Click on the link to the right to avoid the waffle and head straight to the calculator. You'll wonder* how you ever survived this long without it!
So the thinking went something like this: If you have a 7-inch record being played at 33rpm, then the speed that the edge of the label and the edge of the record are moving at is different.
If the diameter of the label is 3 inches, then it would have a circumference of 3 x pi = 9.4 inches, and so at 33rpm any point on the label's edge will travel 311.02 inches every minute. Conversely, a point on the edge of the 7-inch would travel 725.71 inches in the same time.
Now let your mental image of a record on a turntable gently morph. The black vinyl turns to patches of green and shimmering blue, the label turns into the north pole, and the centre of the record is revealed as being the earth's axis.
And sticking out from the edge, like an aerial drawing of a (gently rotating) Mexican taking a piss, you see yourself sticking up from the earth's surface. And it strikes you that your head is further from the earth's axis, and so must be moving faster, than your feet.
Ladies and Gentlemen, welcome to the Headspeed calculator! Created by Rufous, with the help of the magnificent coding skills of Mr M Bee (esq), the Headspeed calculator will work out the respective speeds of your head and feet, and the difference in the distance travelled in your lifetime to date!
The links on the right, will take you either to the assumptions that lie behind the calculations, or head straight for the calculator.
(* May not apply to all readers)