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    Re: Working a lunar. was: lunar distance in Wikipedia
    From: Ken Muldrew
    Date: 2007 Sep 19, 10:58 -0600

    On 19 Sep 2007 at 16:32, George Huxtable wrote:
    > For doing the job using
    > precomputed lunar distances, with logs, 22 minutes is pretty good going,
    > and shows that Henry's brain, as well as eye and hand, is as good as ever.
    > But I wonder, did that answer come out right,and within 22 minutes, at his
    > first attempt? Or did he have to make a few tries at it, with some
    > false-starts, as I usually have to do, and correcting arithmetic errors,
    > before eventually getting things right?
    For what it's worth, I usually end up taking about 40-45 minutes to clear
    a lunar and a time sight because I almost always make trivial mistakes
    where a number of steps have to be recalculated to correct each mistake.
    On the odd occasion when I make no mistakes, I can do it in under 30
    minutes (though I work quite slowly to prevent mistakes). This is with 4
    place logs using Witchell's method, so It would take some practice to
    catch up to Henry.
    > For mariners of the lunar era,
    > repeating the same calculations day after day, it would all become a matter
    > of routine, involving little thought; but for us today , it's rather
    > different.
    Yes, I think 20 minutes would be a fair expectation for a typical
    navigator of the day.
    I should mention that I can clear a lunar using Margetts' Tables
    (available in 1790) in under 5 minutes with very little worry of making
    any errors. If the myth of the 3 hour clearing process kept navigators
    from using lunars, it's really too bad these tables didn't become well
    known back then. I'm not sure history would have unfolded any differently,
    but we'd probably see a lot more lunars in logbooks, looking back on the
    era from today.
    > By the way, observing lunars with Jupiter is a bit different to observing
    > stars, in that Jupiter has an observable disc when seen through the
    > telescope, with a semidiameter of around 20 arc-seconds. I've never tried a
    > lunar using Jupiter. Presumably, the way to do it is to put the centre of
    > Jupiter's disc against the edge of the Moon, not align them edge-to-edge.
    That's the way I've done it; you just try to center the bright smudge
    between the inner touch and the outer touch (for beginners, use a shade
    when taking lunars from Jupiter to reduce the flare).
    Also, I've mentioned this before, but for anyone who likes working lunars
    the old way, I've written a windows program that prints out almanac pages
    much the same way that they were published in the Nautical Almanac ca.
    1800 (but with lunar distances to planets added). Well, it's actually just
    a text file, but it's a simple matter to import it into Excel and print
    off the pages. You can see a sample here:
    The program and the source code are freely available here:
    Also included is a somewhat whimsical star chart updated to use the modern
    navigational stars.
    Ken Muldrew.
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