Do We Need A ‘Drop Second?’ The Worrying Reason Why Earth May Be Speeding Up After Decades Of Slowing Down

Have you got a second? Thought to be gradually slowing down, our planet is suddenly spinning faster than it has for 50 years—and you need to know why. 

Having gotten used to adding a “leap second” every now and then to keep their atomic clocks accurate, international timekeepers are now mulling over whether to add the first-ever “negative leap second” or “drop second.”  

That’s because last year saw the shortest day—so the fastest rotation—since people started counting. In fact, 2020 included the 28 shortest days since 1960. 

So what’s going on? Why is Earth rotating faster? And does it matter? 

The answer, as you might have guessed, could be sinister—the melting of the glaciers may be making Earth spin faster in space.  

How quickly does Earth spin? 

Earth usually rotates about its axis bang on time, with one rotation taking exactly 86,400 seconds (1,440 minutes or 24 hours). That’s one solar day, which we humans judge by the apparent motion of the Sun across the sky, though it’s obviously caused by the rotation of the Earth.

However, our planet doesn’t always work like clockwork. Our planet tends to spin more slowly. 

As of 2020 the planet has stopped slowing down and is now speeding up. A day now lasts for half a millisecond less than 24 hours. 

So what?

Why 2020 was a slow year for our planet

The record for the shortest rotation since 1973 was July 5, 2005, when the Earth’s rotation took 1.0516 milliseconds less than 24 hours, according to Graham Jones and Konstantin Ikos’ blog “Earth Is in a Hurry in 2020” for TimeandDate.com.

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

Why does it matter how fast Earth is spinning?

That might not seem like a lot, but it has big consequences over time because atomic clocks—which are used in GPS satellites—don’t take into account the Earth’s changing rotation.

If Earth spins faster then it gets to the same position a little earlier. A half-a-millisecond equates to 10-inches or 26 centimetres at the equator. In short, GPS satellites—which already have to be corrected for the effect of Einstein’s general relativity theory (the curve of space and time)—are quickly going to become useless.

There are also potentially confusing consequences for smartphones, computers and communications systems, which synchronize with Network Time Protocol (NTP) servers. Defined as the number of seconds since 00:00:00 UTC on January 1, 1970, standard Unix time is not designed to cope with leap seconds, though a “drop second” would be less problematic.

How solar eclipses help

The change in the speed of Earth’s rotation makes estimating split-second events more difficult, such as solar eclipses.

In fact, it’s records of observations of eclipses by Babylonian, Chinese, Arab and Greek astronomers that are one method of working out how fast the Earth must have been rotating in the past.

It’s possible to predict the exact time of a solar eclipse way into the future, but it’s more difficult to predict exactly where it will occur.

Why is Earth rotating faster?

Earth’s rotation is actually slowing over the centuries—right now is an exception.

The Earth’s rotation can change slightly because of a lot of factors. The pressure, seismic activity and the general motions of our planet’s inner molten core is a big factor, and it may even rotate just a bit faster than the planet as a whole.

Other seismic activity such as earthquakes also have a small effect, as do the atmosphere, the weather, the oceans and the Moon. The latter is gradually slowing down the Earth’s rotation because its gravitational pull causes tides and makes the Earth’s orbital path around the Sun slightly elliptical. 

However, one theory suggests that Earth’s slowing rotation may be our fault. Some research suggests that the melting of glaciers in the 20th century put less weight on rock at the poles, which therefore rebound upwards and makes the planet rounder. The effect of that could make it spin more quickly and could even slightly change its tilt.

Why Earth stopped in 2020

So 2020 was an exception, and in most years the average day lasts for slightly more than 24 hours. Having gotten used to Earth’s gradual slowing since 1973, the International Earth Rotation and Reference Systems Service (IERS) has been inserting extra leap seconds into atomic clocks every two or three years. That last happened on December 31, 2016, June 30, 2015 and June 30, 2012. 

(The IERS tends to add (or, in theory, subtract) leap seconds each June or December, announcing its decision many months beforehand.)

Some excess has therefore has been built-up since the last addition of a second, but in 2020, the Earth stopped. Its average day length was almost exactly 24 hours. Over the year it orbited the Sun a mere 1.28 milliseconds faster than average.

So the IERS just announced merely that it won’t be adding a leap second this coming June. It’s possible that it will consider subtracting a second in the future, which would be a first for IERS.

Why Earth will speed-up in 2021

That’s because in 2021 it’s predicting that Earth will continue to speed-up and by the end of the year be in deficit by 35.40 milliseconds. 

The actual length of Earth’s rotation (known as UT1) right now is slightly behind Coordinated Universal Time (UTC), the standard used to keep time across the world (which follows International Atomic Time (TAI).

2021 will see us accumulate a 19 millisecond lag in atomic time, and if the recent acceleration in Earth’s spin continues then we’ll need something new to time-keeping on Earth—a negative leap second. That will mean a day lasting 86,399 seconds. 

Do we really need to add a second to atomic clocks some years and subtract them the next to keep UTC close to the mean solar time?

A faster-spinning Earth has consequences. In our era, one them may be that the very concept of adjusting atomic clocks becomes a complete waste of time. 

Wishing you clear skies and wide eyes.