March 8, 2021

The Moon is moving away from the Earth • Trends21


The Moon is moving away from the Earth, the days are getting bigger and the years have fewer and fewer days due to the dynamics of the Earth-Moon system, which has changed over time: we have had years of 407 days and days of 16 hours, found in the fossil records.

On July 21, 1969, an hour before the end of their last moonwalk, Neil Armstrong and Buzz Aldrin placed a scientific instrument on the Moon. It has been working perfectly for more than 50 years: it is the “lunar laser measurement retro-reflective array”, a set of 100 cubic reflector mirrors pointed at the Earth.

If we send a laser pulse from a ground-based telescope against that mirror, the light will reach the Moon and will be reflected back to Earth in exactly the same direction it arrived. By measuring the time it takes for these laser pulses to come and go from the Moon, we can know at what exact distance our satellite is.

For more than 50 years we have been measuring exactly this distance. The experiment confirms the predictions of the theory: the Moon is getting farther and farther away, increasing in distance at a rate of 3.78 cm per year.

That the Moon would progressively move away from the Earth is something that physicists have known for a long time.

The Moon generates tides in the Earth’s oceans. Tides “consume” an immense amount of friction energy. This friction slows the rotation of the Earth.

In this way, the Earth rotates more slowly and, as a consequence of the conservation of the total rotational energy of the Earth-Moon system, our satellite is moving away from us.

In reality, the Earth slows its rotation very little by little and, consequently, the Moon moves away from us very slowly. But in the deep time of geological ages it shows a lot.

Unusual days

100 million years ago, in the middle of the Cretaceous (the true age of the Tyrannosaurus and velociraptors), the days lasted 23 and a half hours. At the beginning of the Paleozoic, when the extraordinary proliferation of complex plants and animals began 540 million years ago, the day lasted only 21 hours.

This loss of speed of rotation of the Earth and the consequent movement away from the Moon does not occur at a constant speed. It is estimated that the day increases in duration between 8 and 27 seconds every million years (with an average of about 18 seconds per million years).

This is due to the fact that the terrestrial coastline changes shape slowly and the friction of the tides is affected: less friction slows down the rotation of the Earth.

Also the height of the continents (especially their highest mountain ranges) changes little by little during geological eras and affects the speed of rotation of the Earth. It is a phenomenon similar to what an ice skater does: she sticks her arms to the body to spin faster and stretches them to spin slower.

But, despite these small and specific changes, the Earth will continue in its tendency to lose rotation speed. Every time the days will be bigger.

On the contrary, the average distance between the Earth and the Sun remains much more constant. Consequently, the time it takes for the Earth to go around the Sun changes much less.

Thus a curious phenomenon occurs. The years have fewer and fewer days. And this has left its mark on life.

Aldrin walking on the Moon in 1969, carrying in his right hand one of the reflecting mirrors that serve to measure the distance between the Earth and its satellite. Photo: NASA.

Geological footprints

As early as 1963, paleontologist JW Wells found that he could observe annual discontinuities in the growth in thickness of some species of coral (somewhat similar to the annual growth rings of trees). But, looking at them under the microscope, he found that they also had daily growth discontinuities.

Because the coral fossil record is very good, Wells used it to date how old the years were in different geological eras. He was thus able to verify that the fossil corals of the Middle Devonian (375 million years ago) had lived in years that were 407 days old. Most modern corals show years with fewer days. Paleontology could date the length of days to other times.

But we can look for evanescent traces of this loss of Earth’s rotation even older. They left them in various groups of microalgae. Like corals, microalgae left behind their microfossils, by which we can date them.

We know that cyanobacteria, one of the first photosynthetic microorganisms on Earth, originated about 3 billion years ago, when days should only be about 16 hours.

Dinoflagellates are more modern, but old enough that they appeared when days should be around 20 hours. There are also microalgae, such as Conjugates, which appeared very recently in geological terms, when the days were more than 23 and a half hours.

These microalgae show a synchrony of their cell division cycle. If we grow a cyanobacteria in a chamber that generates “short” days of 8 hours of light and 8 hours of darkness, we will verify that they are capable of synchronizing their cell division. They all divide together at the same time, right at the end of the 8 hour dark phase. But dinoflagellates and conjugates can’t do it.

If we grow these algae in a chamber that generates “average” days of 10 hours of light and 10 hours of darkness, both the dinoflagellates and the cyanobacteria will be able to synchronize, but the much more modern conjugated algae cannot.

The Conjugates, which appeared very recently, only manage to synchronize their cell division on days of 12 hours of light and 12 hours of darkness. Like fossil corals, microalgae keep the memory of how long the days lasted at the time they appeared.

Now our devices are so accurate that we can detect even the smallest changes in the Earth’s rotation. Thanks to this we have even been able to measure the effect of climate change: last year the earth’s rotation registered an unexpected acceleration due to global warming and its impact on the earth’s orography.

Much of the snow on the mountain peaks has melted, the mountains are a little lower, so the Earth has reduced its diameter a bit and its rotation speed has been a little faster than if the snow had not melted.

But the loss of the rotation speed of the Earth will continue. And if tidal-based renewable energies proliferate, their effect will also be noticeable by slowing down rotation.

With effects of less than 1 / 50,000 of a second a year we can measure it. But it will have no consequences in our lives.

Reference

Costas, S. González-Gil, V. López-Rodas & A. Aerie. The influence of the slowing of Earth’s rotation: a hypothesis to explain cell division synchrony under different day duration in earlier and later evolved unicellular algae. Helgolander Meeresunters. 50, 117-130 (1996). DOI: https: //doi.org/10.1007/BF02367140

Top photo: The Moon and Earth mark distances. Credit: CC0 Public Domain



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