They had a vertical 'Zenith Telescope' that looked at the same star from two locations. They measured how far from vertical it shifted in the magnified field of

They had a vertical 'Zenith Telescope' that looked at the same star from two locations. They measured how far from vertical it shifted in the magnified field of view. https://en.wikipedia.org/wiki/Ramsden_surveying_instruments#The_zenith_telescope https://en.wikipedia.org/wiki/Ramsden_surveying_instruments#... Similar instrumends measured the wobble of the Earth's axis. https://en.wikipedia.org/wiki/International_Latitude_Service https://en.wikipedia.org/wiki/International_Latitude_Service