# NavList:

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

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Re: Example from About Lunars, Part 2
From: George Huxtable
Date: 2002 Mar 5, 01:17 +0000

```OK, Chuck, just some hints is what you asked for, so I won't work it out
for you.

But remember how fast the Moon moves with respect to everything else,
whereas Mars moves slowly.

Moon motion should be something of the order of a half-degree in each hour,
so over the three hour period the Moon-Mars angle should change by about
1.5 deg. Your numbers quoted below, for D1 and D2, don't change by anything
like that much over the 3-hour period. So something is up with your values
for D1 and D2. Check them again. If you can't find what's gone wrong, post
your working for D1 and D2 and I will try to discover what it is.

Two other things to note.

You shouldn't be subtracting Mars parallax from the lunar distance (they
are in different directions in the sky). One of the weaknesses of Letcher's
method is that it considers the parallax of the "other body" to be
negligible, and so takes no account of it. Whan the other body is the Sun,
then the error is never much more than 0.1 min, but for Mars it can be
greater, especially near perigee. However, there's no way I know of for
accounting for Mars' parallax using Letcher's method, so a possible error
of up to 0.2 min (and somewhat more nearer perigee) just has to be
accepted. So forget that Mars parallax of 0.2 min and see what answer comes
up at the end.

You say "I'm subtracting semidiameter (of the Moon presumably) and Mars'
parallax from the lunar distance". Subtracting the Moon's semidiameter is
correct, because it was stated that the lunar distance was measured ACROSS
the Moon. But somehow, after that subtraction, you have got a result of
65.406, which is greater than the raw lunar distance specified in the
problem, of 65 deg 11.9 min. So have you actually added where you intended
to subtract?

Chuck shouldn't feel despondent. He is going through the same process we
have all had to suffer, when starting with lunars. We all need to
rediscover that facility with numbers that we (well, some of us) had in our
youth, but then lost as calculators and computers took over our arithmetic.

>I'm working my way through the Moon-Mars example now from Part 2 and I'm not
>feeling too sure of myself. I feel pretty good about my calculated D1 and D2,
>65.459 and 65.920 respectively. But, I'm wrestling with using Letcher's method
>to find D and I'm outside of that range. I'm subtracting semidiameter  and
>Mars'
>paralax from the lunar distance and I end up with a D of 65.406. Can
>anyone give
>me some hints?
>
>Chuck Griffiths
>

------------------------------

george---.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
------------------------------

```
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