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Frank,
Thanks for your detailed explanation.
unfortunately the snow is falling here in the Midwest, and I do not
except even the Moon to be visible on this night:-(

Alex.

> Alex, you wrote:
> "Near limb will be the dark one."
>
> Only for the first half of the night. Near closest approach, Jupiter will
> be more or less in line with the terminator, and the choice of near or far
> limb may be tough to make. Not that it matters. If there's any doubt, wait
> it out. Then for a good while after, Jupiter will still be very close to
> the Moon but the illuminated limb will be the near limb.
>
> You asked:
> "Why is this a more "excellent test" than any other Lunar?"
>
> I didn't say it was "more excellent". I said it was "STILL excellent"
> despite the fact that it does not represent a historical case. That is,
> even though they would have avoided such sights for the method of
> longitude by lunars, we can still shoot them today to test the instrument
> and observer and also to understand the basic mechanics and manipulations
> of shooting lunars. And it's a nice next step after measuring the Sun's
> diameter (which I described recently as good "training" for lunars). Most
> beginners find it much easier to start with a short distance lunar because
> there's less flailing about with the sextant.
>
> Parenthetically, you wrote:
> "I understand this could be a test for my conjecture about something wrong
> happening with my sextant first two teeth."
>
> Actually, the suggestion from Bill Morris was a terrific idea. If you have
> really concluded that there is some flaw in the arc near zero, then take
> that portion of the arc out of the puzzle. Just adjust your sextant until
> you have an index error of, say, 5 deg 0.0' exactly or, more likely, 5 deg
> 0.5' (some small extra bit). There's no harm in that. For every sight, you
> take, you just knock off 5 degrees before you even write it down. Treat
> the 5 degree mark as your new zero. Incidentally, I asked if you're more
> "optimistic" because the sights which you recently posted DO NOT display
> that 0.3' offset which you say you always find. The average of the set
> shows an error of less than a tenth of a minute of arc, and the standard
> deviation is right around a quarter of a minute of arc, right in line with
> the numbers that I have described for many, many years in NavList posts.
>
> You asked:
> "Do you mean by the horizontal parallax? How else can the latitude affect
> a lunar distance?"
>
> Yes, exactly. It's that "position fix by lunars when GMT is known" that I
> have described in many earlier posts. You can fix your latitude and
> longitude by measuring a pair of lunar distances at known GMT. Again, this
> was never done in the early history of lunars (meaning 1770-1850 when they
> were in active use at sea). But the Apollo spacecraft was equipped for
> shooting lunars and "earthers" (ugly name which no one used --but you get
> the idea: sights like lunars using the Earth's limb instead of the
> Moon's), and on Apollo 8 in 1968, just to make sure it would work as a
> backup, Jim Lovell shot many lunars for fixing the spacecraft's position.
> So there's still a historical case involved here... not historical in the
> sense of 200 years, but 55 years is starting to be quite a long leap back
> into the history of navigation.
>
> I began the above paragraph with "yes, exactly". I suppose I should
> qualify that slightly by saying that this depends on measuring the Moon's
> "parallax in position" generally rather than saying "horizontal parallax"
> specifically, but that's mostly a question of semantics. It's the Moon's
> parallax that makes this possible.
>
> I mentioned that there were other issues (again, historically) with small
> angle lunars, and you asked: "What are they?"
>
> There's the big one involving non-linear interpolation in the lunar
> distance tables. If you watch the distance reaching a minimum tomorrow
> night, you could easily see that the standard tables giving the distance
> once every three hours would have been difficult to apply (quadratic and
> possibly higher order interpolation would be required). Secondarily, many
> methods for clearing lunars assumed that short distances would never be
> used and by design those methods were allowed to be inaccurate for short
> distances. If a navigator shot a short lunar, that might entail learning a
> whole new procedure for clearing the sight.
>
> You also wrote:
> "I understand that the distance changes very slowly because they have
> different declination. But what else is bad about close lunars? Suppose
> the declination happens to be the same."
>
> I think I answered your question above, but I just wanted to add that I
> think you chose the wrong word here. The distance changes very slowly
> because the two objects are passing each other at different "apparent
> ecliptic latitudes" (not declinations), and hence the Moon is not
> travelling across the sky directly towards/away from Jupiter. I don't mean
> to quibble over a minor distinction. I'm saying this only because there's
> a small chance someone following along might get the wrong idea.
>
> -FER
>
>
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