Total Eclipses Will End as Moon Slips Slowly Away

The moon’s average distance is inching  away from the earth and at some point total eclipses will no longer be possible because the moon will appear smaller and will no longer block the full disk of the sun.

In 1695, Edmond Halley discovered that eclipses recorded in ancient history did not match calculations for the times or places of those eclipses.

Starting with records of eclipses in his day and the observed motion of the moon and sun, he used Isaac Newton’s new theory of universal gravitation (1687) to calculate when and where ancient eclipses should have occurred and then compared them with eclipses actually observed more than 2,000 years earlier.

They did not match. Halley had great confidence in the theory of gravitation and resisted the temptation to conclude that the force of gravity was changing as time passed.

Instead, he proposed that the length of a day on Earth must be slowly increasing. The Earth’s rotation must be slowing down.

If the Earth’s rotation had slowed down slightly, the moon must have gained angular momentum to conserve the total angular momentum of the Earth-moon system. This boost in angular momentum for the moon would have caused it to spiral slowly outward from the Earth to a more distant orbit where it travels more slowly.

If, 2,000 years earlier, the Earth had been spinning a little faster and the moon had been a little closer and orbiting a little faster, then eclipse theory and observation would match.
Scientists soon realized that Halley was right. But what would cause the Earth’s spin to slow?


The gravitational attraction of the moon is the principal cause of the ocean tides on Earth. As the shallow continental shelves (primarily in the Bering Sea) collide with high tides, the Earth’s rotation is retarded. The slower spin of the Earth causes the moon to edge farther from our planet.
From 1969 to 1972, the Apollo astronauts left a series of laser reflectors on the moon’s surface. Since then, scientists on Earth have been bouncing powerful lasers off these reflectors.

By timing the round trip of each laser pulse, the moon’s distance can be measured to an accuracy of several inches. The moon is receding from the Earth at the rate of about 1.5 inches (3.8 centimeters) a year.
As the moon recedes from Earth, its apparent disk becomes smaller. Total eclipses become rarer; annular eclipses more frequent.
Total eclipses are moving toward extinction. …

When the moon’s mean distance from the Earth has increased by 14,550 miles (23,410 kilometers), the moon’s apparent disk will be too small to cover the entire sun, even when the moon’s elliptical orbit carries it closest to Earth.
Total eclipses will no longer be possible.
How long will that take? With the moon receding at 1.5 inches a year, the last total solar eclipse visible from the surface of the Earth will take place 620 million years from now.


The human race has little to fear at present. By the time any change occurred, humans might even have generated technology that could speed up the Earth’s rotation or transport us to other liveable planets within our galaxy.


 A long time ago the Mars-sized object Theia, collided with the Earth and the Moon was formed out of the debris from the collision. …

620 million years ago, a day was only 21 hours long. Now they’ve dragged out to 24 hours and they’re just getting longer, and the Moon is already at a average distance of 384,400 km. It almost feels too far away. …

… there is an end in sight.  50 billion years from now, 45 billion years after the Sun has grown weary of our shenanigans and become a red giant, when the days have slowed to be 45 hours long, the Moon will consider itself all moved into its brand new apartment ready to start its new life.


A full solar eclipse is a pretty amazing thing and I hope you’ve seen one or will get a chance in the future.

Here are the next ones on the schedule. (If you experience an unscheduled total solar eclipse, check that you are awake, check for motherships, etc. )


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