Why Is a Circle 360 Degrees?
Date: 07/01/98 at 02:31:44 From: Chris Perkey Subject: Why is a circle defined as 360 degrees? That's my full question. It came up in class today, and the best answer that I had was that maybe the Babylonians did it to coincide with their base 60 system. This sort of seems incomplete. Is there a better, more complete, or correct answer. Or does anyone know? Thanks in advance for your help. Chris Perkey
Date: 07/01/98 at 02:57:33 From: Doctor Pete Subject: Re: Why is a circle defined as 360 degrees? Hi, As a matter of wording, it is not so much that the circle is defined to be 360 degrees, but the other way around - the degree is a unit of angular measure which is 1/360th of a full "rotation." For instance, the grad is defined to be a unit of angle in which there are 400 in a circle. I believe this unit derives from British military use. But most important of all is the radian. There are precisely 2Pi radians in a circle, where Pi is the ratio of the circumference to its diameter, which is approximately: 3.14159265358979323846264338327950 ... (Sidenote: the importance of Pi lies in the fact that this value is constant, regardless of the radius of the circle.) Anyway, the idea really is that the degree (or grad or radian) is a unit that measures angle, much the way the meter measures length, or the gram measures mass, or the second measures time. But whereas these basic units (m, g, s), are defined in terms of things like atomic vibrations, the speed of light, or a piece of metal sitting in vacuum in Paris, the measure of angle is a mathematical abstraction. One could choose any sort of unit, for instance you could call a "perkey" an angle measurement in which there are 51235071 in a circle. Of course, this is a bit reminiscent of the days when the "foot" was a unit of length corresponding to the size of a king's foot. So in particular, you are quite correct in that the degree is derived from the Babylonian base 60 numerical system. Hours and minutes are similarly divided into 60's (of course, there are minutes of time and minutes of angle - there are 60 minutes in a degree, and, similarly, there are seconds of time and seconds of degree - there are 60 seconds in a minute, 3600 in a degree). And finally, you may wonder why the radian is defined as 2Pi in a circle - this number is not even an integer. The reason is that the radian satisfies certain nice properties, so that trigonometry and calculus are made easier when you let angles be measured in radians. The underlying idea is that the circumference of a circle of radius 1 is 2Pi. So in a circle of radius 1, one radian subtends an arc length of exactly 1. This makes measuring arc length equivalent to measuring angle. Similarly, in a circle of radius 1, Pi radians (which, by the way, equals 180 degrees), subtends an arc length of Pi. And so on and so forth. Now, what arc length is subtended by an angle of 2 radians in a circle of radius 3? I hope that's answered your question in more detail! - Doctor Pete, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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