NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Jupiter and the Moon
From: Alexandre Eremenko
Date: 2013 Jan 21, 10:13 -0500
From: Alexandre Eremenko
Date: 2013 Jan 21, 10:13 -0500
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 > > > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ---------------------------------------------------------------- > > > > > : http://fer3.com/arc/m2.aspx?i=122021 > > >