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On 2017-01-05 17:28, John D. Howard wrote:
> If you set up a telescope ( transit, theolodite ) looking 180 true or 360 if 
south and lock the azimuth.  With only a hack watch needed you note the 
merdian passage of a star then the moon, making note of the time difference.  
Now the time difference is just difference in GHA.

What you have described is the core idea of moon culminations, an old
method to determine longitudes of fixed points. The largest error came
from the Moon ephemeris, so transits were also observed from a point of
known longitude. That cancelled the ephemeris error, and you ended up
with the longitude difference between the observatories.

Lunar culminations generally had lower accuracy than the mechanical
solution of transporting sets of chronometers back and forth between the
observatories.

Chauvenet describes moon culminations and other old longitude methods:
https://books.google.com/books?id=6JlzJHd15vQC&pg=PA13&focus=viewport

Results of lunar culminations and chronometer transportation to fix the
longitude difference between America and Greenwich are in the U.S. Coast
Survey annual reports from the 1850s and 60s.

   

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