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    Re: Lunar Distances with Alex's SNO-T
    From: Bill B
    Date: 2006 Nov 01, 02:48 -0400

    I was writing a reply to Alex when this came in.  Perhaps I shall post some
    of the other points later.
    Alex has in the past looked poorly on natural horizon IE/IC. I still do not
    understand why.  It is two lines. Optical system(s) distortion of two
    spheres or a sphere and line/point are of little consideration.
    Alex has some stringent requirements on accepting sun IE/IC checks with
    semi-diameter figures based on the SNO-T manual and published almanac SD
    figures, and has until recently discounted interpolating the sun's SD when
    any error in the SD would be magnified by 4 in the 4SD. This has been
    discussed on the list.
    Given the above paragraph and the rapid change of the moon's SD, it seems
    foolish to apply sun standards to the moon.  It is not in the moon's nature
    to obey the sun's SD rules.  If I recall, the moon's travels were the last
    to be nailed down--a wild child compared to other bodies.
    There is also a reasonable amount of personal testimony/documentation that
    changing the scope's focus affects the side error and/or IE-IC of the
    You have noted in the past that a bit of side error may be useful (stars).
    Applying common sense, a bit of side error will reduce a sphere's 4SD. (If
    the side error becomes large enough the direct and reflected image will
    never become tangent!)  So why the Alex's acid test on 4SD for IE/IC sun or
    moon checks?
    A question. Side error can change the tangency of the horizon and a body if
    the sextant is not plumb, or lunars, but does 4SD mean anything other than a
    sanity check when doing IE/IC checks with a sphere?
    > Alex, you asked:
    > "Why did not you use the Sun?"
    > Because I get excellent results with the Moon, and it eliminates any issue
    > of shade error. But I repeat, if you want real verification, use a table-top
    > measurement of index error. If you have a large enough work area (a couple of
    > hundred feet) and a decent laser, it should be infallible.
    > And you wrote:
    > "Adding and averaging gives 64.275.
    > While according to  the Almanac, true Moon SD
    > for October 29 was 15.8, which gives  4DS=63.2.
    > So you are off by the whole minute.
    > So much of the Lunar IC correction.
    > Or do I confuse something?"
    > Actually, the correct horizon SD at the time of the observation was 15.9
    > (calculated from 27.27% of the HP --you should always use a calculated SD when
    > doing lunars). Then you need to augment it for altitude which gives 16.0
    > minutes  of arc. The result is then quite a bit closer, but yes, a little off
    > --an
    > error  of 0.14 minutes of arc in the Moon's diameter.
    > Based on your  similar observations you wrote:
    > "Now I derive from my data IC=0.7 which also seems absurd."
    > I checked my notes, and that was the IC of that sextant last time I  checked.
    > Then again, you've adjusted the sextant a bit so don't count on  it.
    > You concluded:
    > "So I am very skeptical about using the Moon for IC correction."
    > Keep at it, Alex! In my experience, I know of no better simple observation
    > than this one. Also, ask yourself this question: how could it possibly go
    > wrong?! This is one of the simplest observations you can make with a sextant.
    > If
    > it doesn't work, what on Earth could explain the error??
    > By the way, for those on the list who are not crazy lunarians  (not yet crazy
    > lunarians, I should  say ), there is, of course, nothing wrong with the
    > tried-and-true  method for getting index correction by lining up the direct
    > and
    > reflected  images of the sea horizon. The reading on the micrometer is the
    > "index error"  and the negative of that number is the "index correction".
    > Equivalently  there's the navigator's mnemonic: "if it's off, it's on; if it's
    > on,
    > it's off".  More explicitly, if the reading on the sextant is above zero when
    > the direct and  reflected horizons are lined up, then that value should be
    > subtracted from every  observation made with the instrument; if it's below
    > zero,
    > that value should be  added to every observation. This horizon test will yield
    > an index correction  accurate to perhaps 0.25 to 0.5 minutes of arc --plenty
    > good enough for standard  celestial navigation, but not quite up to the task
    > for
    > lunars.
    > For the experts on the list, I'm spelling things out here because there  are,
    > in fact, some navigation 'newbies' on the list, and I do hope they'll find
    > something of value in our occasionally obsessive/compulsive discussions of
    > tiny errors.
    > -FER
    > 42.0N  87.7W, or 41.4N  72.1W.
    > www.HistoricalAtlas.com/lunars
    > >
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