I was listening to one of my favorite science podcasts, The Skeptics Guide to the Universe, when one of the hosts casually said that the Earth would someday be tidally locked to the Moon. I’d never heard of this before, so I had to find out when this would happen.
Most of the moons orbiting planets in our solar system are tidally locked. It is much rarer for a planet to be tidally locked to its moon. The only one is Pluto (not technically a planet.) It and its moon, Charon, are tidally locked to each other. However, these two bodies are almost the same size, and Pluto is unusual in many other ways.
So can the Earth become tidally locked to the Moon? Sure. There is a great explanation in Universe Today, but the basic idea is that the Moon causes tides on the Earth, which slows down the Earth’s rotation. The Moon is slowly moving away from the Earth in reaction to this slowing.
In 50 billion years or so, the Earth’s rotation will be slow enough that it will tidally lock with the Moon. When that happens, half the creatures living on the Earth will never see the Moon, and only a fraction on the near side will see a full moon. One could assume that the tides would be drastically different too.
However, no living thing will ever see this phenomenon.
The Earth may become tidally locked to the Moon in 50 billion years, but the sun will expand and become a Red Giant in 7.5 billion years. It will consume the Earth and the Moon at that time.
What a bummer. I like the idea that viewing the Moon would be a destination vacation. And the amazing thing is, if it were possible, no one living at that time would know anything different. They would think a tidally locked Earth/Moon was normal. Like the Plutonians and Charonians do today.