[time-nuts] Time Nuts at PTTI this past week
Tom Van Baak
tvb at leapsecond.com
Sat Dec 9 15:47:38 EST 2006
> 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.
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:
And some cool plots at:
Lunar/Solar Tides and Pendulum Clocks
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.
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