Originally Posted by
lunar 
It's a simple matter of earth curvature divided by the cosine squared,
of the current ionospheric factor.
On Thursdays; that is.
Ah yes, good old Planck's Constant
The original Kilogram, cast in 1889, is kept under lock and key in Paris might be replaced after 2020 as it has gained weight (a few micrograms). Cool object, it is made out of Platinum-Irridium.