# 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: Gary LaPook
Date: 2009 May 12, 23:07 +0200

```You get us back to the drum that I am constantly beating, "just how
accurate do you need your fix to be at the moment"?

First, there is never going to be a perfect celestial fix, accurate to a
nanometer. Now that we have GPS we can compare our celestial fixes to a
highly accurate benchmark, we couldn't do that in the past. Many on this
list try to squeeze almost perfection out of their celestial fixes but I
think it is useful to ask how much accuracy is actually needed and see
if the technique yields that level of accuracy.

sun line with the noon latitude sight. How accurate could we expect this
type of fix to be? Pick your own value for the probable accuracy of each
sextant shot by let's use one half minute for this discussion. So the
morning sun line has an error band one NM wide. Lets say we advance this
for one hour at six knots westbound and add one tenth of the distance
covered as the degradation of the position due to to possible errors in
the DR. (This is a commonly used estimate in flight navigation but a
smaller number may be appropriate on a boat.) This adds another
six-tenths of a NM to the error band making it 2.2 NM wide. (Since the
morning sun line will not have an azimuth of straight east or west not
all of the error will be in the longitude but we have bounded it worst
case, plus and minus 1.1 NM in longitude.) Assuming the same level of
accuracy with the noon sight, you have determined your position plus or
minus one-half NM in latitude and 2.2 NM in longitude. Let's call this
the accuracy level of a traditional noon fix.

We have been taught that you cannot determine longitude from the noon
observation due to the difficulty in determining the exact instant of
LAN. What this actually means is that you cannot determine your
longitude to the "accuracy level of a traditional noon fix." 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. Although this is not to the "accuracy level of a
traditional noon fix" it is probably accurate enough for the purpose of
a backup navigation system. There are techniques to use in an attempt to
identify LAN more accurately (equal altitude sights) and the technique
uncertainty and also accurate enough for backup navigation. (I am
impressed by the level of accuracy achieved by this method.)

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.

gl

> Report of the Secretary of the Navy
>
> "We recognize in navigation two classes of astronomical problems; one
possessing perfect accuracy of computation; the other uncertain, and
approximate only in its results.  The first class is, of course, always
employed when the weather and other circumstances are favorable.  The second
class of problems, that which gives only approximate deductions, is never
resorted to except from necessity.  The latter, however, serves a useful
purpose by enabling the navigator to know his position more nearly than he
could otherwise do, and by often affording perfect security by informing him
of the limit of safety, beyond which he should not venture."
>
> --------------------------
>
>
> I would like to assign the latitude and longitude by the noon sun fix to the
second class of problems.  It can provide us with a useful figure of merit,
albeit without perfect accuracy. Certainly, with a least squares curve fit,
the method affords us a very reasonable fix.
>
> So if Frank wants to "trot out his hobby horse" one more time, I for one will applaud it.
>
> Proven??  A resounding YES!
>
> Best Regards
>
>
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> >
>
>

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