NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Photo sextant sights
From: Frank Reed
Date: 2008 Aug 02, 22:53 -0400
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 --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---