from these large-scale changes, weather and climate have significant impacts on Earth's rotation over shorter periods. As mass moves around the planet on a FORTWEEKLY and MONTHLY basis, the length of the day can change by up to a millisecond.

There is evidence of tidal variation in length-of-day records dating back 18.6 years. Ocean currents and the movement of our atmosphere play a particularly important role. In addition, seasonal changes in snow cover and rainfall, or groundwater extraction, alter the situation.