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Mike.Merriman@alumni.tufts.edu
----- Original Message -----
From: "Fred Hebard" 
To: 
Sent: Tuesday, February 28, 2006 1:30 PM


> Greg,
>
> Welcome!
>
> Mirrors have to be very precisely leveled.  There has been much
> discussion of this on the list, which is in the archives.  Another
> alternative is to use a liquid and let gravity level it.  Unless you
> are up against the shore near tall mountains, such as along the South
> American coast, gravity gives a very fine level surface to liquids;
> the same shortcoming would also apply to bubble levels used to level
> a mirror.  I use a Davis-brand artificial horizon, and fill it with
> dark Caro-brand corn syrup overlain with a thin layer of vegetable
> oil.  I'm not sure that Polaris will be easy to see in the horizon,
> but bright stars such as Sirius, Vega, etc are easy to see, plus the
> planets, sun and moon.  Typically, I can get positions within
> 0.2-0.3' of arc, and sometimes under 0.1' of arc.  That is pretty
> close to the limits of resolution of the sextant (0.05' of arc with
> an artificial horizon.  For objects moving fairly rapidly across the
> sky, timing the sight accurately is an important component of overall
> accuracy.  This would be less true for Polaris, should you be able to
> see it in the horizon.  Mounting the sextant on a tripod would also
> facilitate precise observations.
>
> Yours Truly,
>
> Fred Hebard
>
>
> On Feb 28, 2006, at 12:44 PM, cfi@licfi.com wrote:
>
>> Good Day to the List!
>> My name is Greg & I have an interest in historical navigation. In
>> addition
>> to enjoying
>> sailing I belong to a local Rev. War group, where I have the
>> persona of a
>> surveyor.
>> Since surveyors also had to know how to determine lat/Lon I have
>> started to
>> teach myself the Basics. At this point I have hit a small snag. It
>> should
>> be pretty straight forward to shoot Polaris But living inland I
>> need to use
>> a artificial horizon a "well leveled mirror" as the books put it,
>> and I
>> can't seem to come any closer than 3-4 NM. The sextant I am using is a
>> MAC(" Measure All Co. " - Japan ) that I bought off of  a retiring
>> blue
>> water boater on eBay; it seems to be in quite good condition, and I
>> think
>> $250 was a fair price. All I can think is that there is something
>> wrong
>> with the way I am setting it up. May I ask for some guidance please:
>>
>>
>>         Why does the star look "smeared" through the mirror & 'OK'
>> during
>>         a normal horizon sight?
>>
>>         How optically flat does the mirror really need to be?
>>
>>         How close (or far away) from the mirror should you need to be?
>>
>>         Is a good quality carpenter?s level accurate enough?
>>
>>
>> Many thanks to the list in advance!
>>                                 -Greg
>>
>>
>>
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>

   

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