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    Lunars on Sep 20th. (was Re: Perpendicularity check)
    From: Frank Reed CT
    Date: 2004 Sep 25, 23:40 EDT
    Fred H wrote:
    "That's pretty cool we were shooting at close to the same time.  It looks like you may have planned your shoot rather carefully, as it was right when both bodies were at maximum mutual altitude, about 20 degrees."

    Yeah, I thought it was pretty cool, too. :-)
    It's funny what you say about planning. It didn't occur to me. I don't actively plan shooting lunars except to glance at the current distances to see if there's anything I want to try. But when I step outside in the afternoon and see the Moon and Sun "nicely placed", I'll grab my sextant and head right out. I guess it's subconscious planning.

    Your lunars and mine:
    "Here are my raw data.
    Index error, 33 seconds, off the arc (it was off, so I added it to the
    observed distance).
    Presumed pressure 28.0".  Temp 70F. 36*46.8'N 81*50.7'W.  Sun-Moon,
    both bodies touching on near side.

    time    degrees minutes
    09/20/2004 22:04:49     79      52.5
    09/20/2004 22:05:42     79      53.1
    09/20/2004 22:06:26     79      53.4
    09/20/2004 22:07:32     79      54.4
    09/20/2004 22:08:17     79      54.4
    09/20/2004 22:09:04     79      54.7

    I also reduced your data, which made me rather jealous!  Here they are:
    ClrdD   CalcD   time            lunar   delarc'
    80.15185        80.15086        21:54:30        21:54:36        0.1
    80.16221        80.16762        21:56:20        21:55:44        -0.3
    80.18556        80.18820        21:58:35        21:58:17        -0.2
    80.19380        80.19811        21:59:40        21:59:11        -0.3
    80.20626        80.20951        22:00:55        22:00:33        -0.2
    80.22167        80.22090        22:02:10        22:02:15        0.0"

    I reduced your lunars, too, and I got the same large errors that you did. I agree with you that they don't make sense. When I shoot lunars I can consistently get results as above (about 0.2 minutes typical error in the distance). And you yourself consistently get altitudes off artificial horizons that seem to be equally good. I've been trying to think of what it could be about your sextant that could cause a problem. What power telescope do you use? I have a 7x35 monocular on mine. A while back, I had the opportunity over a few weeks to try out over a dozen different sextants, all from major manufacturers and supposedly quality instruments. Nearly all had problems of adjustment that were inconsequential for standard position line navigation but deadly for lunars. By far the most important non-adjustable feature I found from my experiments was the presence of a nice telescope with good optics. It makes lunars so much easier. Incidentally, the sextant I'm presently using is a "Measure All Co." or "MAC", apparently a Tamaya knock-off and not expensive at all. It has a few design issues, and I had to re-do a calibration table for it (0,0,0,0,0,0,0,0 means "we didn't actually test this"), but with that monocular, it is excellent for lunars.

    A few more thoughts: the biggest physical difference when shooting lunars (like the sets above) is that you're holding the sextant sideways. Everything else should be exactly comparable to a Sun sight off an artificial horizon (except shade error in the horizon shades which you can test this week when the Moon is full). Is the index arm of your sextant tight against the frame. If it shifts a little when held horizontally, that might yield an error. And how do you feel about fatigue when you're holding the sextant horizontally? Do you sit when you shoot these? I have a little hand tremor, and I find it helps a great deal to sit and brace my hands when shooting lunars. Also, as I've mentioned before, I grab the frame of the sextant with both hands instead of keeping one hand on the micrometer. I find that the Moon moves slowly enough that I can do fine adjustments by lowering the sextant to a comfortable posture, nudging the micrometer, and then holding it up with both hands again to check. I would be curious to know if any of this helps.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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