# NavList:

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

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Re: Lat/Lon by "Noon Sun" & The Noon Fix PROVE IT
From: Frank Reed
Date: 2009 May 13, 06:51 -0700

```Gary, you wrote:
"But if you can determine the time of LAN within one minute then the
uncertainty in longitude is only plus and minus 15 minutes of longitude, plus
or minus 15 NM at the most."

But can you really determine the time of LAN within a minute?? Sometimes, yes.
But under general circumstances, not even close. And also, we don't observe
the time of LAN. We observe the time of maximum altitude which is offset from
LAN. At the equinoxes, the motion of the vessel contributes an offset in the
longitude of 4*v*sin(Lat) n.m. where v is the speed of the vessel in the
North/South direction. So if v is 10 knots, Lat=45 deg, that gives 28
nautical miles offset EVEN IF the time of maximum altitude can be estimated
with no error at all.

And you concluded:
"Soooo, why go to the extra complication of the method discussed here,
just mark down the time of LAN, place an error band +/- 15 NM on the
longitude, and plan your landfall based on the most critical longitude
determined? One of the arguments made for this method is that it is easy
to relearn, but it is not nearly as easy to relearn as just taking the
time of LAN."

Well, the problem here is that your 15 n.m. is around the lower limit for the
approach you're describing. You could EASILY be wrong by 75 n.m. when you
factor in the difficulty of guessing at the peak time of the altitude and the
offset due to the vessel's motion. The steps required to fix this up and
bring the error under 5 n.m. (on average, frequently much better) are not
much of an "extra complication" and they provide an easy method for getting a
real fix that navigators can learn at an early stage. Have you tried it,
Gary?

-FER

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