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    Re: Photo sextant sights
    From: Frank Reed
    Date: 2008 Aug 02, 22:53 -0400

    George H, you wrote:
    "But in traditional lunar-distance measurement, the measured angles are kept 
    to 25� or more, so the two bodies are well spaced around the ecliptic, and 
    smaller angles than that wouldn't even be shown in the lunar tables." 
    
    While I'm thinking of it, there is an excellent article written by Guyou 
    back in 1904. The title is something like "Lunars: Past, Present, and 
    Future". It's in French. The history is quite good. The part dealing with 
    the "present" is rather short, as you can imagine, and amounts to saying 
    that their days are numbered. In fact, he quotes Lecky who had written a 
    quarter of a century earlier that they were dead and buried 'never to be 
    resurrection-ized". But Guyou speaks of a future for lunars. In particular, 
    he discusses the idea of placing much greater emphasis on measuring short 
    distance lunars of just a few degrees since it's far easier to "hold" the 
    objects together in the field of view of the sextant --which is true. He 
    notes that this requires some changes in the way the sights have to be 
    analyzed, in particular the altitudes would have to be calculated for proper 
    accuracy, but he sees this as the one possible case where lunars might be 
    sufficiently accurate to retain some contemporary relevance --contemporary 
    c.1904! He suggests that this idea was gaining real popularity in France at 
    the time. 
    
    And you wrote:
    "You would never even consider measuring with a sextant lunar distances of 
    5� and-a-bit, as was being attempted here. And so the bodies were not at all 
    spread around the ecliptic; instead, it was damn-near a conjunction between 
    the Moon and Jupiter, exactly the wrong moment to try a lunar distance." 
    
    What's interesting is that the same photopgrapher had provided a photo with 
    even higher quality of Jupiter and the Moon close together where the objects 
    are, in fact, nicely aligned. Have a look here:
    http://www.starpath.com/resources2/pics/06152005_Nikon-evening-work-sm.jpg 
    
    And you wrote:
    "I'm a bit sad that this example has now shown up in Navigator's Newsletter, 
    because its editor, David Burch, asked my opinion about it, and I explained, 
    back in late 2006, the drawbacks of that particular camera-shot." 
    
    Well, it illustrates the principle despite that drawback, and in any case, 
    who's going to lose GMT in this century?? If your GPS is fried, and you've 
    dropped your best digital watch overboard (and your three backup watches), 
    but you still have a high-end digital camera, the best way to get GMT is to 
    turn on the camera, press the menu button, and read the time from the 
    camera's own internal clock. 
    
    Nevertheless, this photograph with the Moon "passing" Jupiter can be used to 
    generate a very nice "lunar LOP" (at known GMT) even if its geometry is poor 
    for a "traditional" lunar distance observation. 
    
    George, you concluded:
    "There are other drawbacks in making celestial observations by camera rather 
    than by instrument, but that will do to be going on with." 
    
    Which difficulties would you consider serious? It strikes me that a digital 
    camera might make a much better angle-measuring instrument than a sextant 
    (though with a very small angular range). Of course, it has to be calibrated 
    properly, but I would say that's much easier than calibrating a sextant. 
    Maybe the biggest drawback is that a digital camera takes batteries! :-) 
    
     -FER 
    
    
    
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