Circumference of a Globe
Date: 1/27/96 at 12:31:19 From: PAUL H. OLDENBURG Subject: Circumference of globe Dear Dr. Math, I need to get the formula to ascertain the circumference of a large (15- to 20-foot) diameter globe hung 25 feet in the air. My guess is to measure the width of the shadow and follow the formula (which I need) to determine the circumference. Please help. Thank you, Paul Oldenburg
Date: 7/29/96 at 13:21:25 From: Doctor Jerry Subject: Re: Circumference of globe If the shadow is produced by sunlight falling on the globe and we may assume that the rays are parallel (since the sun is so far away this is very nearly true), then draw a figure with a circle above a horizontal line. Draw in two rays, one just grazing the topside of the globe and the other the bottom side. These rays will be separated, since they are parallel, by the diameter D of the globe (from which the circumference can be found). Let the acute angle from the floor towards the sun be called A. With its base on the floor there is a right triangle with hypotenuse w (width of the shadow) and acute angle A opposite the leg of length D. We note that sin A = d/w. So, knowing A and w you can calculate d and, then, the circumference. I'm not sure this answers the questions you asked. Note that the height of the globe doesn't matter. This is a consequence of the assumption that the rays of light are parallel. -Doctor Jerry, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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