[time-nuts] Information on the Danjon Astrolab

[Hal Murray]

> Hmm, but doesn't the local value of "horizontal" affect the precision
> also ?

> You would really need to know the direction of local gravity for this
> to be any good... 

An interesting problem.

How do you measure the direction of gravity?  How far off does it get?

With modern equipment, I'd expect something like comparing time with 
observations of the stars would be a good approach.  What did they do before 
they had atomic clocks?


Several years ago at the local USGS open house, a guy had a neat topo map of 
the SF peninsula.  The lines were gravity, magnitude not direction.  They 
were trying to estimate rock density.

They measured gravity and corrected for elevation.  We asked him how they got 
the elevation that accurately?  He smiled and said from the sewer people.
  



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