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For the dip,
you don't have to take any measurements,
Dip (under normal conditions)
it is a simple geometric problem.
Such dip tables could (in principle)
be made by any mathematician in 2-nd century BC.

Refraction is another matter...

Alex.

> I'm not so sure how tricky it was to make those measurements because they
> sure got accurate results. I am absolutely blown away by the accuracy of
> the refraction and the dip tables in the 1799 edition of the New Practical
> Navigator, edited by Bowditch. I compared a sample of the values in the
> 1799 tables with the modern tables and only rarely did the discrepancy
> exceed six seconds of arc, 0.1'. See attachments.
>
> gl
>
>
>
>
> ________________________________
>  From: Marcel Tschudin 
> To: garylapook---.net
> Sent: Friday, December 20, 2013 1:24 AM
> Subject: [NavList] Measuring Dip in the 18th Century
>
>
>
>
> ________________________________
> Andy Young draw my attention to the following publication:
>
> Huddart Joseph (1797): Observations on Horizontal Refractions Which Affect
> the Appearance of Terrestrial Objects, and the Dip, or Depression of the
> Horizon of the Sea. Philosophical Transactions of the Royal Society of
> London, Vol. 87, (1797), pp. 29-42
>
> a pdf copy of it can be obtained from JSTOR
> http://www.jstor.org/betasearch?Query=Huddart+joseph&fq=py:[1796+TO+1798]
>
> Andy wrote: "Huddart managed to measure the altitudes of the Sun's limbs
> above both the northern and southern horizons around noon, interpolating
> to find the apparent altitudes exactly at culmination. This was evidently
> a difficult and tricky measurement to carry out, especially with the
> rather primitive instrumentation available to him; he comments that the
> instrumental limitations prevent it from being done except in a restricted
> zone of latitude, and that it can't be done near the equator because of
> the rapid change in azimuth as the Sun passes near the zenith.
> Nevertheless, he apparently was able to get useful information from this
> work."
>
> He thought that this publication may eventually also be of interest to
> some members of NavList.
>
> Marcel
>
> : http://fer3.com/arc/m2.aspx?i=125858
> Attached File: http://fer3.com/arc/img/125869.1799 refraction and dip
> tables.jpg
>
> Attached File: http://fer3.com/arc/img/125869.img_0002.jpg
>
> Attached File: http://fer3.com/arc/img/125869.img_0001.jpg
>
>
> : http://fer3.com/arc/m2.aspx?i=125869
>
>
>
>

   

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