How the Moon Is Making Earth Days Longer – 03/13/2023 – Science

Throughout human history, the Moon has always been a mysterious and inseparable presence over the Earth. Its gentle gravitational pull sets the rhythm of the tides, and its pale light illuminates the nocturnal nuptial rituals of many species.

Entire civilizations have set their calendars around the Moon as it comes and goes. Some animals, such as dung beetles, use sunlight reflected off the moon’s surface to help guide them.

More fundamentally, some theories claim that the Moon may have helped create the conditions that made life possible on our planet, and even started life on Earth in the first place.

Its eccentric orbit around our planet is also believed to influence some of the important weather systems that dominate our lives today.

But the Moon is also moving away from us. With each turn of its extraordinarily balanced space ballet around Earth —in circles, but always without pirouettes, which explains why we always see the same side of the Moon—it gradually moves away from our planet, in a process known as “lunar recession”.

By firing lasers at reflectors installed on the lunar surface by astronauts on the Apollo mission, scientists have recently been able to measure with absolute precision the exact speed of the Moon’s retreat. They confirmed that the Moon is receding at a speed of 3.8 cm per year. And as that happens, our days get longer and longer.

“It’s all about the tides,” says David Waltham, professor of geophysics at the University of London Royal Holloway, who studies the relationship between the Moon and Earth.

“The tidal force on Earth reduces its rotation and the Moon receives this energy in the form of an angular impulse”, he adds.

Basically, as the Earth rotates, the gravity of the Moon orbiting our planet pushes the oceans to create high and low tides. These tides, in fact, are “volumes” of water that extend in an elliptical shape, against or in favor of the Moon’s gravity.

But the Earth rotates on its axis much faster than the Moon’s orbit, which means that friction from our planet’s moving ocean basins also drags the water with it. With that, the volume of water moves slightly ahead of the Moon in its orbit, which tries to pull it back.

This process slowly sucks energy from our planet’s rotation, slowing it down, while the Moon gains energy, causing it to move into a higher orbit.

This continued slowdown in our planet’s rotational speed means that the length of the average day on Earth has increased by about 1.09 milliseconds per century since the late 1600s, according to the last available analysis.

Other estimates put the figure slightly higher, at 1.78ms per century, based on observations of older eclipses.

None of these numbers seem worrying, but over the 4.5 billion years of the planet’s history, the accumulated change is very significant.

The Moon is believed to have formed about 50 million years after the birth of the Solar System.

The most accepted theory is that a collision between the Earth in formation and another object with the approximate size of the planet Mars, known as Theia, ripped a piece of material and fragments that aggregated to form what we now call the Moon.

Longer distance and longer days

What is clear from geological data preserved in rocky bands on Earth is that the Moon was much closer to Earth in the past than it is today.

Currently, the Moon is 384,400 km from Earth. But a recent study indicates that around 3.2 billion years ago — when Earth’s tectonic plates were starting to move and ocean microorganisms were devouring nitrogen — the Moon was only 270,000 km from Earth, or about 70% of your current distance.

“Earth’s faster rotation reduced the length of the day, so that [em um período de 24 horas]the sun rose and set twice, not just once, as it happens today,” explains geophysicist Tom Eulenfeld, who led the study at the Friedrich Schiller University in Jena, Germany.

“This could have reduced the difference in temperature between day and night and affected the biochemistry of photosynthetic organisms”, according to Eulenfeld.

What studies like his reveal is that the speed of lunar recession isn’t constant either — it has waxed and waned over time.

A study by geologist Vanina López de Azarevich, from the National University of Salta, Argentina, indicates that, around 550-625 million years ago, the Moon was perhaps receding by up to 7 cm per year.

“The speed at which the Moon moves away from Earth has definitely changed over time and will change in the future,” says Eulenfeld. But for most of its history, the Moon has moved away at a much slower rate than it does today.

Indeed, we are currently living in a period where the speed of recession is abnormally high.

The Moon would need to have moved away at its current speed for only 1.5 billion years to reach where it is today. But this process has been going on since the formation of the Moon, 4.5 billion years ago. Clearly, his removal was much slower in the past.

“The current tidal force is three times greater than expected,” says Waltham. The reason could be the size of the Atlantic Ocean.

The current configuration of the continents means that the North Atlantic basin happens to have exact proportions to generate a resonance effect, so that its water moves from one side to the other at a speed close to that of the tides. As a result, the tides are steeper than they normally would be.

Waltham explains that it’s like pushing a child on a swing. It will reach higher if each impulse is coordinated with existing movement.

“If the North Atlantic were a little wider or a little narrower, this wouldn’t happen,” says Waltham. “The models seem to show that if you go back a few million years, the tidal force was less because the continents were in different positions.”

But this shift is likely to continue into the future. Models predict that a new tidal resonance will emerge 150 million years from now. And a new “supercontinent” will only be formed in about 250 million years.

Can we then expect that, one day, the Earth will no longer have the Moon?

Even at its current rate of retreat, the Moon is not likely to completely leave Earth. The Sun’s tragic end will likely occur much sooner, in about 5-10 billion years. And humanity will likely have been extinct long before that.

But in the short term, humanity itself can help make the days a little longer by reducing the amount of water trapped in glaciers and ice sheets due to melting caused by climate change.

“The ice basically suppresses the tides,” explains Waltham. He notes that around 600-900 million years ago, our planet is believed to have entered a particularly freezing period known as the Ice Age. And at that time, the speed of the Moon’s retreat was dramatically reduced.

But it’s hard to predict the future impact, which will be partially offset by the landmasses that will be reclaimed as the weight of the ice sheets is lifted off them and by other complications.

Theoretically, the next group of astronauts who will fly to the moon in NASA’s Artemis program will be able to verify that they have seen their home planet from a greater distance than their Apollo program predecessors saw 60 years ago — although the point of its arrival during the elliptical orbit of the Moon is probably more decisive, since the distance between the apogee and the perigee varies by 43 thousand kilometers every 29 days.

And, for those who stick around, our lives are too brief to observe the picoseconds that are added to the duration of each passing day. If you blink your eyes, they’re gone.

Read the original version of this report (in English) on the BBC Future website.