NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Dip observations by Carnegie Institution
From: Marcel Tschudin
Date: 2013 Jun 13, 23:54 +0300
From: Marcel Tschudin
Date: 2013 Jun 13, 23:54 +0300
Frank, please receive below a few replies to your comments: > > Marcel, you wrote: > "I encourage you to continue with your direct measurements of the dip, > hopefully arriving at a data set covering all seasons, with dips resulting > from positive and negative temperature differences between air and sea." > > And you two can have roasted wild goose for dinner next New Year's Eve. Yes, why not? Realising that one uses refraction and dip values based on measurements and analyses made 50 to 100 years ago I wonder how these previous values compare to own measurements and to what extend the estimations could be improved by correlating them with meteorological data as they are available these days. I was from the beginning aware that this requires a lot of data and that it will therefore be a long term project. > > You also worte > > > "This would unfortunately not help much because one would use for this > verification the (astronomical) refraction which itself is not exact." > > What? The astronomical refraction is most certainly "exact", as far as > sextant observations are concerned, except for altitudes below two or three > degrees. Your personal, specific choice of a problem to analyze --sunsets-- > is indeed subject to this problem combining the worst case scenario of > astronomical refraction with the uncertainty of dip. But this problem does > not affect normal sextant observations at sea. I always mentioned here that I investigate refraction near the horizon, and - if I remember right - always supported your warnings related to sextant observations at low altitudes. > > Bruce had asked if celestial fixes might benefit from using "smaller dip" > values. I've said it before, but I will say it again: this is a wild good > chase. Even so, setting that aside, you can always treat dip as an unknown > if you have more than four (better yet dozens of) observations from the same > height of eye. Just count it as a systematic error. Then you adjust that > common offset to all observations until the scatter in the plot of LOPs is > minimized. Naturally any other systematic error, for example a small error > in IC, will be included in the result. There is no way to separate this from > a systematic error in dip. > > But is this news?? Of course not. The basic principle has been known for > decades. Nautical astronomers have been making experimental observations to > better understand dip for over two hundred years --since the very beginning > of modern scientific celestial navigation. The idea that there is some > large, obvious correction to be made to the dip tables borders on the > absurd. There are variations in dip on the order of half a minute of arc or > more that arise from well-known causes, but which cannot be corrected in any > practical way. Sorry but I can not see what should be absurd when comparing own results with published ones. We obviously rate what I am doing differently. I do this study out of a general interest, not limited to practical navigation as you seem to look at it. It is my personal opinion that a data set with measured dips would help finding out to what extend its estimation could be improved, e.g. with (additionally) found correlations. It is for this reason that I encouraged those who do have the instruments to measure and collect such data. Because the final data set should have e.g. some several hundred observations, spread over all seasons this would rather be a longer term commitment. I was wondering whether there is someone on NavList who likes measuring dip as a hobby and who would also share my interest in investigating again the dip estimations. Marcel P.S: Regarding some of your comments further down. I never pretended that dip or refraction near the horizon would not be variable which your comments may allude to. Considering additional correlations help however reducing the spread of the scattered values. > > You concluded that: > > > "The "Nautical Almanac"-model (1) is not the best one." > > This statement is so minimalist that it cannot literally be false. > Unfortunately, it "suggests" that there is something wrong in practical > terms with the normal dip tables. And that is not true. It IS true that the > uncertainty of the horizon has always been the principal uncertainty in > altitude observations. But "fixing up" the dip tables or adding more > independent variables is not going to remove that principal uncertainty. > > Also, there is a small technical error here. You've put the number "(1)" in > this description referring back to your original list "Model 1: Refraction > with Bennett formula and Dip [etc. ...]" That could easily be misleading. > The refraction tables are not computed from some variation of the so-called > "Bennett formula". It is the other way around. The refraction tables have > been derived from a combination of modelling of atmospheric physics and > astronomical observations. They are generally computer by numerical > integration. This was a difficult task as little as thirty years ago, but > today it is simple. The editors threw the Bennett formula into the > explanatory pages in the Nautical Almanac decades ago as a token offering to > amateur calculation enthusiasts. The Bennett formula and its cousins have > value when you need a quick, computationally cheap way of generating > refraction values (for example, it's easy to type in a spreadsheet cell), > but it is not a "fundamental" equation. > > Marcel, you concluded: > > > "A noticeable improved estimation can be attained by considering additional > environmental parameters." > > You've spent several years collecting a mountain of data on sunsets. From > your perspective, there damn well better be some improvement in estimation > that results from all that hard work. Or... maybe... it's a wild goose > chase. > > I have so far seen nothing with respect to dip variability that could be of > any practical value to anyone attempting practical celestial navigation > under any circumstances that I can imagine, whether manually or in > automation. Dip IS variable, especially approaching land, at a level of some > fraction of a minute of arc. Occasionally, under extreme environmental > conditions, the dip is MUCH MORE variable. It is important to know that it's > variable. It's potentially useful to understand how simple variations in the > temperature structure of the lowest level of the atmosphere can influence > the dip. It's certainly useful to be aware of other "signs" that may promote > extraordinary dip. But unfortunately, the only observable measure of that > temperature structure over the distance scale we require, is the terrestrial > refraction itself: the dip or some equivalent. You can observe the dip to > estimate the terrestrial refraction. And from the terrestrial refraction you > can calculate the dip. > > -FER > > > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ---------------------------------------------------------------- > > : http://fer3.com/arc/m2.aspx?i=124333