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A thin crescent Moon highlights the western sky the next couple of evenings. It’s quite low as darkness falls this evening, but a little higher by Friday.
Sunlight illuminates only a small fraction of the lunar hemisphere that faces our way. Such a pronounced crescent is sometimes described as a “fingernail” Moon — the crescent looks like the curved edge of a fingernail.
And in a surprising way, that’s an appropriate description for the Moon itself. It’s moving away from Earth at a rate of about an inch-and-a-half per year. According to a study a few years ago, an adult’s fingernail grows at that same rate.
The Moon is inching away from us because it “steals” a bit of momentum from Earth through the tides. The Moon’s gravity raises tides in Earth’s oceans, and smaller tides in the solid crust. That causes Earth to spin slower on its axis. To maintain the overall balance in the Earth-Moon system, the Moon’s orbital speed increases, which pushes the Moon farther away.
Eventually, the Moon will move far enough away, and Earth will spin slowly enough, that the two will match. At that point, the system will be gravitationally locked, so the Moon will stop moving away from Earth. When that happens, the same side of Earth will always face the Moon, just as the same side of the Moon always faces Earth. So the Moon will always hover in the same spot in the sky from one side of Earth, while the other side never sees the Moon at all.
Script by Damond Benningfield