NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Henry Halboth
Date: 2010 Jan 13, 22:53 -0800
| Hi all, Latitude at Noon, or thereabouts,
This matter has again raised its head. Jeremy has put his oar in, and George has come forth with his usual number of insightful questions. Yet it seems that we go further astray with each posting. Let’s for the moment, forget about the various Lunar methods of determining GMT, or Longitude, and allow ourselves the luxury of a well equipped ship, vessel, boat, yacht, or whatever you can afford, possessing a well rated chronometer, as well as the other appurtenances of modern Celestial Navigation. We can now look into the Noon, or any Meridian Transit, observation as it was dealt with at sea, utilizing the sea horizon, before the advent of universally available electronic means. The body to be observed, whether Sun, Moon, Planet, or Star,
will be on your meridian at such time as its GHA is equal to your Longitude. By
reference to a Nautical Almanac, or other appropriate publication, you may
readily deduce the corresponding GCT of transit, or if you like you can use the
old style LAT +/- EqT +/- Long method for the same result. Regardless of any
other consideration, this is the time to observe any body for the most accurate
determination of Latitude by the traditional Meridian Altitude method. It will
be the time of the body’s highest altitude for a specific location on the
surface of the earth. At this point in time the body appears at rest – for a
short time neither rising nor falling before the altitude begins to decrease, i.e. it "hangs". You may, if you wish and as some textbooks do advocate, wait until the body has “dipped” to insure having read its greatest altitude – this constitutes more of a “security blanket” as assurance against having made a mistake in calculation of the time than any sort of technical exercise; it is, however, essentially incorrect and representative of the “good enough attitude” which crept into the teaching of celestial navigation with the advent of electronic methods. I can well remember articles on the various electronic methods denigrating the position finding accuracy of celestial navigation, as practiced at sea, to the order of 15 to 20 miles – a premise with which most experienced navigators, then or now, would hardly agree. Waiting for the “dip” has no great effect on the ultimate outcome of the solution IF YOU ARE STATIONARY, or even moving slowly. If, however, you are moving at any significant rate of speed there is a different tale attached to the horse. With any ship movement, other than due East or West, the observer will be either approaching or receding from the body – neglecting possible declination change of the Sun or Moon, in approaching the altitude will be raised and in receding lowered commensurate with the ship’s motion, and the body will appear at rest when its rate of change due to the earth’s rotation is equal and opposite to the apparent rate of change due to the vessel’s motion. Therefore, at upper transit, in moving toward the body it will appear to be at rest when its true altitude is really diminishing, i.e., after its actual meridian transit – conversely, in moving away from the body it will appear at rest while its true altitude is increasing, i.e., before its true meridian passage. It should be readily apparent from the foregoing that the maximum altitude of a body is not always the true altitude at meridian transit and, when such is desired, the altitude should be taken at the predicted time of the phenomena to attain maximum accuracy of result – otherwise, in either case meridian angle will have developed and, when observing the Sun or Moon, there will be declination change. Conventional wisdom and teaching allows that vessel movement is not as significant factor in East/West ship movement at meridian transit, as respects waiting for the “dip”. This appears to be essentially true; East/West movement at high speed may affect the actual time of the phenomena, however, will not affect the altitude, thus making no difference in the Latitude ultimately deduced, except as may be occasioned by declination change of the Sun or Moon. Traditionally speaking, a relatively close off-meridian observation, or a declination change, should theoretically be dealt with by ex-meridian technology, i.e., utilizing the altitude at dip and the corresponding time, calculate the correction to the meridian, as well as the declination, for the time of observation by the methods described in vintage texts, and therefrom determine the Latitude at the TIME OF OBSERVATION, which reduced to the time of LAN will agree favorably with a Latitude as if then determined. This all may sound complicated, but is really quite simple in practice – it becomes of more relevance as the ship’s speed increases and a greater north/south direction of movement becomes involved, and is of less significance in east/west movement. The ex-meridian, or Reduction to the Meridian, observation
was a standard and traditional method of obtaining the Latitude when, for
whatever reason, the altitude of the body observed, and particularly the Sun,
could not be obtained at LAN – even for the simple reason of the interference
with the lunch or dinner hour. Later teaching emphasizes the traditional sight
for Latitude at Meridian Transit to be but a special solution for a LOP and
suggests, even recommends, that conventional sight reduction methods be employed
in its place to determine a LOP running E/W, or nearly so, to be crossed with
an earlier or later LOP in the determination of a conventional running fix. As
the older methods disappear and celestial navigation is streamlined to suit
solely back-up requirements, as well as educational simplicity, I feel sure
that this latter methodology will ultimately prevail, more or less as one
solution to fit all purposes. This latter concept has just about done away with
the use of the Reduction methodology. Actually, what the MM does will be largely governed by USCG License examination requirements, a matter on which I diplomatically decline comment. IMHO postings to this List frequently seem to consist of “sound bites” which do not take into consideration the whole picture. It should be recognized that a Navigator’s Day’s Work at sea was not a series of isolated sight takings, but rather an integration of sights, positions, speed checks, course checks, and compass error checks, as well as a carefully advanced Dead Reckoning continuously upgraded as evaluated astronomical data dictated. It was seldom, indeed, that the Noon Position was not well known long before LAN. Given:- a reasonably good AM star fix, indicating an overnight speed from the previous PM star fix: hourly Sun lines during the AM, serving both as speed and/or course checks and for advancement to Noon; and, when available, a simultaneous Sun/Venus or Sun/Moon cross - it becomes readily apparent that the Latitude sight at noon was often largely anticlimactic and an acquiescence to tradition. Please don’t misunderstand, it wasn’t always this way – there were times when position finding became a struggle; when it became necessary to use every trick in the book. Regardless, as Jeremy indicates, the Noon position, i.e., the LAN position reduced to 1200 hours Standard Time, had and probably has, in the MM become more of an Administrative Position, used for the standardized calculation of a vessel’s performance in various categories. Yet, I feel obliged to ask what he, or anyone else, did or would do if AM and PM star fixes were not available, as North Atlantic winter conditions so frequently dictate – assuming of course the absence of electronic equipment, i.e., GPS, et al. Lastly, I would briefly address the matter of Longitude determination at Noon by equal altitudes – recently rediscovered by this List and considerably expanded upon by the graphing methods discussed at length. While I am unable to historically comment on graphing methodology, it is possible to cite excellent documentation on equal altitude use, including all relevant corrections for course and speed between observations – for whatever reason, it just died a natural death, apparently by simple neglect or misuse. Suffice it to say, I occasionally used the method at sea and found it to be quite practical; those interested may find further information in Chauvenet’s , Vol. I of Spherical and Practical Astronomy or, if a less “land bound” authority is desired, in a small volume entitled Ex-Meridian Altitude Tables, Declination 0 – 70 degrees, authored by Captain Charles Brent, RN, and others, published by George Philip & Son, Ltd., London, in 1914. Regards, Henry --- On Sun, 1/10/10, anabasis75@aol.com <anabasis75@aol.com> wrote:
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