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    Re: Celestial Navigation as a college course
    From: John Huth
    Date: 2010 Apr 13, 19:40 -0400
    Sometimes the mistakes of science can be more enlightening than the successes.  

    So, there is the famous measurement of the radius of the Earth by  Abū Rayān Muammad ibn Amad Bīrūnī (973-1048).   Supposedly this was carried out from a mountain top in India and used the dip angle.   The difference between his value and the modern value is something like km.    I wonder if he did any corrections for refraction or was perhaps the lucky recipient of the right atmospheric conditions?

    John H. 

    On Tue, Apr 13, 2010 at 7:26 PM, Frank Reed <FrankReed{at}historicalatlas.com> wrote:

    John, you wrote:
    "I'm reading these comments closely as I'm trying out a course next fall that uses primitive navigation as a theme to do physics. The "primitive" hook allows me to go beyond simple celestial navigation to include more items."

    Sounds great! By the way, since you mentioned bioluminescence again, I should add that I thought of you several times while I was reading Bligh's logbook from the Bounty. He comments quite a few times on luminous animals in the oceans at night. On one occasion, he says that the light is from small transparent animals about two inches long which dry up to nothing overnight (I'm thinking comb jellies here but they may have been salps). Later he says that some other luminous animals are "blubbers" about a foot across. That's English-English for jellyfish. If you can somehow manage to talk about bioluminescence in September, before they disappear, you can easily scoop up some 'Mnemiopsis leidyi' from the waters around New England. These are comb jellies that are great fun to see flicker when disturbed in an aquarium in a darkened room, but you have to get everybody dark-adapted for a solid five minutes --I think that's the main reason more people don't see bioluminescent marine organisms these days; they're just never sufficiently dark-adapted. I used to bring them into planetarium shows in a jar back c.1980 and shake them up during the program while talking about the potential for life in the seas of Jupiter's Europa... And Mnemiopsis also has some interesting modern history. Within the past thirty years, it became North America's revenge for the Zebra Mussel and now infests the Black Sea.

    Also, on dip, I like to point out that it is a nice example of how science does NOT work. Pardon me for going on about this... :)

    The dip that we all know, IF it calculated from straight-forward Euclidean geometry based on the known diameter of the spherical Earth gives incorrect results. In other words, simple trigonometry predicts a dip table that does not match observations. It's wrong by about 10%. Therefore, if we follow the precepts of mathematics, we have to throw out either Euclidean geometry or the theory of a spherical Earth (flat-earthers rejoice), or at least we have to deduce a different diameter for the Earth. But science doesn't work that way --it doesn't work like mathematics despite often looking like math-- and we often keep theories around that fail the immediate test of observation, and in the end we know that the dip model has to be corrected by terrestrial refraction. So geometry is safe, and the Earth really is round. No physical theory exists in isolation.

    This "failure of dip" was, in a way, at the center of a bizarre "bet" over the sphericity of the Earth back in the 19th century. A small group of modern flat-earthers, including one zealous fool at their head, back then claimed that they could see no curvature of the surface of water in a six-mile stretch of a canal in England. Therefore the Earth is really flat. A simple test showed otherwise, and the winner was supposed to get a big payoff. The flat-earthers claimed fraud, and a later photo even seemed to prove their case. Was the Earth really flat, and was science engaged in some elaborate self-deluding fraud? Isn't science supposed to work by throwing out hypotheses that are falsified by evidence?? Of course, we know today, and many knew back then, that the right refraction conditions, in particular a temperature inversion near the level of the water, can make the Earth appear very nearly flat, every once in a while, as far as all optical observations are concerned. Dip really can be ZERO for any height of eye under the right atmospheric conditions! Here's a Wikipedia article for some background on this craziness: http://en.wikipedia.org/wiki/Bedford_Level_experiment. And don't think it was just a few kooks in Bedford. Joshua Slocum found a flat-earther in South Africa in 1897 after he had sailed three-quarters of the way around the world...

    -FER

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