Except that guestimation rocks your socks off.
For example, no calculator or anything beyond what the average person knows, how fast is the Earth spinning? Well, the Earth spins one time a day. Or once per 24 hours. That’s a speed, now to change that to MPH. USA could be about 3000 miles across. Sounds good. Then looking at a globe or map of the world, it looks like I can fit about 7 USA’s across the Earth if I put them next to each other. So I’m guessing the Earth is about 21000 miles around. (it’s actually about 24900, I looked it up afterwards) 21000 miles per 24 hours, quick estimation I can say it’s a little bit less than 1000 MPH. Whew pretty fast. Maybe wrong? http://ask.yahoo.com/20020411.html says 900 MPH, pretty close for quick questimation :)