Why Earth will one day have its last total solar eclipse – 04/10/2024 – Science

The total solar eclipse visible last Monday (8) in parts of Mexico, the United States and Canada was a perfect confluence of the Sun and the Moon. But this type of event comes with an expiration date: sometime in the distant future , Earth will experience it no more.

That’s because the Moon is moving away from Earth, so our closest celestial neighbor will one day, millions or even billions of years in the future, appear too small in the sky to completely obscure the Sun.

“We’re only going to have annular eclipses,” says 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 last year.

But determining an exact date for Earth’s last total solar eclipse is a serious computational challenge involving a variety of scientific disciplines.

Since the Moon formed more than 4 billion years ago, it has been moving away from Earth. This process results from its gravitational interactions with our planet.

Tides raised by this gravity cause water in the oceans to slide over the seabed and along the edges of continents. This creates friction that causes the Earth to rotate more slowly on its axis, explains ocean scientist Mattias Green of Bangor University in Wales.

The Moon moves outward in its orbit in response to Earth’s slowing. Imagine a skater extending her arms and slowing down, Green said. “It’s the same physical principle, but in reverse.”

One of the first people to predict the Moon’s expanding 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 robotic rovers left devices known as retroreflectors on the lunar surface.

Researchers could fire laser pulses at the mirrors of these briefcase-sized instruments and time how long it took the light to make a round trip. This gave scientists a way to precisely measure the distance to the Moon. Until the early 1970s, researchers discovered that the satellite was moving away from Earth 3.8 centimeters per year.

This is approximately the rate at which human fingernails grow. “We’re dealing with extremely small changes,” says planetary scientist Robert Tyler of 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 cover the Sun, and total solar eclipses will become a thing of the past.

To calculate the date of the last total solar eclipse, it is important to remember that the Moon’s orbit around the Earth and the Earth’s orbit around the Sun are elliptical. This 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 appearances of the Moon differ by about 14% in size, while the corresponding difference for the Sun is about 3%.

The last total solar eclipse will occur when the Moon with its largest appearance covers only the Sun with its smallest appearance. A little math involving the diameter of the Moon and the apparent sizes of the Moon and Sun provides an estimate for this event of approximately 620 million years.

But there is uncertainty in this number, researchers warn. This assumes, to begin with, that the Moon will move away from Earth at its current rate. And that almost certainly won’t happen, Green said.

The Moon’s receding rate is affected by a number of parameters, he points out, including the length of a day on Earth, the depth of ocean basins and the arrangement of our continents. These things change over time, Green adds, so it’s too simplistic to assume the Moon will always move away at the same rate.

If Tyler’s simulations are correct, total eclipses will remain visible for about 3 billion years. He cautioned that there is significant uncertainty in this estimate.

And while we probably have eons to experience total eclipses, that’s no excuse not to seek out their splendor, says Petro. After all, they are a celestial phenomenon unique to our earthly existence.

“No other planet in our Solar System has total solar eclipses,” Petro said. “We have this wonderful opportunity.”