On our last program we mentioned the startling experience of five British monks from the twelfth century who saw an enormous explosion occur on the moon.

In their records they wrote that the upper horn of the new moon split in two and emitted a pillar of flame. Modern astronomers, working on the assumption that the monks witnessed an asteroid collision, have found a crater of recent origin in the spot where the monks said the explosion occurred.

There is also more evidence to support the theory that the moon was struck by a passing object that night in June. In fact, the moon may still be vibrating from the impact.

When the Apollo astronauts traveled to the moon, they placed instruments on its surface called "laser retro reflectors." These instruments are essentially sophisticated mirrors that allows us to shine laser beams at the moon and receive the bounced-back light. Multiply the amount of time it takes to get the signal back by the speed of light and you have a highly accurate measurement of the distance to the moon.

This method of measurement has revealed that the moon is, in fact, vibrating, like a bell that has been struck by a rock. The vibration makes the moon move back and forth by about ten feet every three years.

Not all astronomers agree that an asteroid strike set the moon vibrating; some argue that it is a natural effect of the earth's gravitational pull. However, if the asteroid hypothesis is correct, it seems likely that the British monks of eight hundred years ago saw the blow that left the moon shaking even today.