NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Latitude by Lunar Distance
From: Frank Reed CT
Date: 2006 Nov 12, 19:21 EST
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From: Frank Reed CT
Date: 2006 Nov 12, 19:21 EST
Henry H, you wrote:
"I cannot help but be somewhat amused by the responses to Frank's
interesting postings regarding Latitude determination by Lunar distance -
whether the concept is old or new notwithstanding. There have been over the
years many interesting manipulations of the astronomical or
spherical triangle proposed..."
Well, sometimes it's amusing... :-)
Of course, it really is all just one "manipulation of the spherical
triangle", as you say. At a fundamental level, it's all the same problem. And
what's amazing is that even one of the highest tech applications of navigation
is very close to this process of "latitude by lunar distance" that I've been
talking about. Highest tech, you say?? Sure. How about navigating the asteroid
belt...
Imagine yourself out among the asteroids between Mars and Jupiter. Lots of
mini planets, many large enough to see with a good sextant telescope passing
within a few hundred thousand miles of your spacecraft. You've brought your
sextant and a rather detailed "Deep Space Nautical Almanac", so you're all set.
Of course, you have to pop the hatch and go EVA to make this work with a
traditional sextant since you can't shoot through windows and expect any kind of
reasonable accuracy, but that's ok. As you're floating around outside your
capsule, with no horizon and no definition of "vertical" at all, you measure the
angles between a couple of nearby asteroids and bright, distant stars. The
stars, in fact, could be the 57 traditional navigational stars. There are no
corrections required for these angles except those due to instrument
error. Each angle places you on a cone of position with its apex at the
asteroid's known position at that instant of GMT, very much like the "cones of
position" that I drew up for the earth-based Moon observations recently. By
measuring two star angles for each of two nearby asteroids, you find that you
can only be in one place in space. If the asteroids are each a quarter of a
million miles away, similar to the Moon's distance from Earth, and the sextant
measures to 0.1 minute of arc accuracy, then the position in space will be
accurate to about six nautical miles, just like the terrestrial case I've been
talking about. Is this too far out there? Nobody will be navigating the asteroid
belt anytime soon, right? Well then, how about a navigating robot...
From late 1998 through the end of 2001, a small NASA test spacecraft, known
as "Deep Space One" maneuvered out into the asteroid belt and eventually
photographed Comet Borrelly. This was not covered much in the media at the
time since the flyby occurred less than two weeks after
9/11/2001. Deep Space One tested an autonomous navigation system that
used the same basic principle of celestial navigation outlined above. But there
were some important differences. First, the spacecraft didn't use a
sextant of any sort. Since the spacecraft's computer carried a database of
stars down to magnitude 9.0, and since you can always see faint stars where
there's no atmosphere, you're no longer limited to the bright stars as in
traditional celestial navigation. Because you can use faint stars, you never
need to measure angles greater than about a degree, and therefore angle
measuring is greatly simplified. You use a digital camera in place of a sextant.
The computer aboard the spacecraft aimed itself at known nearby asteroids, based
on the spacecraft's self-calculated DR position, and then photographed those
asteroids against the background of faint, distant stars. It's then a
straight-forward matter to "read off" the small asteroid's position in right
ascension and declination. The spacecraft then must be somewhere along a ray
that starts at the asteroid and extends into space in the opposite direction
from the measured RA and Dec. Cross two of those lines of position, and you've
got your position fix. This is real celestial navigation applied in space. The
sextant has been replaced with a digital camera. The 57 navigational
stars have been replaced by 250,000 catalogued stars. The target
selection and clearing process are 100% automated. And the horizon, or the
limb of the Moon in the case of lunar distances, is replaced by a set of nearby
"beacon" asteroids. The technological differences are substantial, but this is
the same concept fundamentally as the process for fixing latitude and longitude
at known GMT by lunar distance observations that we've been discussing on the
list.
Neat, huh?
You added:
"Regardless, I sincerely enjoy Frank's postings"
Thank you. Glad to hear that.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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