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Re: Angles *ARE* Ratios
From: Frank Reed CT
Date: 2005 May 16, 23:25 EDT
From: Frank Reed CT
Date: 2005 May 16, 23:25 EDT
Peter you wrote: "any chance of some more detail on this issue?" Sure. I should say up-front that it's no big deal, and you're probably familiar with all of this but I'll just write a bit, and see if it goes anywhere. Angles ARE ratios. This is the fundamental definition of an angle. Almost all of us learn about angles at a very early stage of our education and we learn how to use angles, how to picture them and compare them, and we learn to count them in degrees. But the idea of defining an angle is usually left for much later, and by then students are so familiar with angles that the importance of the definition gets lost. An angle is the ratio of the length of an arc of a circle to its radius. That's all --nothing more. And because of this, angle have no units. Like any fraction, any ratio, they are pure numbers (no feet or meters or other physical units). So if I have an object that's a foot across and it's a hundred feet away from me, then its angular size, the angle it subtends, will be 1/100. Most of the time, people refer to this as an angle "measured in radians" but this is really an accident of history and education. An angle is just a ratio, and it's not really "supposed to" have any name after it. Of course for those of us who enjoy sextants and angle measuring, we like to convert angles to the familiar ancient sexagesimal terms. And to do that, it's very convenient to memorize the number of degrees or minutes or arcseconds in a unit angle. If an angle is 0.001, for example, we can express it in minutes of arc by multiplying by 3438 to get 3.4 minutes, or in degrees by multiplying by 57.3, or in arcseconds by multiplying by 206265. If I want to know the angular size of my fist is at arm's length, I just divide 4 inches by 28 inches. That's an angle of 1/7 (or, multiplying by 57, just about 8 degrees). And note that none of this has anything to do with sines and tangents and all that. We only need those when we know that the geometry is a true triangle. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars