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I had mentioned , about the CD on celestial navigation, newly available from the IoN-

| "One of the morsels available on this IoN disc is "Lunar parallax
| method of astro navigation", by J S Thompson
| ...
| That may strike a chord with some of us on NavList. There's very
| little that's really new..."

and Frank responded-

| Yes. Perhaps you missed it -- I brought up this article on the list
| back in November. Here's my brief review in NavList message #1784:

I may not have missed it, but I had certainly forgotten it. Sorry about that. 
This is what Frank wrote in Navlist 1784-

"As it turns out, it's an interesting article though only tangentially related 
to what I've been discussing. The fascinating aspect
is that the author does a nice job explaining how to track the Moon at one 
arcsecond accuracy, with automated telescopes aboard a
missile, using technology available in 1950. It's rather impressive that it's 
possible at all. At this level of accuracy, you have
to worry about the fact that the Moon's visible center is not quite the same 
as its center of mass. You also have to worry about a
couple of arcseconds of aberration of light arising from the missile's high 
speed of travel. Thompson correctly concludes that it's
not worth exploring the question further, even though the technology might be 
feasible, since the technique would only work when the
Moon is in the sky. You can imagine the problem: "I'm sorry, sir, we cannot 
launch a counter-strike until the Moon is at least five
days old...""

I agree with every word of that assessment.

Frank added, in #2497-

"in fact, the same
| automated, analog light sensing technology that he is describing could
| have been used to determine the location of the horizon. To do this,
| you put a light sensor on a rotating platform. It sweeps around
| looking at a great circle. Assuming we're no more than a couple of
| hundred miles up, any random great circle will intersect the Earth.
| The sensor will alternately see light and dark as it rotates around.
| The platform automatically wobbles about, changing the orientation of
| its axis, until the light signal becomes uniform. At that point,
| you're looking at a great circle parallel to the horizon. That
| observation, combined with ordinary altitude measurements of stars,
| would have yielded a better position fix --and one that worked every
| day of the month (but only in daylight)."

That's an interesting suggestion. However, I doubt if that would have been 
practical, because of the Earth's atmosphere, and surface
topography, though it would on a cloudless billiard-ball planet. Compared with 
the precision, to an arc-second, that the parallax
method required, this would ask for no better than one arc-minute accuracy to 
get a similar result. But that would be achievable,
from "no more than a couple of hundred miles up", with clouds and mountains 
affecting that horizon in different ways, in different
directions?


Frank added-

 "The key is that
| the book informally known as "THE Nautical Almanac" worldwide was
| actually the astronomer's almanac. And that almanac gave the Moon's
| position to a precision of 0.1 seconds of arc."

Thanks. That was what I had suspected.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.




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