How Far Away and How Big is the Moon?
The story of this measurement is a classic example of the way in which the most important piece of equipment owned by an astronomer is his or her wit. This was not a planned measurement at all. I noticed something that surprised me, and followed it up.
One night in February 2007 I happened to notice that the Moon and Saturn were quite close together. If you know from a star chart roughly where Saturn is, you can easily identify it with the naked eye – not by its rings which require about 30x magnification – but by its creamy color. Like Mercury, Venus, Mars, and Jupiter, it is bright. These planets, unlike stars, do not twinkle.
Anyway, that night, I went to visit my parents. When I left, I looked up and saw the Moon again, but no Saturn. Puzzled, I got the telescope out and could then see Saturn right next to the Moon. This looked so cute it just had to be photographed with my brand new webcam.
The result was a disaster. Either the Moon was overexposed or Saturn was...