Why is a Circle 360 Degrees? Date: 2 Jan 1995 15:16:33 -0500 From: Roy P. Sachs Subject: Origin of degrees Over the holiday, the question came up in a family discussion, and could easily come up in one of my geometry classes. What is the origin/basis of the degree measurement? Why is a circle divided into 360 degrees rather than some other number? Any light that you could shed would be appreciated. Roy Sachs rsachs@umd5.umd.edu Date: 2 Jan 1995 16:34:20 -0500 From: Dr. Ken Subject: Re: Origin of degrees I'm glad you asked this question, because I've been wondering it myself. I figured it had something to do with the Babylonians, who used a base 60 number system. But it sure took a lot of digging in several books to find out anything concrete about it. I finally found what I was looking for in a book called "A History of Pi" by Petr Beckmann, a mathematician from Czechoslovakia. Here's the passage: In 1936, a tablet was excavated some 200 miles from Babylon. Here one should make the interjection that the Sumerians were first to make one of man's greatest inventions, namely, writing; through written communication, knowledge could be passed from one person to others, and from one generation to the next and future ones. They impressed their cuneiform (wedge-shaped) script on soft clay tablets with a stylus, and the tablets were then hardened in the sun. The mentioned tablet, whose translation was partially published only in 1950, is devoted to various geometrical figures, and states that the ratio of the perimeter of a regular hexagon to the circumference of the circumscribed circle equals a number which in modern notation is given by 57/60 + 36/(60^2) (the Babylonians used the sexagesimal system, i.e., their base was 60 rather than 10). The Babylonians knew, of course, that the perimeter of a hexagon is exactly equal to six times the radius of the circumscribed circle, in fact that was evidently the reason why they chose to divide the circle into 360 degrees (and we are still burdened with that figure to this day). The tablet, therefore, gives ... Pi = 25/8 = 3.125. So that's who gave us the 360 degrees in the circle. See, assignment of degree-measure to angles is somewhat arbitrary. Some choices are more natural than others, though, and when you're working in base 60, 6x60 is a pretty natural choice. As a sidenote, the actual ratio that the Babylonians talk about is 6r/(2r*Pi) = 3/Pi, which is about 0.95493. They say it's 24/25 = .96. And you might ask why we chose Pi as the letter to represent the number 3.141592..., rather than some other Greek letter like Alpha or Omega. Well, it's Pi as in Perimeter - the letter Pi in Greek is like our letter P. I hope this helps answer your question. Write back if you have more! -Doctor Ken, The Math Forum Check out our web site! http://mathforum.org/dr.math/ Date: 8/28/96 at 16:58:39 From: Herbert S. Friedman Subject: Re: Origin of degrees There is more to this than the six sixes for the 360 from the Babylonians. It has to do with Claudius Ptolemy (100-170 AD), who divided the circle into 360 parts for his sine table. He actually used the length of the chord for each central angle in steps of 1/2 degree in a circle of radius 60 rather than sines. See p. 212, problem 4 of Burton's "The History of Mathematics" (1985, Allyn and Bacon). I still have the feeling that the Babylonians were partially responsible.