# NavList:

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

**Re: Time of meridian passage accuracy**

**From:**Douglas Denny

**Date:**2009 Sep 27, 16:46 -0700

George. At last you have addressed the question posed; and, (surprise?) I can agree with most of what you say in this posting, as it seems we are saying the same thing mostly. My edition of Bowditch is 1995, and states exactly the same as Andreas quotes, including the paragraph number 1801, with the one minute time constraint for Local Apparent Noon. (LAN). I note you now agree with me that Latitude from LAN within one minute constraint is acceptable; and I agree with you the Longitude from a sight one minute either side of LAN is dubious. The LAN case is a special case with LHA at zero where Latitude can be obtained quite accurately, and all that is required is 1) an accurate declination for the time of LAN (easily obtained from almanac or calculated) and 2) accurate sight at the time of the true LAN, (which can be easily calculated from Mer Passage of Sun at Greenwich with longitude correction). Actually for "extraordinary accuracy" this is not good enough but for practical navigation it is quite good enough. I have not disagreed with you that the true LAN is not the same as max altitude observed. This is, or should be, common knowledge. Where I disagree is that the difference for a stationary observer is small, and for practical use one minute either way of a calculated LAN is acceptable - for Latitude anyway. Using a sight at a calculated time for the actual LAN is accurate enough so long as the observer uses the calculated time of sight and not the max altitude per se - though there will be little difference between the two. It would not normally be the case however in modern practical navigation that Longitude would be obtained with anything other than accutate timings for the observed sight - noon or at any time. There is no other method worth using other than Marcq St Hilaire in my opinion - which is a universal method with few limitations unless you are into Polar navigation. The whole business using the Sun at LAN or near LAN is similar to the so-called 'Ex-Meridian' problem or 'Equal altitude' problem, where the main difficulty is difference in declinations between sights due to it's constantly changing. These methods were only developed because of the practical navigator's historical preference for the almost religious "Noon Sight", and not being able to take a sight if the cloud cover did not allow it at exactly local noon. ----------- Latitude can be obtained accurately without a knowledge of the absolute time of two sights, of the same body separated in time, so long as an accurate time between them is known, in other words, having a clock which you do not know the actual accuracy of the absolute time but is accurate to measure the interval. I have done this myself for the fun of it using marine sextant and mercury artificial horizon. It is an interesting exercise in spherical trig. ------------- To address some points in an earlier post: George: "If we rephrased Douglas' statement, above, to read- "Local Apparent Time is, by definition, 12 hours at the instant when the true Sun crosses the observer's meridian", then I suggest both of us might accept those words" Agreed. I was lax in fast writing. I spotted afterwards too late I should have put '... when true Sun crosses the local meridian'. I was referring to the Local Transit Time. ---------- George: "But that is not the instant of greatest altitude of the Sun; not even for a stationary observer, because of the Sun's changing declination. The difference may never be much more than a quarter of a minute of time, but it isn't zero (except at the solstices)". I have not disagreed with this anywhere. It is not a large quantity however. ----------- George. "(For the Moon, because its declination changes much faster, the time difference between meridian passage and maximum altitude can amount to many minutes.)" Agreed. The Moon is always a nuisance - it moves so fast. Newton declared whilst trying to analyse Her position in the Heavens as the only problem that gave him a headache! ---------- George: "In many situations, such as deciding on the best moment to observe for a noon Sun latitude, that time difference can indeed be ignored, for an observer who is stationary or slow-moving. But when the longitude is to be derived from timing the Sun's changing altitude, it must be considered; otherwise, it will just add a systematic error to the result, which varies with time-of-year. That's what Douglas has neglected to take into account. If he is simply claiming that such a correction is too small for him to bother with, in view of the limited precision of the observation, then that would be fair enough. However, he appears to be denying the case for such a correction at all, as I read his postings. Is that his position?" No it is not my position. You are extrapolating and making unwarranted suppositions. I am claiming Bowditch makes a reasonable satement about a one minute criterion for LAN in practical navigation terms. (And that I would say, only if a proper LAN is calculated). This is certainly OK for Latitude, but like you I am uncomfortable about any such use for longitude and do not think it reaosnable for this. I wonder in fact if Bowditch makes an error in suggesting this use of LAN for longitude. --------- George: "I ask again: what happened to the claim, in [9932], that- "The difference is 0.000737068 degrees, or 0.04422408 minutes, or 2,6 seconds of arc. Utterly negligable " Does Douglas wish to defend that claim, or perhaps withdraw it? What does it mean?" It means the differenece in declination of the Sun between the three minutes of change of time from Mer Passage at Greewich to Mer Passage at Bosham is only 2.6 seconds of arc. You missed the point that the example and graph of results has a calculation for latitude which uses a declination calculated for Mer passage at Greenwich (three minutes earlier than at my location). The use of Dec for Greenwich is negligable compared to Bosham three minutes later: it does not affect the results for declination used to calculate latitude. You will note that 2.6 seconds of arc in three minutes is equivalent to 0.8 minutes of arc in one hour - which is what the rate of change of declination was at the time of observation and what you estimated somewhere in a previous posting by a simple calculation. ----------------- George: "Thanks for bringing that Cotter book to my attention. I hadn't come across it before. Cotter also treats these questions in his "History of Nautical Astronomy", (1968), but some aspects are confused, and several of his equations are wrong. Perhaps he had got things clearer by 1969." I have not yet looked to compare Cotter's 'History of Nautical Astronomy' with his 'Complete Nautical Astronomy' and check your claim that he was inaccurate in some way. I have all of his books, some with personal annotations to me as I knew him from my Cardiff student days. It is due to Charles Cotter that I have an interest in navigation and nautical astronomy today. He was a most charming and generous man and an excellent teacher. I would be very surprised indeed if he is in any way "Confused" or with "wrong equations" as he was a recognised expert in the theroy of nautical astronomy. He was a lecturer teaching at Cardiff University and wrote extensively about the subject in books and in the Journal of the Institue of Navigation over many years. This subject of Meridian Passage not being the same as maximum altitude was written about in the 'Journal' by him extensively and in detail. He also wrote extensively about the Ex-Meridian and double- altitude problem amongst other things. ----------- In another posting you suggest taking Smart's Equation of Time formula with "a pinch of salt". (?). Smart is clear and says it is ..."accurate up to the order of approximation adopted"... explained in the text. It is quite accurate. Perhaps your edition is an earlier one than mine of 1977, in which he uses parameters for epoch 1975. The values used in the final formula of ecentricity and longitude of perigee are for 1975 and can be ammended going back to the earlier parts of the formulae if wanted. The mean longitude of the Sun I used in Smart's formula was calculated with up to date parameters for obliquity and nutation included. Douglas Denny. Chichester. England. P.S. I will have to start putting in a caveat to ignor obvious typos and/or simple blunders as one cannot correct them after they are posted. I always seem to notice silly errors which I miss with the quick scan through after writing which I pick-up on re-reading later. It is a pity one cannot have say fifteen minutes to go back to a posting and make corrections. Douglas. ========================= Original Post:- Andres wrote- "BOWDITCH says this: 1801. Equation of Time To calculate latitude and longitude at LAN, the navigator seldom requires the time of meridian passage to accuracies greater than one minute. Therefore, use the time listed under the "Mer. Pass." column to estimate LAN unless extraordinary accuracy is required. Pub. No. 9 THE AMERICAN PRACTICAL NAVIGATOR. 2002 BICENTENNIAL EDITION Any opinion? ============== That's an interesting question. I can't find those words in my earlier 2-volume edition of 1977 , not in para 1801 or in para 1809, which it devotes to equation of time. It's certainly true that to calculate LATITUDE from Sun altitudes at LAN, there's no need to know Sun-time to better than a minute. But in the (unusual) situation of trying to deduce LONGITUDE from altitudes-around-noon, knowing Sun-time only to the nearest minute adds an unnecessary error of +/- 7.5 arc-minutes to the result. Perhaps Bowditch presumes that in general, attempts to deduce longitude that way are going to be rough-and-ready ones, in which such an additional error wouldn't matter. And that any attempts to do better come into the "extraordinary accuracy" category. But it seems silly to me, to introduce such unnecessary error by taking the predicted "mer pass", given only to the nearest minute, when right alongside it is the prediction for equation of time, given to the nearest second. Why not use that? Or, if needing to be even more precise, why not use the Sun GHA prediction for noon that day at Greenwich, and convert the difference from 0-degrees into time? Of course, all those predictions are for noon that day at Greenwich, and as equation-of-time is continually changing, though slowly, by the time it's local noon where you happen to be, the EoT will be a bit different. It's pretty easy to allow for this by using the tabulated value in the almanac for that day, and also for the previous day or the next (depending on whether you are East or West of Greenwich), and interpolating accordingly. There are other ways to get Equation of Time. It can be computed from "first principles", in a similar way to that described by Douglas Denny, who wrote, in [9923]- "The Equation of Time is quite rigorously dealt with in W.M. Smart's book 'Textbook on Spherical Astronomy'. A formula is quoted based on the mean longitude of the Sun." I have the 5th edition of Smart's textbook, printed in 1971, which gives the equation of time in equation 32 of chapter VI, "Time". And yes, the calculation of equation of time has been treated rather carefully. However, if precise results are needed, it needs to be taken with a pinch of salt. Smart treats the tilt of the Eart's axis, the eccentricity of the Earth's orbit, and the longitude of perihelion as constants, adopting their values as they were in 1931. In fact, all three change slowly over the years, altering the shape of the curve of equation of time quite dramatically when taken over several hundred years, as Meeus illustrates in his chapter 28. Over the ensuing 78 years after 1931, those changes will be less dramatic, but will be there. For any precise work, that Smart formula should not be used blindly, without taking some care to check whether it still remains valid (I haven't done so). 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