# NavList:

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

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Re: equal altitudes
From: Bill Murdoch
Date: 1997 Dec 04, 3:57 PM

```We have been back and forth over the last couple of weeks over using different
methods of reducing sun sights.  It seems that no one objects to using a noon
sun sight to determine latitude.  It is not hard to find the altitude where
the sun stops rising, and the math is simple with the navigational triangle
collapsed to a line.  You can reduct the sight with simple addition and
subtraction.

The rub comes with using the noon sight to find longitude.  It seems to me
(and I have never done it anywhere but in my backyard) that if at noon the sun
is passing somewhere near overhead it would work and if the sun is near the
horizon it would be difficult at best.  With the sun passing overhead it will
be rising at 15 deg/hr before noon and setting at 15 deg/hr after noon.  For
only the shortest of instants would it not be moving.  That should be an easy
spot to find.  Just take a sight while the azimuth is still east and another
at the time the azimuth is west and the altitude is the same as the first
sight.  Split the difference and you have the time of noon; a little
arithmetic and you have the lngitude.  However, if the sun were near the
horizon at noon, it would be rising very slowly in altitude before noon and
setting in altitude very slowly after noon.  It would be hard to tell the time
when it stopped rising and started setting.  In fact for someone near the pole
for whom the sun neither rises nor sets it would be impossible to tell the
time of noon and thus impossible to tell the longitude by this method.

The use of sight reduction tables or a calculator to solve the navigation
triangle as was pointed out gets around all of this, but books and trig scare
most people to death.  To bad.  They miss a lot.

That little calculator can do more.  It can not only replace the sight
reduction tables; it can also replace the almanac.  Try the attached form.  It
can be used to calculate the GHA of Aries.  Any trig calcualtor with one
memory should work so long as it keeps 12 digits or more internally.  It is
not hard to do the almanac calculations.  100 years ago they were all
calculated by hand.

(I hope the form is without mistakes.  I have not had time to exhaustively
check it.  Let me know if you find something wrong.)
```
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