# NavList:

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

**Re: Classification of the methods for clearing the Lunar Distances**

**From:**Herbert Prinz

**Date:**2003 Apr 9, 00:00 +0000

Hello Jan, Thank you for your historically and mathematically illuminating analysis of L.D.s. Especially the section about approximate methods is rich in details that I have not seen before. Here are just a few minor comments as they occurred to me on first reading: Jan Kalivoda wrote: > > We can distinguish four classes of these methods, which are remotely similar to the classes of the methods for reducing sights by "Marcq St Hilaire (intercept) method", the only method for using celestial lines of position surviving in today's navigation. These are in the order of their increasing length, difficulty and logical clearness and beauty (in my eyes): > > - software solutions; quite common now and not unknown in the first half of 19th century! > > - inspection tables (compare HO 214, 218, 240, 229 and ancient Ball's tables, firstly edited in 1907) > > - "short" methods (compare Ageton's method in HO 211, Dreisenstock's method in HO 208, Smart, Ogura, Aquino etc.; in these methods short tables with auxiliary values are provided that are combined to obtain the end result; these tables were much less bulky and expensive than the inspection tables, but their use was more difficult and time-consuming) > > - rigorous solutions (compare cosine-haversine formula) I do not quite see the analogy, because in one case you are talking about various methods of solving a given problem (that of clearing the distance), and in the other case you are talking about various implementations of a given method (the modern intercept method). This method, in turn, is just one of many that solve a particular problem, namely the "position from combined altitude" problem. Moreover, the modern intercept method, as well as the original Marcq St Hilaire method are both approximative methods. Therefore it is misleading to speak of their rigorous solution via the cosine-haversine formula. A method can be rigorous or approximative. But an implementation of a method is either correct or incorrect (and should always be the former!). Maybe, this is just irrelevant nit picking of the former programmer in me. Jan Kalivoda wrote: > Another [approximate/short method] was the method of the French astronomer Chauvenet that replaced all other older methods in "American Practical Navigator" in the year 1888 [...] William Chauvenet, 1820 - 1870, was an American professor of mathematics and astronomy. He was instrumental in founding the Naval School in Annapolis, which later became the U.S.N.A. Besides teaching, he also served there as the first librarian. He even got a U.S. battleship named after him. He wrote a standardwork on spherical and practical astronomy (see below) which is still a valuable reference and which I highly recommend. I use it all the time, although it goes without saying that his numerical methods are irrelevant today. It also includes chapters on nautical methods. Chauvenet's method for clearing the distance had already been published in the American Ephemeris and Nautical Almanac for 1855 and 1856, and - according to his own statement in the cited book - also in the Astronomical Journal, Vol II, which must have been even prior to that (1850?). Later, it was reprinted in "New method of correcting lunar distances, and improved method of finding the error and rate of a chronometer by equal altitudes", Washington, Bureau of Navigation, 1864. It was again described and fully explained in "A manual of spherical and astronomy, embracing the general problems of spherical astronomy, and the theory and use of fixed and portable astronomical instruments. With an appendix on the method of least squares", Philadelphia, J.B. Lippincott & co.; London, Tr�bner & co., 1864. I believe there exists a Dover reprint, not sure whether it is available. Ian, are you sure of that given date, 1888? It is quite amazing that the method, devised in 1855, was incorporated virtually unchanged into the "Bowditch" edition of 1888 for the first time, apparently as Ian explains, in order to keep up with the progress of time! Chauvenet had been dead for 18 years by then. This fact totally escaped me until now. Back in 2001, I said in a message to this list: "Appendix V in the edition of 1909 describes the [lunar distance] method, contains all necessary auxiliary tables and a worked example using the distances shown in an extract from the Nautical Almanac 1855. Without having checked, I would therefore assume that any edition since 1855 would not differ too much in the way the subject is treated [...]" In early editions, data of worked examples were regularly updated to more recent dates for purely cosmetic reasons even when a method did not change. Seing the 1855 method and data, I jumped to a wrong conclusion. In the mean time, of course, I came to realize that it was the general trend of this publication after the death of Bowditch to let new technology mature for a quarter of a century before taking it into consideration. (First brief mentioning of Sumner line in 1855, intercept method around WWI) Jan Kalivoda wrote: > Borda (1778): > cos A squared = cos M cos S sec m sec s cos[(m+s+d)/2] cos[(m+s-d)/2] > sin D/2 squared = sin[A + (M+S)/2] sin[A - (M+S)/2] > > I cannot understand, why this cumbersome method gained such popularity. The riddle disappears once you know that the above isn't Borda's method. He, in fact, has sin A = Sqrt{cos M cos S sec m sec s cos[(m+s+d)/2] cos[(m+s-d)/2]} / cos[(M+S)/2] sin (D/2) = cos[(M+S)/2] cos A which requires one table opening, one addition, and one halving fewer than the other method. Fellow mathematicians and ordinary seamen alike may have appreciated the "economy of means". The former may have admired the artful substitution, while the latter will certainly not have minded the inherent obscurity. They never cared to understand their recipes, anyway. But how popular was the method really? And popular with whom? I often see it quoted in learned works, textbooks and the like, but can it be found in ship logs? > > Maybe the method of Bruce Stark is the last method invented in this [the approximative] class, but I don't know anything about it. As Bruce already said himself, it is rigorous. He forgot to mention that it is highly original. Most rigorous methods differ from each other only in the detail of how they eliminate cumbersome additive terms of trigonometric functions. (These prevent efficient use of logarithmic trigonometric tables and force the user to additionally use a set of natural trig. functions.) However, all the methods in the Borda-Delambre-Young-Krafft-... category use trigonometric substitutions for the purpose of this elimination. Bruce uses Gaussian logarithms. These have been favoured by astronomers, specially German ones (e.g. D�llen) in the late 19th century, exactly to avoid such ocscure substitutions as the one above by Borda. I have never before come across the idea of using them in the context of a lunar distance. A certain George W.D. Waller proposed this idea in 1946 for altitude calculation (see Bowditch 1984, p.593). Nothing came to fruit. The main drawback of Gaussian, or addition logarithms is that they are yet another table to carry (and to produce in the first place!) So I believe Bruce's method is in a category all by itself. I am sure he could tell us more... Best regards and thanks again Herbert Prinz