# NavList:

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

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Almanacs, theory and use.
From: Mike L
Date: 2007 Nov 20, 03:39 -0800

```We live in Scotland and my mother lives in England, so I thought it
would be  pretty simple to prove to my kids using a sextant that the
world was  spherical - so I bought an EBBCO on eBay. Whilst I've
proved to myself I can  use a sextant to find out where I am, I'm
still to convince the children either that you can, or that you would
want to.

I started by using some software into which I put the time,
approximate  longitude, latitude and sextant reading, and (after
working out I had to  subtract half the sun's diameter) I finally got
something average on our  location on average about 3 miles from our
location. Unfortunately, as far  as a kid is concerned, if you have a
PC, you may as well look up  google/streetmap rather than waste time
with a sextant, so I need to find a  PC-less way to find out where I
am.

So, using a bit of trig (with some software from the web) I created my
own  single-page weekly tables (the sun don't shine everyday!), giving
altitude  and direction of the sun for a given location for each
minute of the day.  This allows me to create a table for any given
place which most children who  can add two digit numbers, and use a
ruler/protractor could use by  themselves (with instructions) to plot
a line giving their location (to  within 10miles I hope!), which if
repeated twice in a day should give an  "exact" location.

Now, I know how my "Almanac" works, but even having figures for every
minute  of the day, for a known location and interpolating results for
seconds, I  will still be pushing it to get tabular errors less than
1'. From what I  have been able to discern about real almanacs they
contains a fraction of  this information with only hourly figures for
worksheets to "calculate" the figures, I  can't understand how these
are used (I neither have a worked example, nor do  I have an almanac,
nor do I have a theoretical explanation for the tables -  but I don't
see that as a fundamental problem!) Surely getting from these  figures
in the Almanac to one at any time for a particular location but
involve some complex trigonometry and rather hectic sinusoidal
interpolations - neither of which are apparent on the worksheets!

What I really want to know is how my "almanac" relates to a real
almanac,  and how, could and should I make my "almanac" more like a
real almanac and  still have it useable by children? I've tried
searching the internet, for  any explanation of how to use an almanac
(with the theoretical background) -  any help would be greatly
appreciate (remembering I am not familiar with  SHA, GHA, and whilst I
learnt spherical geometry at University, I'm a little  rusty)

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