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Alex raised these questions about artificial horizons-

>This question was discussed a lot in October 2004,
>and I want to add something.
>
>Recently I made many experiments with Davis art horizon
>sold by Celestaire.
>
>I tried various combination of Caro syrup and oil,
>Caro syrup alone, and oil alone.
>(Caro syrup was recommended on this list).
>
>The best "combination"
>seems to be pure vegetable oil, without any syrup.
>I really don't understand why this sirup was proposed.
>(To make the bottom darker? What for? And if this is
>indeed useful, why not to paint the bottom of the horizon
>vessel black with ordinary paint?
>
>The syrup hardens when the weather is cold (it was 23 F last
>weekend), always has impurities and causes double reflection
>unless the oil layes is extremally thin. Such thin layer of
>oil is usually not enough to smoothen the surface of sirup.
>
>I Have not tried the dark (mahine) oil yet, but why it is
>considered useful to have dark substance in general?
>
>Alex.
>
>P.S. I DO understand why Bauer and others recommend sextant
>mirrors painted black on the back side (to prevent second
>reflection from that back side) but with liquid filled art horizon
>with plastic botom it is different: there is no reflection
>from the bottom.

================

Dealing first with his question-"why it is considered useful to have dark
substance in general?": the reason is to avoid reflections from the bottom
of the pan, as Alex surmised when he wrote-

"To make the bottom darker? What for? And if this is indeed useful, why not
to paint the bottom of the horizon vessel black with ordinary paint?".

The problem is this. At present, Alex is seeing a Winter Sun, at rather low
altitude, and with the Sun at that angle, there's plenty of reflection from
the top surface of a liquid. Things get worse as the Sun gets higher in the
sky, though an ordinary sextant limits such altitudes to 60 degrees. A
larger fraction of the light penetrates the top surface, and if the liquid
is clear (such as pure water) most passes through it to reflect off the
bottom of the pan, and provide another, and perhaps brighter, view of the
Sun. No precautions are taken to align (or deliberately misalign) the plane
of the pan bottom with the horizontal, so that the lower reflection might
be close enough to the wanted surface reflection to confuse the
measurement.

That's why the other (Norwegian?) type of artificial horizon, a piece of
flat glass with levelling screws and a sensitive spirit-level, is always
made with black glass, so there's no need to question the parallelism of
the two surfaces.

How is this reflection from the pan bottom to be avoided? Alex gives one
answer; to give the bottom of the pan a non-reflecting surface. "Ordinary
black paint", however, has quite a shine to it, and even matt-black paint
tends to show a bit of mirror-reflection. Best might be to line the bottom
of the pan with a material that is made up entirely of angled facets, none
being horizontal, ribbed like the surface of a coarse file. Perhaps a piece
of glued-down coarse emery paper would do the trick. Maybe a
retro-reflective surface would be the answer, to send most of the light
which strikes the bottom back in the direction it came from. These are
suggestions, but I've never tried them in practice.

Another approach might be to tilt the pan deliberately away from the
horizontal, so that any shine from the botton gives an image that's well
away from the desired reflection.

Anyway, it's to avoid such problems that the Australian explorer Augustus
Gregory used black tea for his superb sextant observations; and then drank
it.

Alex says- "but with liquid filled art horizon with plastic botom it is
different: there is no reflection from the bottom."

I don't know the Davis model art. horizon but perhaps some precautions have
been taken in its design to make the bottom non-reflective at any angle of
incident light. That would indeed be sensible. In which case, there's no
need at all for the liquid to be blackened.

It strikes me that a combination of two non-mixing liquids ("caro syrup and
oil") in a shallow pan would be a bad idea, unless the pan was deep enough
to ensure that the interface was always well below the surface. And might
it not introduce another problem, of reflection at the interface between
them?

The arguments for using filtered used engine oil are-
It reflects well from a nice shiny surface
It transmits no light.
It's viscous enough to that the wind doesn't (much) ruffle its surface, so
usually no cloche is required above it.
It's designed to keep its viscosity reasonably constant over a very wide
range of temperatures.
It's free.
On the other hand, it's filthy stuff to use or dispose of.

============================

There's no doubt that the best reflecting fluid is Mercury, which reflects
nearly all the light from a beautiful shiny surface, and even allows stars
to be readily observed. The snag is that it tends to freeze in Arctic
weather: does anyone know its freezing point? Does it's surface become dull
as it nears its freezing point?

Perhaps there are other possible reflecting liquids which remain
sufficiently fluid at such Arctic temperatures, which can reach extremes of
-45 deg. Celsius. Any suggestions?

George.

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