“ Our moon’s rotation is locked to its orbit around the Earth due to tidal forces, which is why we only ever see one side of it. For the same reason, the Earth’s rotation is also slowing down and will eventually match the orbit of the moon. At that point, billions of years from now, both a day and a month will last for around 47 of our current 86,400-second-long days. One side of the Earth will face the moon; the other will never see it again. ”
— Dave Hayden from Panic, in his great post about the origins of the phrase Blue Moon, and the calendar.
(via Daring Fireball)
