> Theres no way to accurately predict g, only measurements will do if you > want precision, but a first order aproximation can be derived from > a topological representation and rudimentary knowledge of the geology.
PHK, I think a first-order value comes simply from the mass and radius of the earth, which gives you 9.8 m/s^2. What several of you are talking about are 2nd and 3rd order effects; minor things like equatorial bulge or mountain geology or soil density variations (like when there's an oil field below your feet). And then there's the Bouguer correction (someone is welcome to explain that one to me offline). I mean, go even further and below the ppm level you get lunar/solar tidal variations and Love numbers. There's a nice treatment and useful equations at: http://en.wikipedia.org/wiki/Acceleration_due_to_gravity And some cool plots at: Lunar/Solar Tides and Pendulum Clocks http://www.leapsecond.com/hsn2006/ch1.htm The point is, for calculating the effects of relativity on a road trip the nominal value of g is important, but at the ten percent or one percent level where I use g, these subtle space or time variations in g play no part. /tvb _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
