The far side of the moon was unexplored until it was photographed by space probes, then later seen and photographed by astronauts.

The far side looks quite different from the side we see from Earth. It's very rugged and much more heavily cratered. Just about all those flying rocks would have hit Earth, if not for the "deflector shield" called the moon!

At any one time you can only see 50 percent of the moon, but a phenomenon called libration gives earthbound observers a peek at the lunar far side. Libration is a kind of wobbling or oscillating motion of the moon that lets you see 59% of its surface over time. But you have to be very patient!

To see the effect of libration, press the P button under the Time and Date display to start time running forward.