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Courtney,

They are making an artificial horizon.

What is an artificial horizon?  When you use a sextant to measure the
altitude of a star at sea, you generally measure the angle between the
star and the horizon visible from your ship.  Unfortunately, not all of
us have the opportunity to look at the horizon over the ocean, since we
are land locked.  We then have to resort to some contrivance to
substitute for the horizon.  Most often, people use a reflective
surface, such as a mirror or a bowl of water or oil, and superimpose
the image of a star seen through the index mirror of a sextant on the
image of that star reflected in the artificial horizon, viewed through
the horizon mirror of the sextant.  You end up with an angle which is
twice as large as the altitude of the star, ignoring index and sextant
error.

A bowl of water or oil is self leveling, since it is leveled by
gravity.  (Gravity is also what "levels" the horizon at sea; ignoring
waves, the sea is flat, at least that part of it you can see looking
straight out from a ship at a normal height of eye for sextant
observations).  But bowls of oil or water suffer from the defects of
being not very reflective and being ruffled by the wind.  The lack of
reflectivity makes most liquid artificial horizons unsuitable for use
with all but the brightest stars and planets, and, of course, the moon
and sun.  A mirror, on the other hand, has plenty of reflectivity, but
must be leveled precisely and accurately in order to give good results.
  These guys are discussing constructing a mirror-type artificial
horizon out of black plexiglas.  The 6-32 machine bolts are for
altering the tilt of the mirror until the Dow RTV 3145 bonded-levels
indicate it is perfectly level.

These lists are archived at http://www.i-DEADLINK-com/lists/navigation/.  If
you read through the threads from this month on artificial horizons,
you might be able to figure out what I just told you, but please, keep
on asking.

Yours Truly,

Fred Hebard

On Monday, Jul 14, 2003, at 23:04 US/Eastern, Courtney Thomas wrote:

> This is the first message I received as a new subscriber and am
> intrigued with your activity. What are you making and for what purpose
> please ?
>
> Appreciatively,
> Courtney
>
>
>
> Royer, Doug wrote:
>
>>> -----Original Message-----
>>> From:         Royer, Doug
>>> Sent: Monday, July 14, 2003 16:49
>>> To:   'navigation-l@listserve.webkahuna.com'
>>> Subject:      Plexiglass horizon
>>>
>>> Looks like Jared was thinking along the line I was.Over the weekend I
>>> bought a piece of black plexiglass 7x7 in.square,3/8 in. thick for
>>> $12.00
>>> at Ridout Plastics(any plastics store should have it).Drilled and
>>> tapped
>>> the plexiglass in each corner for a 6-32 machine bolt 1.25 in.
>>> long.Got 2
>>> more liquid levels and useing Dow RTV 3145 bonded the levels to the
>>> plexiglass.I left the paper on one side of the plexiglass in case the
>>> reflecting side gets scratched.That way I can just turn the whole
>>> thing
>>> over,reverse the the bolts in each hole,rebond the levels on the new
>>> reflection surface and use it.This is much lighter than the glass
>>> one I
>>> made.No problems in seeing clearly and sharply the points of light
>>> from
>>> stars as I used it last night to test just that.
>>> I'm leaveing this week to take a vessel from San Francisco through
>>> the
>>> Panama Canal to Jacksonville,Fla.I'll sign off the list Wen. before I
>>> leave and sign back in when I get access to a shorebased computor
>>> somewhere along the route.Take care gentlemen.
>>>
>>
>
>
> --
> Courtney Thomas
> S/V Mutiny
> Lying Oriental, NC
>
>

   

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