As the Earth rotates, the moon’s gravity moving over the oceans pulls the oceans toward it, causing tides. Tides are actually upwellings of water that expand in an oval shape both on the near side and on the far side of the moon’s gravity.
But the Earth spins on its axis at a faster rate than the Moon, which means that the friction of the subsurface ocean basins drags the water along. In this way, the Ascendant moves slightly ahead of the Moon in its orbit, which tries to pull it back. This slowly reduces the Earth’s rotational energy and slows its rotation, while the Moon gains energy and causes it to go into a higher orbit.
According to the latest analysis, this incremental braking of the planet’s rotation means that the length of an Earth day has increased by about 1.09 milliseconds per century on average since the late 1600s. Other estimates, using ancient observations of eclipses, put the figure slightly higher at 1.78 milliseconds per century. Although these numbers may not seem like much, they make a noticeable difference over the 4.5 billion year history of the Earth.
The moon is thought to have formed in the first 50 million years after the birth of the solar system. The most widely accepted theory about how the Moon formed is that the collision between the early Earth and another Mars-sized body (known as Theia) caused chunks of material and debris to break off, which then coalesced to form the Moon.
What we know from geological data preserved in rock bands on Earth is that the Moon was much closer to Earth in the past. The moon is currently 384,400 km away from us. But a new study shows that about 3.2 billion years ago (just as the tectonic plates began to move and microorganisms living in the oceans consumed nitrogen), the distance between the Moon and the Earth was only 270,000 kilometers, or about 70% of the current distance.
“A faster rotating Earth shortened the length of the day, so that there were two sunrises and two sunsets (in a 24-hour period),” said Tom Olenfeld, a geophysicist who led the study at Friedrich Schiller University in Jena, Germany. “This may have reduced the temperature difference between day and night, and it is possible that it may have affected the biochemistry of photosynthetic organisms.”
What studies like Ollenfeld’s reveal, however, is that the moon’s retrograde speed has not been constant and has increased and decreased over time.
A study by Vanina López de Azarevic, a geologist at the National University of Salta in Argentina, shows that around 550 to 625 million years ago, the Moon could have moved away from the Earth at a rate of 7 centimeters per year. “The speed at which the moon is moving away from Earth has definitely changed over time and will continue to change in the future,” says Olenfeld. “However, for most of its history, the Moon has been moving away from Earth at a much slower rate than today.”
We are currently living in an era where the rate of retrenchment is unusually high. At its current speed, the Moon would only have to retreat for 1.5 billion years to reach its current position. But this process has been happening since the formation of the moon 4.5 billion years ago, so it must have been slower at some point in the past.
“The tidal pull is now three times greater than we expect,” Waltham says. This may be due to the size of the Atlantic Ocean.
The current configuration of the continents means that the North Atlantic Basin happens to be exactly the size to produce the resonance effect, so that the water in it moves back and forth as fast as the tides. This means that the tides are larger than otherwise. To understand this, Waltham gives an example: think of pushing a child on a swing; If each push is in sync with the existing movement, he will rise.
“If the North Atlantic was a little wider or narrower, this wouldn’t have happened,” Waltham says. “According to these models, if you go back a few million years ago, the tides were less intense because the continents were in different positions at that time.” But this will probably change in the future. Modeling predicts that a new tidal intensification will appear over the next 150 million years and then disappear as a new supercontinent forms in about 250 million years.