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The passage around this formula

​ σ y ​ , gives $h = \sqrt{\frac{\sqrt2\,\sigma_y}{\phi_{,zz}}} = \sqrt{\frac{1.414\times10^{-18}}{3.43\times10^{-23}\,\mathrm{m^{-2}}}} \approx 203\,\mathrm m$. The fourth-derivative Taylor remainder at this height is smaller than the leading term by a factor of roughly 10^{-8} , confirming that the quadratic approximation used above is, for this purpose, exact. Two hundred metres of half-spacing — roughly four hundred metres end to end — is not a laboratory bench, but it is not exotic either: it is close to the height difference a transportable optical lattice clock actually used, linked to a second clock at the base of the Tokyo Skytree tower by fibre, to measure the ordinary first-derivative geopotential difference directly against independent levelling [ 9 ] . Comparable transportable-clock campaigns have…

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