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George Huxtable wrote:
>
> Geoffrey has said nothing at all about the context of the observations for
> which he was claiming such accuracy, and I think it's about time he did.

He hasn't replied, so I would like to mention the Danjon impersonal
prismatic astrolabe. Danjon himself described the instrument in this
1958 article in the Monthly Notices of the Royal Astronomical Society:
http://adsabs.harvard.edu/abs/1958MNRAS.118..411D

By the 1980s, the U.S. Naval Observatory had built a transportable
Danjon astrolabe system for astro-geodetic work. This 1986 article says
it can "determine position to within one meter accuracy." Unfortunately,
the article focuses on the electronic data acquisition and processing
equipment and has little to say about the astrolabe.
http://adsabs.harvard.edu/abs/1986IAUS..109..397L

It's interesting that both articles say the observations may be plotted
like celestial lines of position. In fact, Danjon's article includes a
couple plots. Because of the large number of LOPs at different azimuths,
the "cocked hat" looks more like a circle, the LOPs being tangent to the
circle. The regularity of the figure is remarkable. I suppose the radius
of the circle corresponds to the index error of a sextant.

Of course an astrolabe is affected by deflection of the vertical, so its
latitudes and longitudes are a little different from the "geodetic"
values shown on a map or GPS receiver. This is true of all instruments
that rely on gravity. Deflection of the vertical is usually a few
seconds of arc, but sometimes tens of seconds. In the mountains above
the Los Angeles basin, at the Mount Wilson Observatory, it's nearly 30".

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