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On 2019-08-09 8:31, Peter Monta wrote:
> For example, how stable were the ground marks at transit telescopes?  As I
> understand it, they put a target, collimator, and light source some
> distance from the telescope to the south, to serve as an azimuth
> reference.  If it habitually moved around multiple seconds, I would think
> that would render it useless, and they would have gone with something
> internal instead, such as a rotary encoder on the telescope's vertical axis
> (microscope-reading scales).  Did they maintain statistics on
> reference-mark performance at places like Greenwich, Pulkovo, or USNO?  Of
> course they dealt with only a few marks directly to the south, not spread
> out over the entire horizon, but at least it's a data point.

Bear in mind that when an astronomical transit instrument observed its
collimators, the distance was extremely short (by surveying standards)
and presumably pains were taken to minimize temperature gradients around
the building. This 1920s description of the Greenwich instrument
mentions collimators north and south of the transit. Furthermore, when
the latter was set vertical, openings in the tube give a clear line of
sight so one collimator could observe the other.

I don't see how the high stability attained in such a controlled
environment can be relevant to geodetic triangulation, where the sights
were many kilometers long over varying terrain.

Determination of the collimation error:


http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1926GOAMM..86A...2D&db_key=AST&page_ind=13&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES

At sextant accuracy I'm not worried about horizontal refraction. Anyway,
it wouldn't be detectable since you don't have the internal checks of
triangulation, where the angles must sum to 180 plus the "spherical excess".

   

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