mapug-digest wrote:

> mapug-digest Sunday, September 23 2001 Volume 01 : Number 3785
...

>Date: Sun, 23 Sep 2001 18:07:41 -0400
>>Subject: Re: [M]: Calculation of Field Rotation
>I am sure you can justify what you explain below however, any 1st year
>physics learns that you cannot divide by zero. To divide by cos(90)
is >totally invalid. Any equation that lends itself to division by zero
is >mathematically invalid.

Not true. For example, the simple (plane parallel) equation for "airmass"

which estimates the amount of air starlight passes through is

X=1/cos(z). Zenith distance (z) can range from 0 deg at the

zenith to 90 degrees at the horizon.
Z cosz X
0 1 1

90 0 infinite


This makes both mathmatical and physical sense. Assuming a plane
parallel atmosphere, starlight would have to traverse an infinite
pathlength through the air at the horizon. (Note: better equations exist
that include the curvature of the Earth, the nature of the variation of
pressure with altitude, etc. but that does not make this equation
invalid, nor does the fact that this equation can result in 'divide by
zero' make it invalid.


--
John Oliver

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