# NavList:

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

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Re: Working a lunar
From: George Huxtable
Date: 2009 Aug 6, 08:14 +0100

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contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Hanno Ix"
To:
Sent: Thursday, August 06, 2009 6:57 AM
Subject: [NavList 9382] Re: Working a lunar

Gentlemen:

I am a novice to CelNav, and I certainly have no experience in lunars.

Some algorithm occurred to me, though, that I would like to share and
discuss. However, given the age of this business, if it is a valid one I
doubt it is new. If anyone has seen it before, please let me know, so I
could read up on it. The objective is to find GMT and location.

Let's make a Gedanken experiment:

A ship reaches land of unknown coordinates. Land makes it practical for the
navigator to measure the meridian passages of Heavenly Bodies rather
reliably. Given GMT, he can calculate LAT and LONG. (One shot method.)

But now we pose GMT as unknown. Sitting on land, measure the meridian
passages of, say, sun and moon which moves. Can I find GMT, too, using the
now available data not using the classical moon distance methods?

If I see things right, there must be a LOP which connects all locations on
Earth with a given, fixed difference DT between the meridian passages of sun
and moon.

However, along this LOP, the same DT occurs at a different GMT. In this
scenario, the LOP referring to a given DT is pre - calculated, listed in an
almanac and annotated with GMT at each LAT. So, by having found the LAT
before we just read the GMT of the DT-specific LOP.

There is another opportunity:

By accepting preliminarily this GMT, we can calculate LAT again, namely from
the meridian passage of the MOON, and compare both values found. If there is
a gross difference we must have made an error. This, by itself, would be of
value. Otherwise, though, we have good reasons to accept the GMT we found.

I appologize if I am talking about a method I have not gone through myself
yet! I fear there is a hick-up in this somewhere. But I would like to hear
the critique of you specialist navigators before I spend alot of time trying
to do something long known as wrong.

If, however, you find it sound, and has not done before I will pusue it.

Best regards

--- On Sun, 8/2/09, Paul Hirose  wrote:

From: Paul Hirose
Subject: [NavList 9301] Re: Working a lunar
To: NavList@fer3.com
Date: Sunday, August 2, 2009, 3:13 PM

Antoine wrote:
> So, my results would not be that bad ...

Your results are only 1 second different from my program.

But it recommends the upper limb of the Moon, because the light is
coming from almost directly above (position angle = 350�). Of course, if
the "observation" was calculated, this doesn't matter. Even if it was
really observed, I don't think there is much error because the Moon is
nearly full (phase angle = 4�), and the cleared lunar distance is not
very sensitive to altitude.

My corrected time is 2001-04-08 at 03:21:10.5 UT1 (03:22:14.5
Terrestrial Time). Position fix from the altitude observations is
N44�50.48' W011�01.08'.

I checked the solution by entering the Terrestrial Time, latitude, and
longitude W11�17'07.2" into the USNO MICA 2.0 program. Longitude equals
my solution, moved west by delta T (64 seconds) converted to arc (= 64 *
15 * 1.002738 arc seconds). With these inputs, the outputs from MICA
should match the observations. (MICA does not compute refraction, so I
used my program's values.)

31.4506 Moon altitude
- .2718 semidiameter
+ .0263 refraction
---------
31.2051 lower limb (calculated)
31.2050 lower limb (observed)

17.7536 Mars center
+ .0497 refraction
---------
17.8033 refracted center (calculated)
17.8033 refracted center (observed)

64.9266 refracted separation angle
+ .2718 Moon semidiameter
--------
65.1984 lunar distance (calculated)
65.1983 lunar distance (observed)

The check is good.

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