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    Re: Lunar distance measurement in ideal conditions: attainable accuracy.
    From: Frank Reed
    Date: 2013 Jan 18, 12:08 -0800


    During the period when you were not following NavList messages, I described a method for checking index correction which is more reliable and accurate than any other I have used. It's fairly simple, but it requires a "spotting scope" --a small telescope with a magnification of perhaps 30x. The method is simple. Remove the sextant's scope, and place it on its side on a table where you can see some suitably distant well-defined vertical pole or tower. Then put the spotting scope on a tripod in line with the usual optical path to the horizon mirror. You may need to build a cardboard shade or "umbrella", as Maskelyne put it, to keep out direct images. Then use the usual method to test the index error, lining up the direct and reflected images of that distant marker. Your results should be identical to the tenth of a minute of arc on roughly four out of five trials (based on my experience). I know of no better way to check index correction. I don't know if that would explain your 0.3' bias (which does not appear to be present in your current observations), but it might help.

    But in any case, your results with lunars are just fine. You're getting roughly two-thirds of individual observations within a quarter of a minute of arc (that's another way of saying that the s.d. is about 0.25' on single observations, which, of course, is what I have been saying for years). And your average on sets of four is within a tenth of a minute of arc most of the time. You mentioned a couple of weeks ago that you are somewhat "pessimistic" about lunars. Doesn't all of this make you even a little more "optimistic"?

    For others following along, if you shoot four lunars and average them (with a properly-adjusted modern sextant under good conditions with a 6x or better scope), as was common practice "back in the day", you can expect an error in your observed distance of about +/-0.1' most of the time. This is equivalent to an error in the resulting GMT of +/-12 seconds, which, of course, is equivalent to an error in the corresponding longitude of +/-3' at the equator, or at, e.g., 40 degrees latitude, an error of +/-2.3 nautical miles in position.

    PS: Also, Alex, that same method setting a spotting scope in line with the instrument on a table can be used to measure arc error if you add another sextant with a KNOWN arc error (or better yet zero arc error).

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