The Final Total Eclipse Is a Long Way Off
The total solar eclipse visible on April 8 over parts of Mexico, the United States and Canada was a perfect confluence of the sun and the moon. But at some point in the distant future, Earth will experience its last total eclipse.
That is because the moon is drifting away from Earth and will one day, millions or billions of years in the future, appear too small in the sky to completely obscure the sun.
“We’ll only ever have annular eclipses,” said Noah Petro, a planetary scientist at NASA Goddard Space Flight Center.
But putting an exact date on Earth’s final total solar eclipse is a serious challenge.
Ever since the moon formed over four billion years ago, it has been spiraling away from Earth. The moon’s retreat results from its gravitational interactions with Earth. Tides raised by that gravity send the water in our planet’s oceans sliding over the seafloor and along the edges of continents. That creates friction that causes Earth to spin more slowly on its axis. The moon moves outward in its orbit in response to the slowing of Earth.
By the early 1970s, researchers had discovered that the moon was receding from Earth by about 3.8 centimeters each year.
Both the moon’s orbit around Earth and Earth’s orbit around the sun are elliptical, meaning the distances between Earth and the moon and between Earth and the sun are not constant. The apparent sizes of the moon and the sun as seen from Earth vary accordingly; the largest- and smallest-looking 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 largest-looking moon just barely covers the smallest-looking sun. A bit of math involving the moon’s diameter and the apparent sizes of the moon and the sun yields an estimate for that last eclipse of about 620 million years.
But there is uncertainty in that number. It assumes, for starters, that the moon will recede from Earth at its current rate. And that almost certainly will not happen, said Mattias Green, an ocean scientist at Bangor University in Wales.
The moon’s recession rate is affected by many parameters, including the length of a day on Earth, the depth of the ocean basins and the arrangement of our continents. Those things change over time, Dr. Green said.
Most researchers agree that the moon’s recession rate will probably decrease. “The tides of the future will probably get weaker,” said Brian Arbic, a physical oceanographer at the University of Michigan. Weaker tides translate into slower lunar retreat, buying our planet
As Earth spins slower, the moon drifts farther away.
more opportunities to bask in the moon’s umbral shadow.
There is evidence the moon retreated more slowly in the past as well. Margriet Lantink, a geologist at the University of Wisconsin-Madison, has analyzed sedimentary rocks in Australia that record climatic changes caused by fluctuations in the Earth-moon distance.
Her team’s findings have been used in simulations that suggest the moon receded by about one to three centimeters per year for much of its history. Those simulations also reveal that during some periods lasting a few tens of millions of years, the moon hurtled away from Earth at more than 10 centimeters per year.
Simulations by Robert Tyler, a planetary scientist at NASA Goddard Space Flight Center, suggest that the moon will drift away around 0.75 centimeters per year on average for the next several billion years. And the moon’s retreat in the future will not be as variable as it was in the ancient past, he said.
If his simulations are correct, total eclipses will remain visible for about three billion years. He cautioned that there is significant uncertainty in that estimate.