Pictures I've seen of using an artificial horizon show the observer
squatting cross-legged with the trough placed close in front on the ground,
or else the trough
placed on some
sort of stool or
table or tripod to bring
it nearly against the sextant of an observer who is standing or perhaps
stooping close by. Such closeness does not affect the reading, and would
allow a smaller trough to be employed.
I agree with Jeremy that it would be interesting to see how good such a
rafted mirror could be, and I would not wish to put anyone off from trying
it out. My intention was just to point out the problems that might arise,
which would need to be overcome.
George.
contact George Huxtable, at george{at}hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: <
Anabasis75@aol.com>
To: <
NavList@fer3.com>
Sent: Saturday,
January 22, 2011 4:02 PM
Subject: [NavList] Re: Making an artificial horizon, and leveling thereof
|I would think that it would be a nice experiment for someone to build a
| rafted mirror and float it in a artificial horizon and see what kind of
errors
| result since they can be compared to the known position. We can then get
| a good set of numbers that indicate the real result of errors that
George
| pointed out.
|
| I suspect that standing close to a artificial horizon would require you
to
| be quite low to the ground for lower altitudes. Standing further back
| would allow for you to stand erect and still see the sun. This is
assuming
| that you don't put it on a reasonably level table.
|
| Jeremy
|
|
| In a message dated 1/22/2011 5:29:46 A.M. Central Asia Standard Time,
|
george@hux.me.uk writes:
|
| Alan
wrote-
|
| "In any event, re "leveling", if I remember correctly, I read somewhere
| that this was NOT critical barring setting the thing up on a steep
| hillside, as "water, I suppose ditto for oil and or mercury, seeks it's
| own
| level". Is this, or is this not the case re using an artificial
horizon?"
|
| Yes, that's correct.
|
| In guessing at how little Mercury one might be able to get away with, I
| wrote, on 20 Jan-
| "Mercury is VERY dense (over 13) so an ounce of the stuff won't go far;
| occupying about 2 millilitres. My guess is that around 10-15 ml would
be
| required in the trough of a sensibly-sized art. horizon, to make it easy
to
| use without having to be over-careful about levelling. That would
| correspond to 5 to 8 ounces.
Maybe it would be possible to penny-pinch
and
| get way with somewhat less."
|
| Since then, Bill Morris has actually tried it out, to see how much
Mercury
| is required to get uniform coverage over the floor of an artificial
| horizon, without the liquid gathering into blobs, and has assessed it as
| 750 grams. This is about 55ml, which is very much more than my own guess
| that 10-15 ml might suffice. I've no doubt that he is right, and accept
| that judgment. There's nothing like practical trial, to get a reliable
| result.
|
| And normally, the levelling of such an artificial horizon is very
| non-critical, just as Alan says. It's only if skimping on the Mercury,
| that
| any tilt might result in the liquid gathering in one part of the trough,
| leaving another bare, or affected by meniscus.
|
| Alan continued-
|
| "I've
done sun shots with mine, in the spring and summer, standing in a
| reasonably level parking area at our apartment complex, taking sun
sights
| several hours apart, that when plotted show quite small displacement
| between my calculated fix and GPS coordinates."
|
| This list thrives on numbers, Alan. Without numbers, even approximate
| ones,
| a statement such as "show quite small displacement" has no meaning to
| anyone other than you.
|
| "Seems that orienting the ah properly is an important factor, as is
being
| able to stand far enough away from the ah so as to be able to see the
| reflected and sextant suns."
|
| That's a surprise, to me. On what basis do you deduce that any such
| discrepancy is the result of mis-orientation? And why do you need to
stand
| back to see the two views? Surely, the closer you can get,
the larger is
| the solid angle that's available in the liquid reflector. I see no such
| advantage in standing back, as long as the wind-shield isn't interfering
| with the direct view.
|
| George.
|
| contact George Huxtable, at george{at}hux.me.uk
| or at +44 1865 820222 (from UK, 01865 820222)
| or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
|
|
|
|
|
|
