Imagine a line extending from the center of Earth out to the equator. Now imagine another line extending again from the center of the Earth to Chicago. Those two lines form an angle at the center of the Earth, and the measure of that angle is called latitude.
Similarly, imagine a line extending from the center of the Earth to the prime meridian. (This is a line that extends from the north pole to the south pole and passes through Greenwich England.) Now imagine another line again extending from the center of the Earth to the point on the equator that is directly south of Chicago. Again you have an angle at the center of the Earth, and the measurement of this angle is called longitude.
Click for full size picture.
Now you know why lat and lon are given in degrees, because they really are measuring friggin degrees! Isn't that cool? I think it is also cool that the thing that is being measured, the angle formed by the two lines, is actually located in the very center of the Earth.
In my previous post with the formula for figuring out the great circle distance between two coordinates there was a lot of use of the cosine, sine, and arctangent functions. If you recall from your high school geometry days those are used when performing calculations on angles.
For example cosine is simply the ratio between the length of one of the sides of a right triangle to the length of the hypotenuse. Nothing scary there.
Is it all starting to jell together for you? Why lat and lon are measured in degrees. Why cosine, sine, etc. functions are used to calculate distance across the surface of a sphere (Earth)? Now you know why I get all excited about this stuff. Because it is so damned cool!
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