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oops, I should have said 1 arcminute of latitude = 1 nautical mile).

On Nov 20, 2007, at 9:31 AM, Fred Hebard wrote:

>
> You do need to know the semidiameter of the sun when using a
> reflected artificial horizon for precision work, as you match up the
> edges of the reflected and direct images.  I suppose centering the
> disks would work to a few minutes of arc (1 arcminute = 1 nautical
> mile).  Matching the edges you can easily get the error under 0.2' of
> arc, with practice.  Timing is the most important parameter as the
> sun moves about 1 arcminute in 4 seconds.
>
> The refraction table on the inside cover of the Nautical Almanac
> would work (it's inside the cover of the the orange almanac, but
> further in on the blue one).  The table also incorporates additional
> corrections, if I recall correctly.  The dip table is separate.
>
> Running a level string at eye height is a good way of making an
> artificial horizon and would be the best for kids, in my opinion, due
> to the difficulty of finding the image in the artificial horizon.
> You might want to calculate the error involved in having the string
> deviate from eye height.  Obviously, positioning of the string and
> observing point with respect to the sun's amplitude at various times
> of day are important.
>
> Good luck with getting kids interested for very long!  Soccer or
> basketball might be a better activity for long-term participation,
> even if you're a lousy athlete!
>
> Fred Hebard
>
> On Nov 20, 2007, at 6:43 AM, Isonomia wrote:
>
>> Refraction errors on false horizons
>>
>> The great advantage with a false horizon for children (as opposed to
>> the sea), is that there is no need to pay for icecreams! AKA, taking
>> the children to the sea side - but  more importantly there is no need
>> to know the size of the sun, the height of  the waves, the height of
>> the person or anything else like that as they all  cancel out.
>>
>> The great disadvantage is that you still have refraction through the
>> atmosphere (though not of the horizon!) and the only table I have
>> doesn't  list refraction of the horizon separately from that of the
>> direct view of  the sun so I would appreciate some information on
>> this
>> error!
>>
>>>
>
>
>
> >



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