Why the Earth started to spin faster

You didn't realize it, but July 19, 2020 was perhaps the shortest day of your existence!

On that date and on 27 other occasions last year, the Earth broke the daily rotational speed record that it had recorded in 2005. An acceleration each time imperceptible, of the order of a few fractions of a second .

But such a shortening of days would not have been observed since quartz clocks in the 1930s revealed these subtle variations.

In an article in the Telegraph, the British physicist Peter Whibberley announced in early January "that the Earth is spinning faster now than at any other time in the past fifty years".

If the phenomenon is of concern, these changes in the speed of rotation of the Earth are not new.

In recent decades, our planet has sometimes tended to slow down, before accelerating again, more or less in cycles of about 13 years.

"These decadal variations are a phenomenon that is poorly explained," says Christian Bizouard, astronomer at the International Earth Rotation Service at the Paris Observatory.

The commonly accepted hypothesis is that they would be induced by interactions between the fluid core of the Earth and its mantle.

But we only have theoretical models.

"

The lesson of eclipses and corals

Other deviations, of lesser magnitude (about a millisecond, on average), are repeated every month under the influence of the Moon.

“The tides explain about 20% of seasonal oscillations.

The rest is mainly linked to atmospheric variations, ”continues Christian Bizouard.

Other, more marginal factors can intervene such as the melting of ice and snow!

And then there are very long trends.

On a millennium scale, the Earth would rotate less and less quickly.

The experts of the time realized this by crossing their dating of the eclipses of the past with the accounts of distant civilizations, from the Babylonians to the Romans, including the Egyptians, the Greeks or the Chinese!

Current models allow us to know when precisely these alignments with the Sun and the Moon occurred, not where they were observed.

A gap filled thanks to the ancient chronicle of these events which have always fascinated humanity.

Hence this incredible discovery: in the space of 4000 years, the days would have lengthened from 60 to 70 seconds!

In total, it is a dozen hours that the Earth would have let slip by in forty centuries!

The study of streaks on coral fossils has shown that 400 million years ago, a day lasted 22 hours!

How is it possible ?

"In the very long term, it is believed that the Earth is decelerating due to friction between the oceans and the seabed", underlines Christian Bizouard.

One second less in 2026?

The problem with humans today is that they insist on seeing specific periods of 24 hours in the day!

They even organize their activities around Coordinated Universal Time (UTC), against which the world's watches lock and shift, to the nearest half hour.

In 1972, the international authorities decided to synchronize this UTC with international atomic time (TAI), developed from hundreds of ultra-precise atomic clocks around the world: between the two, there may be a few seconds of lag but not a nanosecond more.

However, aware of the unstable nature of the Earth, they agreed that this UTC could not deviate too much from the “real” duration of the day, the time that the planet takes to turn on itself.

However, for decades, this celestial top has rotated in just over 24 hours.

Since the 1970s, it has therefore been necessary to add 27 "leap seconds" to universal time in order to eliminate this slight delay.

But there was 2020. The average length of the "real" day then became less than 24 hours.

If the phenomenon persists, the International Earth Rotation Service in Paris could this time have to remove, for the first time, a second from the reference time.

“I would make the decision,” recalls Christian Bizouard.

“I did my calculations.

If the trend continues at the rate of -0.3 milliseconds per year, I will have to remove one second in 2026. "

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Accused of generating bugs or of being a vestige of another time, the "leap seconds" are no longer in the odor of sanctity and many are those who militate for their elimination. “This system is a bit obsolete,” admits Christian Bizouard. "It corresponded to a technical need in the 1970s. Today, atomic time, more regular, is unbeatable for all kinds of applications, civil, military ..." But this "master of world clocks", title that Christian Bizouard politely rejects, tempers: “We have time to see. Nothing says that the trend will continue after 2021. "