The Moon will move far enough away from Earth that it will no longer completely obstruct the Sun. But predicting when this will happen poses numerous challenges.
The total solar eclipse visible Monday over parts of Mexico, the United States and Canada was a perfect confluence of the Sun and Moon in the sky. But it’s also the kind of event that has an expiration date: At some point in the distant future, Earth will experience its last total solar eclipse.
This is because the Moon is moving away from the Earth, so one day, millions or even billions of years from now, our nearest celestial neighbor will appear too small in the sky to completely obscure the Sun.
“We will only have annular eclipses,” said Noah Petro, a planetary scientist at NASA’s Goddard Space Flight Center, referring to “ring of fire” eclipses like the one that crossed the Americas in October.
But putting an exact date on Earth’s last total solar eclipse is a serious computational challenge that involves a wide variety of scientific disciplines.
Since it formed more than 4 billion years ago, the Moon has been moving away from the Earth. The Moon’s retreat is due to its gravitational interactions with our planet. The tides caused by this gravity cause the water in our planet’s oceans to slide over the seabed and along the edges of the continents. This creates friction that makes the Earth spin more slowly on its axis, said Mattias Green, an oceanographer at Bangor University in Wales.
The Moon shifts outward in its orbit in response to the Earth’s slowdown. Imagine a figure skater extending her arms and slowing down, Green said. “It’s the same physical principle, but in reverse.”
One of the first to predict the expansion of the lunar orbit was George Darwin, one of Charles Darwin’s sons. But his hypothesis, published in 1879, would not be verified until American astronauts and Soviet robots left devices known as retroreflectors on the lunar surface. Researchers could fire laser pulses at the mirrors of these suitcase-sized instruments and time the time it took for the light to make a round trip. In this way, scientists could accurately measure the distance to the Moon. In the early 1970s, researchers discovered that the Moon was moving away from Earth by about 3.8 centimeters a year.
About the speed at which human nails grow. “These are extremely small changes,” said Robert Tyler, a planetary scientist at NASA’s Goddard Space Flight Center.
But over hundreds of millions of years, the Moon will become noticeably smaller in the sky as it recedes. At some point, it will appear too small to completely block the Sun, and total solar eclipses will be a thing of the past.
To calculate the date of the last total solar eclipse, it is important to remember that both the Moon’s orbit around the Earth and the Earth’s orbit around the Sun are elliptical. That means that the distances between the Earth and the Moon and between the Earth and the Sun are not constant. The apparent sizes of the Moon and Sun as seen from Earth vary accordingly; The largest and smallest moons differ in size by about 14 percent, while the corresponding difference for the Sun is about 3 percent.
The last total solar eclipse will occur when the larger moon barely covers the smaller sun. A mathematical calculation of the diameter of the Moon and the apparent sizes of the Moon and Sun gives an estimate of approximately 620 million years.
However, researchers caution that this figure is uncertain. To begin with, it assumes that the Moon will move away from the Earth at the current rate. And that almost certainly won’t happen, Green said.
The speed of the Moon’s retreat depends on a number of parameters, such as the length of the day on Earth, the depth of the ocean basins and the arrangement of the continents. According to Green, these factors change over time, so it is too simplistic to assume that the Moon will always recede at the same rate.
Most researchers agree that the Moon’s rate of retreat will likely slow. “If I had to guess, future tides will probably be weaker,” says Brian Arbic, a physical oceanographer at the University of Michigan. Weaker tides translate into a slower lunar retreat, which would give our planet more opportunities to enjoy the Moon’s threshold shadow.
There is strong evidence that the Moon also receded more slowly in the past. Margriet Lantink, a geologist at the University of Wisconsin-Madison, has analyzed sedimentary rocks from Australia that record climate changes caused by fluctuations in Earth-Moon distance. “I read the fingerprints of those astronomical variations,” Lantink said.
His team’s findings and those of other researchers have been used in simulations that suggest the Moon receded between 1 and 3 centimeters a year for much of its history. These simulations also reveal that, during some periods of several tens of millions of years, the Moon moved away from the Earth at more than 10 centimeters per year.
Tyler’s models face the difficult task of predicting the future lunar recession rate. They suggest that the Moon will move away an average of 0.7 centimeters a year over the next billion years. And the Moon’s retrograde in the future will not be nearly as variable as it was in the distant past, she said. “Most of the interesting stuff has already happened.”
If Tyler’s simulations are correct, total eclipses will remain visible for about 3 billion years. He cautioned that there is significant uncertainty in that estimate.
And while we’re likely eons away from experiencing total eclipses, that’s no excuse not to pursue their splendor, Petro said. After all, it is a unique celestial phenomenon in our terrestrial existence.
“No other planet in our solar system has total solar eclipses,” Petro said. “We have this wonderful opportunity.”
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