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CHRISTOPHER CRUISE: “Blue moons, you saw me standing alone.” The cow jumped over the moons. “Shine on Harvest Moons.” These expressions sound unusual. Why the choice of “moons” and not just “moon?” There is only one moon, right? Yes, there is only one, but long ago there might have been two!

American and Swiss researchers say the Earth may once have had a big moon, the one that is still there, and a little moon. And then, about four billion years ago, they hit each other. The little moon exploded into rock and dust, and much of it landed on the dark side of the larger moon. The researchers call it the “big splat.”

Scientists from the University of California at Santa Cruz and the University of Bern reported their findings in “Nature” magazine. The scientists say the little moon was very small, only about one thousand two hundred kilometers across. They say the little moon and the big moon were just fine together for one hundred million years. Then something happened and the little moon lost the battle.

Four possible stages of a collision between the Moon and a smaller moon about 4 billion years ago. This image was provided by Martin Jutzi and Erik Asphaug, University of California, Santa Cruz through "Nature" magazine.

BOB DOUGHTY: The Earth and the moon turn in a way that keeps the dark side of the moon away from us. From earth, we see only one side. But we know what the other side looks like. Spacecraft have taken many pictures of the dark side. Those pictures show that the side of the moon we see is a lot different from its other side.