NavList:

I read the paper and, though I glaze over with some of the mathematical parts, I think the key point is that gravity, and therefore what we call level, varies with the local mass of the earth.  At Greenwich it caused, or still causes (?), an offset from a GPS of today.  Other traditional observing spots around the world also have varying offsets as shown in Frank's bar graph.

My conclusion is that a great circle determined at multiple points by a Mercury AH or a bubble level will not be a perfectly straight line around the earth and none of this detracts from practical CN.

A question: Using the best mechanical equipment available today - not GPS - would Greenwich still be at zero longitude?

Regards, Noell