You’ve probably heard a lot about the upcoming solar eclipse on August 21, 2017. You may have also heard that solar eclipses are a rare phenomenon. Total solar eclipses (when the Moon completely covers the Sun) occur approximately once every year and a half—that works out to two to three eclipses of all kinds each year and only two total solar eclipses every three years. This might not sound so rare to you, but the shadow the Moon casts on the Earth is, at most, 274 kilometers (170 miles) across. That shadow covers roughly 37,015 square kilometers (23,000 square miles) at any one time: only 0.01% of the total surface area of Earth! At most, a total eclipse can last about 7 minutes in one place, so the chances of a total solar eclipse coming to you during your lifetime are pretty low. For instance, the last total solar eclipse that passed through Washington, D.C., was in 1478, and the next one is not coming until 2444!
Now, what if I told you that total solar eclipses are becoming even rarer? What if I told you one day, the Earth will have its LAST total solar eclipse? Sorry to give you the news folks, but it’s true! See, total solar eclipses are only possible because the sizes of the Moon and Sun have the same ratio as the distance between them.
The Sun is 400 times wider than the Moon, so if the Moon was close to the Sun, it wouldn’t block very much of the Sun, which would make for a very wimpy “eclipse.” However, the Sun is about 400 times farther away from us than our Moon is. Therefore, when the Sun, Moon, and Earth align, the Moon is just the right size to cover up the Sun’s surface from the perspective of someone on Earth.
When this alignment completely covers up the surface of the Sun, we call that a total solar eclipse. However, sometimes the Moon appears a little smaller in our sky and can only cover the center of the Sun, leaving the outer ring of the Sun’s surface visible. We call this type of eclipse annular, since annular means ring-shaped. You might be thinking, “Wait, didn’t you just say the Moon was the right distance to cover up the Sun? Why doesn’t the Moon always cover the Sun, then?” The answer: The Moon’s orbit around the Earth is not circular.
The Moon follows an elliptical, or oval, orbit around Earth, so sometimes the Moon is closer to Earth and other times it is farther. So, when the Moon is farther from Earth when the Sun, Moon, and Earth align, the Moon appears smaller to us, and we only see an annular eclipse. In the distant future, annular eclipses will be the only type of eclipses we will see from Earth because the Moon’s orbit around Earth is getting larger. What can we blame for our Moon drifting away? Our oceans, sort of.
The Moon and Sun’s tidal gravitational forces on Earth pull on our oceans, creating our daily tides. Tidal force just means that the gravitational force varies, getting weaker when the Moon is farther from the Earth. Since liquids like our ocean water move around more easily than the rocky material that make up our planet, the water gathers in big bulges. This creates two big bulges of water on the Earth: one bulge slightly ahead of the Moon in its orbit, and one on the opposite side of Earth. Even though the Moon’s pull is what creates the water bulge, the bulge leads the Moon because the Earth’s rotation pulls it ahead. Since the leading bulge makes the mass of the Earth lopsided, it creates an extra gravitational pull on the Moon. That extra gravitational pull slows down the Earth’s rotation and accelerates the Moon into a larger orbit, both to conserve angular momentum.
Caption: The black bulges on the Earth represent Earth’s oceans bulging out from the planet because of the Moon, and partially the Sun’s, tidal gravitational pulls. The bulges do not line up with the Moon because the Earth is spinning and pulling the bulges ahead.
Source: Tufts (https://ase.tufts.edu/cosmos/print_images.asp?id=43 )
If the Moon’s orbit is getting larger, then it is getting farther and farther from Earth. One day, the Moon will be so far that it can no longer completely cover the Sun from view, no matter where the Moon is in its elliptical orbit around the Earth. That means only annular eclipses for Earth that cover less and less of the Sun. So, total solar eclipses are truly in limited quantity. The total eclipse this August will check another off the list that is ever shortening, so catch a total solar eclipse when you can!