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Re: Latitude by Lunar Distance
From: Henry Halboth
Date: 2006 Nov 11, 13:04 -0500

Hi gang,

I cannot help but be somewhat amused by the responses to Frank's interesting postings regarding Latitude determination by Lunar distance - whether the concept is old or new notwithstanding. There have been over the years many interesting manipulations of the astronomical or spherical triangle proposed, seeking to establish position from a single sight, or perhaps closely spaced observations of a single or multiple objects, but most if not all of these methods rely on either some reasonable knowledge of time or position. For example: to clear a Lunar Distance normally requires (at sea at least) observations of the respective altitudes which, with multiple observers, does not present much of an obstacle, but with only one observer requires additional calculation; yes, I know it's possible to calculate the altitudes - however, does not that require a position/time of reasonable accuracy - some authorities stating these calculated altitudes should be within four arc minutes of the truth, and does not such accuracy require a position/time accuracy better than is ultimately determinable by the Lunar Distance. I am obliged to say that backyard Lunars, taken under antiseptic conditions from a known position, although they may be a fair exercise in the determination of sextant accuracy/capability, do not replicate conditions of observation at sea; if the altitudes are obtained by use of the artificial horizon, they do replicate conditions probably faced by land explorers. To the best of my knowledge, gleaned through both experience and research, the best average accuracy attainable at sea by the use of Lunars was/is in the range of fifteen arc minutes of Longitude.

As far as I am concerned, the greatest practical accuracy of observed position, by closely spaced observation, say 10 to 20 minutes, of a single body, is attainable by solar equal altitudes at or about Local Apparent Noon, either on land or at sea. This method is quick, easy, readily understood, and requires little calculation; I have on several occasions demonstrated this method on List. It slso does, however, depending on intended use, whether Longitude or time, require a knowledge of position (in rating chronometers) or time (in determining Longitude).

Most postings related to this subject have drawn some critical fire, and this is perhaps a good thing, so long as that fire is of a constructive and impersonal nature. I am purely a practical navigator, and do not respond to many of the postings dealing with accuracy and precision of observation as, at least at sea, these considerations are subject to existing circumstances and conditions not under the navigator's control - circumstances which must be evaluated at each observation. Regardless, I sincerely enjoy Frank's postings, and view them in the light of challenge and excellent mental exercise which, at my stage of the end game, is of great value.

Regards,

Henry

----- Original Message -----

From: Wolfgang Köberer

To: NavList@fer3.com

Sent: Saturday, November 11, 2006 4:13 AM

Subject: [NavList 1665] AW: [NavList 1663] Re: Latitude by Lunar Distance

Dear all,

Frank's up-tempo reaction to me referring to an article describing his new idea 94 years ago, causes me to give some more information:

The "Annalen der Hydrography und Maritimen Meteorologie" were the monthly journal of the "Deutsche Seewarte", published between 1872 and 1944. During the period of interest here, prior to the "Great War" (as I think they say in your part of "Old Europe", George) it contained articles and reports on hydrographic subjects (ocean drifts and water depths etc.) as well as on weather forecasting - some colorful descriptions of storms in the North Sea area - and, of course, celestial navigation. The articles on celnav did not make up the bulk of the contributions, as the intercept method had been almost universally adopted in Germany. Those articles were mostly concerned with refining measurement - I recall lengthy reports on dip experiments developing formulae - but some treated celestial navigation in the air (mostly for the benefit of determination of the position of a balloon) and therefore without a usable horizon. There were also discussions about new tables or graphical means of shortening the necessary calculations.

The article by Jäger proposes - as the title says - to find one's position by determining the parallax of the moon. The explanation is about the same as given by Frank - and it does not contain a worked example. The article also discusses the shortcomings of this method: an error of one arc second leads to an error in position of one nm at best. In our latitudes the error may amount to as much as 40 nm - due to the fact that the moon is not as high in the sky - on the condition that sextant reading is accurate to 10 arc seconds (which is the smallest division of the scale of the Plath Navistar Professional for instance - and Plath only guarantees an accuracy of +/- 20 arc seconds). Best results can be obtained when the moon is near culmination and when the star observed is above or almost above it. When taking the distance to a second star the second star distance should form a right angle with the first (which is self explanatory) and this may be a rare condition.

Jäger mentions that he has tested the method on board of "Herzogin Sophie Charlotte" himself - it is interesting, though, that he does not say that these test were satisfactory, as one would expect if they were. There was no reaction or discussion of the method in the "Annalen" afterwards; it obviously sank like a "lead duck" as we say in this country - only to be discovered by Frank almost a century later.

If anybody is interested in a scan of the article, as ever: feel free to say so.

Regards,

Wolfgang

-----Ursprüngliche Nachricht-----
Von: NavList@fer3.com [mailto:NavList@fer3.com]Im
Auftrag von Peter Fogg
Gesendet: Samstag, 11. November 2006 01:23
An: NavList@fer3.com
Betreff: [NavList 1663] Re: Latitude by Lunar Distance

Frank wrote:
>
> I think [position via moon parallax is]
> quite a novel concept for this group of people. I suppose that's why a
> couple of people were determined to attack it instead of trying it out.

Having followed this topic with some interest, as was also shown by
others, I don't recall 'attacks' as such. What I do remember are
requests for more detailed information, and perhaps the expression of
a little frustration when this was not forthcoming. Don't be so quick
to assume the mantle of the martyr, Frank. Do feel encouraged to
explain fully new concepts.

The other interesting aspect here is that we now know this idea was
described in 1912...how?

Because it was published in a peer-reviewed journal and has thus
endured 94 years so far. I seem to remember some pooh-poohing of this
suggestion when proposed as appropriate for Frank. Perhaps the value
of publication in such journals has been supported by this revelation.

The advantage of peer review before publication is that the process
(should) ensure that the idea is complete (whole), is well expressed,
and (hopefully) able to be demonstrated. It makes a good idea better,
while weeding out the others.

Which brings us full circle to the shortcomings of the expression of
this idea as presented.

100% for ingenuity, Frank; but only 30% for clear expression.

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