The Moon's orbit has been growing ever larger-estimated at a slow rate of 3.8 centimeters annually-since prehistoric times. Factor millions of years past and Luna was closer to Earth; but factor millions of years hence, and Luna will be farther from the Earth.
"Tidal friction, caused by the movement of the tidal bulge around the Earth, takes energy out of the Earth and puts it into the Moon's orbit-making the Moon's orbit bigger, but a bit paradoxically, the Moon actually moves slower," according to Dr. Britt Scharringhausen of Beloit College in Wisconsin. Scharringhausen is a professor of astronomy and physics.
"The Earth's rotation is slowing down because of this. One hundred years from now, the day will be 2 milliseconds longer than it is now. This same process took place billions of years ago, but the Moon was slowed down by the tides raised on it by the Earth. That's why the Moon always keeps the same face pointed toward the Earth. Because the Earth is so much larger than the Moon, this process, called tidal locking, took place very quickly, in a few tens of millions of years," she said.
While it won't be as large in the sky as it is today, the Moon of the far future will-with a nod to Lord Byron-be still as bright 500 million years hence.
Scharringhausen writes that just because the Moon is moving away from us inexorably, it will most certainly not recede so far from us that it will fade from naked-eye view.
"Changing the Moon's distance by a few percent won't have any significant effect on our ability to see it," she notes. "Changing the Moon's average distance by a few percent-which is what will happen over the next 500 million years or so-will similarly not prevent us from being able to see the Moon, and to see it quite easily with the unaided eye."