A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction at the horizon.
From: Frank Reed
Date: 2008 Mar 15, 23:12 -0400
From: Frank Reed
Date: 2008 Mar 15, 23:12 -0400
George H, you wrote: "Greg's postulated value of 6 minutes of arc was indeed unreasonable, and based on a misunderstanding, as I have explained above." His reasoning, which he explained later, was invalid, but nonetheless, the range of variability he quoted is NOT "unreasonable" though we could argue over a factor of two depending on whether we're talking about mean variation, a standard deviation in variation, two s.d., etc. And you wondered: "And even if that assertion is valid, how on Earth does a navigator know, when trying to use a sunset time to ascertain his position, whether Frank's restrictions apply or not, within the "variation within the variability" that he conjures up?" Well, come on. There are many ways that this information can be available. Consider the case of a navigation enthusiast practicing from land: I look around and I see a snow-covered landscape in New England, and in front of me I see the warming waters of the Atlantic in early Spring. That's a case where you need to worry about temperature inversions and unusual refraction. On the other hand, consider the case of a navigator at sea: I know I left Bermuda two days ago, and I'm travelling east. The weather is moderate and I am well south of the Gulf Stream. Those are conditions where I can expect relatively little variability of refraction at the horizon. And of course, there is one very useful clue: the appearance of the Sun itself. If the Sun is strangely distorted just after it rises or before it sets, then that's a darn good indicator that the timing will be off. And: "If we are talking about use of timing a sunset for real navigation, requirements for position knowledge can be very relaxed in mid ocean. It is only when a land mass is being approached that navigation becomes critical. So observations made at coastal sites, with a view over the ocean, will be particularly relevant in assessing the navigator's problem." Yep, that's the irony about celestial navigation. It works best where you need it least. Then again, these problems are not significant when you're sailing towards a mid-ocean island. They're also not as significant in warmer climates. It's true that there are areas and times of year where the refraction variability will be a show-stopper --in the high Arctic, for example, but it would be silly to worry about such issues when they don't apply. In addition, if you were in mid-ocean, in reality, let's say this summer, are you really saying that you wouldn't care about an accurate position? Do real navigators and/or real celestial navigation enthusiasts just ignore the navigation out in the opean ocean?? Just from the anecdotal evidence of talking to people, I find that real people try for accuracy as often as possible. And you wrote: "And those are exactly the sites from which Schaefer and Liller have timed sunsets, at sea, in temperate climes." I guess not. As has become apparent in later messages, many of these observations were made from the site of the observatory at Cerro Tololo, atop a 7000 foot peak in the foothills of the Andes 50 miles from the Pacific. I will be very blunt here: observations from an inland mountaintop have ALMOST ZERO RELEVANCE to celestial navigation at sea. Calculate the dip there. It's around a degree and a half below the true horizon. That means, in the first place, that the MEAN value of the refraction there will be significantly greater than the usual 34 minutes at the horizon (this is partially offset by the fact that the observatory is in thin air due to its altitude). Much more important is the fact that we are looking down into the atmosphere; this has a large effect on refraction from high altitude locations (as Marcel has already described) and leads to some strange and rather beautiful phenomena. Additionally, this location is a special case where there will be significant horizontal variations in the atmosphere. From Cerro Tololo, you're looking out across a dozen valleys towards the Pacific, fifty miles away (and the horizon far beyond that). Some areas will already be in shade at sunset and cooling off as night approaches. Other areas will still be in direct sun. The air over the sea will have yet another temperature profile This is a much more complicated and much more extreme refraction situation than one would ever encounter doing celestial navigation at sea. Again referring to the advice in Bowditch, you wrote: "Not just weak. Absurd." Ummm, no. That is an exaggeration. When I noted that the variability in refraction doesn't matter above about three degrees altitude, you wondered: "Without disputing that assertion, I would like to see some evidence to support it. What precision is implied by "exact for all practical navigation"?" I'm saying that from around three degrees, even if you look at significant variations in the low-altitude temperature profile, you will get the same refraction values to the nearest tenth of a minute of arc starting at about three degrees altitude (above the TRUE horizon). For example, suppose the temperature lapse rate (Lr) is -8 deg C per km from 1km to 13km altitude (constant temp above that, but it doesn't matter at such high altitudes). Then consider varying the lapse rate from -10deg/km to +10deg/km in the lowest kilometer of the atmosphere in steps of 5 deg/km. When you do the integrations, you'll find this table of refraction from 0 to 4 degrees: alt(deg): refr(min) for each Lr (-10, -5, 0, +5, +10) 0: 33.2, 35.0, 36.9, 38.8, 40.7 1: 24.1, 24.4, 24.8, 25.1, 25.4 2: 18.2, 18.3, 18.4, 18.5, 18.6 3: 14.4, 14.4, 14.4, 14.5, 14.5 4: 11.7, 11.8, 11.8, 11.8, 11.8 I should emphasize that these various lapse rates (the columns in the table) are not all equally probable. At sea, the lapse rate is typically close to the lower limit of this table, and sure enough, the values in the Nautical Almanac are very close to the left-most column. The positive lapse rates (implying the air warms with altitude) are relatively rare at sea but quite common on large landmasses, especially when the ground is very cold (e.g. in the early morning in winter). Probably, the next thing that one might worry about is more complicated atmospheric structure. For example, what happens if the temperature falls for a few hundred meters, then inverts sharply at a higher altitude, and then falls again (as it always inevitably does, up to the tropopause). It is not difficult to run integrations of those more complicated situations. The results are qualitatively similar and differ only in the details. You can trust the standard tables for altitudes above three degrees (true altitude). I wrote: " (incidentally, Greg's "6 minutes" is close to the geometric mean of these other claims, for whatever that's worth
)." And you replied: "Not much." You probably know this, but to avoid misunderstanding... That little notation " " is an old online thing meaning "grin", equivalent to the slightly cheesier "smilie" :-). Point being, I was making a little JOKE. I wrote previously, "By the way, there are sometimes temperature inversions even at sea. Calm weather especially is associated with them. It's worth knowing that the same conditions which would lead to anomalous low altitude refraction at sea will also often lead to "sea fog"." And you dismissed: "Can Frank reliably deduce, from absence of sea fog, that there's therefore some limit on the variability of refraction? If not, that comment is of little use." Well, George, you seem to have interpreted my phrase 'it's worth knowing' to mean 'it's a law of nature'. Of course, that would be a most anomalous interpretation. It is indeed "worth knowing" that sea fog is often associated with the same conditions that can lead to unusual refraction, but it's no guarantee. In satellite photos in winter, you can often sea feathery bands of the stuff in the western Atlantic. They start right on the edge of the Gulf Stream as cold air passes over that warm water. There's no guarantee you'll have anomalous refraction there, but it's something to watch out for. And again, the best clue is the appearance of the rising/setting Sun itself. And: "I have often seen a highly distorted Sun disc, at altitudes of well over 3 degrees, that leads me to question it as a general rule." I'm going to doubt you on one detail here, George. Three degrees is six Sun diameters. Now think back... have you REALLY "often" seen a "highly distorted Sun" at six full Sun diameters away from the horizon?? In my experience, and yes, I DO look for these phenomena every chance I get, all the action occurs at much lower angular altitudes. And you wrote: "Evidence requested to support that assertion, please." See the table above. And: "I understand (personal communication from Schaefer) that "our atmosphere always have rapidly and wildly varying thermal structure (i.e., temperature inversions come and go on all time scales and are generally present at some level)." " Well, that's just plain hand-waving. All time scales?? Micro-seconds to decades?? And consider the observational evidence that we have ALL SEEN: the setting or rising Sun. Is it a crazy swirling mass of changing appearances like a mirage?? No. There are small changes that occur on a time scale of a minute or so, but the overall appearance is relatively constant (for a fixed altitude --that is, as the Sun passes through the half-degree altitude line, it might develop a "pinched" appearance, and that pinch will rise up the Sun's disc as the Sun sets). And every once in a while, it looks really odd. And: "In that, he does not distinguish between over-land and over-sea, though no doubt such effects are significantly greater over land." First of all, maybe he doesn't know or care about effects at sea. The phenomena one will see from mountaintops is much more fun. Second, it sounds like you're just assuming something here from the fact that he did not include a bunch of detailed caveats. I wrote previously: "Incidentally, I base these conclusions, in part, on a large number of refraction integrations that I ran back in 2005 (I think) when Marcel got me interested in the production of refraction tables. It's quite straight-forward to take an observed temperature profile and generate a refraction table just for those special conditions." And you replied: "Indeed, Schaefer himself published such an integration procedure (which I haven't seen) in Sky and Telescope, 77, 311 (1989), and the procedure Frank used may have been that one, or one based on it. It was to test the unexpectedly large fluctuations that it predicted, that Schaefer's survey of horizontal refraction was undertaken." No, no connection with the code I wrote. The code published in S&T back in 1989 is trivial (this was the era of the 80386 and the wonderous 387 "math coprocessor"!). Here's just a short sample from that column to remind everyone of the good old days (by then, very few people were using line-oriented Basic, but the editors of S&T insisted on it, which I told them was a mistake then and I still think it was a mistake today. Would you like sauce with your spaghetti?): "350 REM REFRACTION SUBR 360 REM 370 N=N0: DH=S/200: IF H<0 THEN DH=-DH 380 H1=0 390 T=T1: GOSUB 1110: T1=T 400 IF (H1-H)/DH>=0 THEN 450 410 H1=H1+DH: GOSUB 1110 420 EX=-1-(T0/(S*LR)) 430 N=1+(N-1)*((T/T1)^EX) 440 T1=T: GOTO 400 450 REM INITIALIZE PARAMETERS 460 RF=0: L=0: LZ=0: R=R0+H 470 L1=LR: Z=Z0 480 REM" And you wrote: "Schaefer's survey involved 116 observations at 4 different sites, worldwide. If Frank PREFERS simulation evidence from integrating observed temperature profiles" etc. etc. Ya know, George, atmospheric refraction is a completely solvable physics problem. There is today still a bit of new science to tease out from the issue of atmospheric refraction simply because we have far more computational power today, and that's why there are a few specialists working on a few interesting problems. We can easily pick whatever profiles we want, including complicated horizontal variations, if you really want to get fancy (though that requires a different integration procedure). And you wrote: "to such direct measurements, then perhaps he will explain to us the statistics of the population on which his conclusions were based. Just how many temperature profiles did he consider? What was their geographical distribution? How many were taken at the relevant times of sunrise/set? How many measured air temperatures some tens of miles out from the coast, relevant to Schaefer's survey and to navigators' needs? And what was the resulting scatter that Frank calculated?" Well, George, since you're the one making the assertion that the statement in Bowditch is "absurd" (I agree that it could use clarification), then maybe you should be the one digging up data and calculating "scatter". Have you looked at any temperature profiles? Did you try looking at the balloon data for Bermuda that I've mentioned several times? And no, before you ask the obvious, I did not look at JUST Bermuda --there are quite a few oceanic sites with temperature profile data. In reply to Peter, you wrote: "On those occasions when the sky was clear right down to the horizon, with no sign of distant cloudbanks, the time of the last (or first) glimpse of the Sun could be logged to the GMT second, from a well-known height of eye, with a precise GPS position at that moment, with a note of the datum in use. With the closeness of horizon that can be seen from a small craft, it could only work reliably in calm conditions without significant swell. How about that for a project on the next cruise, one that calls for no special instrumentation? It wouldn't be hard to collect that data together from a number of navlist members, and boil it down. Perhaps that's been done already, but if it has I haven't heard of it." Sure. That sounds like fun. Now where do I sign up for my free ocean voyage? -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavListemail@example.com -~----------~----~----~----~------~----~------~--~---