* Just to be clear, mass and weight are different. Mass is a property of matter; you can think of it as how much matter there is, and we measure it in grams or kilograms. Weight is the force of gravity on that mass, and we measure it in pounds. A cannonball has the same mass whether it's on the Earth or the Moon, but on the Moon it weighs one-sixth as much because gravity is one-sixth as strong; on the Earth 1 kilogram equals 2.2 pounds, but on the Moon it weighs about 0.36 pound.I wish to argue this point with my American friends (and that means you, Farmer Pete). The distinction between mass and weight is valid, but you noodles must remember that you guys are just about the only ones in the world who measure weight in weird, old-fashioned units, such as pounds and slugs.
The way Phil has put it is confusing to those of us in the real (read: rest of the) world, where mass is measured in kilograms and weight is commonly measured in kilograms. For instance, in Australia, 1 kilogram of mass equals 1 kilogram of weight. Except perhaps at the top of Mount Kosciuszko, where it might weigh just a little more, or at Lake Eyre, where it would weigh a smidgeon less, but probably not enough to tip even the finest calibration of kitchen scales.
Furthermore, to be really technical, weight is a measure of the force of gravity on the mass of an object (weight = mass x acceleration, for those of you who can't recall high school physics) and it is actually measured in newtons, the SI unit of force. 1 newton is, appropriately, approximately the weight of a small apple at the Earth's surface -- 102g -- and therefore the weight of 1 kilogram at the surface of the Earth is 9.8 newtons, and on the moon, 1.63 newtons.
I raise this point--not as a criticism of Dr Plait's book or his reasoning, which is otherwise entertaining and excellent--but only because that small footnote cost me a couple of hours of brain strain and kept His Honour, AFSM, awake for at least half an hour while I bent his ear about it in bed. The text above is a distilled version of my research and reasoning that I hope will save you the same brain strain when you read the book. Which you should, because it's a great read.
I also wouldn't mind if it alerted some of our US friends that they are about two centuries behind the rest of the world when it comes to a sensible system of weights and measures, and they shouldn't just assume that "we measure it in pounds" when they are writing for an international audience.
On another astronomical note, the Hair Dude and I went to see "Time Warp", a show about space, time and magic with Dr Fred Watson from the Anglo-Australian Observatory and Matt Hollywood, magician. It was entertaining and informative, and the Hair Dude got his copy of Why is Uranus Upside Down? autographed by the star himself. I'll bet Dr Watson doesn't fool around with pounds and slugs in that book, but I'll let you know when we've read it.