# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Traditional navigation by slide rule**

**From:**Gary LaPook

**Date:**2016 Oct 31, 09:50 -0700

Yep, that's pretty funny, isn't it. Those two formulas came directly from the prior Air Force navigation manual, AFM 51-40 and, apparently, nobody gave it any thought then or when it was copied to the current manual. The manual says that the "60*(Long2-Long1)*cos(C). formula applies only when the course is zero, straight north. But in that case the formula is simply the difference in latitude times 60 so this makes no sense. Try it. If the course is zero then long1 and long2 are the same so you end up with "0" which multiplied times 60 is still zero and when multiplied by cos lat1 is still zero! I already pointed out that the formula for course has an error too in that it needs another set of parentheses in the denominator because the 180 mulitplies the result of the subtraction of the two terms so I have just ignored that inaccurate distance formula.

gl

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**Re: Traditional navigation by slide rule****From: **Sean C**Date: **2016 Oct 31, 07:05 -0700

Gary,

The second equation for rhumb line distance in the link you posted for AFPAM 11-216 seems to first be given as "(60*(L2-L1))/cos(C)", and then as "60*(L2-L1)*cos(C). Or am I mistaken?

Regards,

Sean C.