© Stocktrek Images via Getty Images
Many of us wish we could get through this difficult year as quickly as possible. Seems the Earth feels the same way — it has been spinning unusually fast lately. 2020 included the 28 shortest days since 1960.

Atomic Clocks Expose Earth's Irregular Speed

The Earth is an excellent timekeeper: on average, with respect to the Sun, it rotates once every 86,400 seconds, which equals 24 hours, or one mean solar day.

But it is not perfect. When highly accurate atomic clocks were developed in the 1960s, they showed that the length of a mean solar day can vary by milliseconds (1 millisecond equals 0.001 seconds). These differences are obtained by measuring the Earth's rotation with respect to distant astronomical objects, and using a mathematical formula to calculate the mean solar day.

2020 Had Shortest Days on Record

Before this year began, the shortest day since 1973 was July 5, 2005, when the Earth's rotation took 1.0516 milliseconds less than 86,400 seconds.

But in the middle of 2020, the Earth beat that record no less than 28 times. The shortest day of all came on July 19, when the Earth completed its rotation in 1.4602 milliseconds less than 86,400 seconds.

The speed of the Earth's rotation varies constantly because of the complex motion of its molten core, oceans and atmosphere, plus other effects.

Variation of day length throughout 2020. The length of day is shown as the difference in milliseconds (ms) between the Earth's rotation and 86,400 seconds.
2021 Is Predicted to Be Even Shorter

Scientists monitoring the Earth's rotational speed expect the trend of having shorter days to follow us into 2021 as well.

According to their calculations, an average day in 2021 will be 0.05 ms shorter than 86,400 seconds. Over the course of the entire year, atomic clocks will have accumulated a lag of about 19 ms. For comparison: in past years, they ran fast by a few hundred milliseconds per year. See yearly averages since 1973

In fact, the year 2021 is predicted to be the shortest in decades. The last time that an average day was less than 86,400 seconds across a full year was in 1937.

Leap Seconds Keep Us in Sync with Earth

If the Earth's rotation gets too far out of sync with the super-steady beat of atomic clocks, a positive or negative leap second can be used to bring them back into alignment.

Since the system of leap seconds was introduced in 1972, the Earth's rotation has generally been a bit sluggish. So far, there have been 27 leap seconds, and they have all been positive. In other words, they have all added an extra second to our clocks, enabling the Earth to catch up.

Recently, however, the Earth has been getting quicker, and no leap second has been required since 2016. If the Earth's rotation continues to quicken, we may at some point require a negative leap second. If this happens, our clocks would skip a second, in order to keep up with the hurrying Earth.

How We Know That Earth Is Speeding Up

Official measurements of the length of day are made by the International Earth Rotation and Reference Systems Service (IERS).

To determine the true length of a day, IERS scientists determine the exact speed of the Earth's rotation by measuring the precise moments a fixed star passes a certain location in the sky each day. This measurement is expressed as Universal Time (UT1), a type of solar time.

UT1 is then compared to International Atomic Time (TAI), a highly precise time scale that combines the output of some 200 atomic clocks maintained in laboratories around the world.

The true length of a day is expressed by the deviation of UT1 from TAI over 24 hours.