# NavList:

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

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Re: Need dip? Throw a rock...
From: Frank Reed
Date: 2011 Jun 20, 15:21 -0700

Hello Antoine, you wrote:
"Multiply the number of seconds by 2 ???"

Well, how about two squared. :)

dip = 4*t

with t in seconds and dip in minutes of arc. Using 3.9 would be a little more exact, but the real limiting factor here is air resistance. If you drop a two-inch ordinary rock from fifty feet, air resistance is close enough to zero that it won't affect the result. But what if we launch the same rock horizontally from the top of a 100 foot bluff with the water's edge a couple of hundred feet out in front of us? Would it still be safe to use that factor of 3.9? What are the real limits of this method? I don't see any problem doing the timing to the nearest tenth of a second or better with modern technology available and we could even time to a quarter of a second with "traditional" technology. The latter would be good enough to estimate the dip to within one minute of arc which might be better than a wild guess at height of eye, but not much better than that.

-FER

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