Shooting the moon
Two astronomers were making a study of the moon. One was looking through the telescope and the other one demanded that he get a chance to look. The first astronomer said:"Come back later." The second one said:"But I want to see what is happening now!" The first one said:"Then you must come back later." When we look at the moon, all we see is light being reflected off the moon's surface. Given that the moon is 384, 000 km away, and light can travel 300, 000 metres in a thousandth of a second, when should the second astronomer come back? Answer in milliseconds.
Refrences: Brandreth, Gyles (1981) - The Puzzle Mountain