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    Re: Lunar Distances with Alex's SNO-T
    From: Alexandre Eremenko
    Date: 2006 Oct 31, 08:27 -0500

    Dear Frank,
    Thanks for reporting the first SNO test.
    I hope more will follow.
    Later today I will report on my experience with your
    sextant. I also took several lunars and star distances,
    using the few days of good weather.
    One question. Your Lunar web site has "Height of eye"
    as one of the parameters. I played with it, introducing
    various numbers from 0 to 3000 feet, and this does not seem
    to have any effect. Why then you have this parameter?
    In general, refraction should indeed depend on your height over the
    sea level. (The ray travels less in the air
    if you are high). Is there a good reference for
    such a formula?
    The formulas I've seen were all derived for the sea level.
    Related question: the barometric pressure in the refraction formula.
    Should I use the actual pressure at my observation site,
    or should I "reduce it to the sea level"?
    The actual pressure at my site reflects not only the deviation
    from the standard atmosphere but also my altitude over the sea level.
    On Tue, 31 Oct 2006, Frank Reed wrote:
    > Yesterday, October 29, a little before sunset here in Chicago, I did
    > some lunar distance sights using Alex E.'s SNO-T sextant by the lake
    > near Diversey Harbor. First I did a series of index correction sights
    > off the Moon. Carefully aligning the Moon images pole-to-pole, my
    > observations were (on arc, off arc):
    > 33.1, 31.7
    > 32.9, 31.3
    > 32.7, 31.4
    > 32.8, 31.2
    > Subtracting and averaging, I settled on an index correction of -0.75
    > minutes of arc.
    > The Sun was peeking between buildings near the lakefront and rather low
    > in the sky, so I had time for only three lunar distance sights as
    > follows:
    > GMT        LD
    > 22:21:00  89d 57.8'
    > 22:24:05  89d 58.8'
    > 22:26:26  89d 59.4'
    > The last one looked a little unusual through the sextant since the Sun
    > was half-eclipsed by one the apartment buildings. Air temperature was
    > 50F, reported pressure was 29.90in. Hg. Averaging the three
    > observations and clearing the result, I find an error of about -0.1
    > minutes of arc. That is, the observation was short by a tenth of a
    > minute of arc.
    > Generally, I like this sextant a lot. It's the first chance I've had to
    > try out this design. The inverting telescope is a great help for lunars
    > since it has good magnification, but it certainly takes a little
    > getting used to. For anyone on the list who may be unfamiliar with the
    > SNO-T, it is derived from the German Freiberger with little
    > modification, if I understand correctly, and it was apparently the last
    > production model for the Soviet Navy. The only flaw I find in it so
    > far, which I think bothered Alex, too, is that the micrometer has a
    > significant amount of static friction. It has to be nudged a bit to get
    > it moving. As it happens, the way I shoot lunars (and recommend
    > shooting lunars), this is not a real problem.
    > -FER
    > www.historicalatlas.com/lunars
    > >
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