We all know that a day on planet Earth is 24 hours long — except it isn’t. It’s actually a few fractions of a second longer, which is why we have February 29th every four years. Still, Earth isn’t a clock, and the actual length of a day can vary slightly. Scientists now say that the days have started trending shorter because the Earth is spinning faster, which could require additional, potentially confusing adjustments.
Before you get stressed about yet another existential threat to the future of the planet, scientists don’t believe the increased rate of spin is a danger. The mechanisms for this effect are well-understood. Factors like lunar gravity, snowfall levels, and mountain erosion can affect the speed at which the globe rotates. The effect is that days can be a few milliseconds shorter or longer than the 84,400 number we use to keep track of time.
Over the past several decades, the availability of precise atomic clocks has allowed humanity to make more accurate adjustments to our measurement of time. For example, on several occasions, we’ve added a “leap second” to keep clocks aligned outside of the traditional leap year adjustment. Since we began measuring time like this, the length of days has been trending longer, usually by a fraction of a millisecond. However, that trend has now reversed.
The moon’s gravity is one of several things that can change the Earth’s rotation speed.
Scientists note that days in the last year have been on the shorter side by the same small margins. However, July 19 was a notably shorter day, clocking in at 1.4602 milliseconds below the standard. The earlier record for the shortest day was set in 2005, but it’s been beaten 28 times in 2020.
Should this trend continue, we may very well need a negative leap second in the next few years to keep our clocks synchronized with “real time.’ That would be a first as all previous adjustments have added time. While it’s impossible to say for certain that this speed-up will continue, most scientists believe that it will. Regardless, the quicker rotation is itself not a problem — the causes might be, though. Some have started to wonder aloud if large-scale changes from global warming have started to have a noticeable impact on the spin. Shaving a few milliseconds off the day won’t hurt anyone, but it’s not exactly a good sign.
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