TITLE: Have the values of constants ever changed before?
QUESTION [1 upvotes]: Example sub-questions:

Was the speed of light always that value?
Was the acceleration due to gravity always been 9.8m/s/s?
Etc

By "change" I mean both actual change in the physical world because of multiple factors and "change" in the numerical value (because the initial value discovered was not precise or accurate).

REPLY [3 votes]: Look to large redshift values with the Hubble telescope.  Then do spectroscopy.  Primary physical constants have not changed within visible time within experimental error - more than 14 significant figures for the hydrogen atom hyperfine transition.  Look up the half-life of that spin flip, then Heisenberg for slop in the gears.
Terrestrial acceleration due to gravity is not a constant,
http://www.splung.com/content/sid/2/page/gravitation 
http://www.typnet.net/Essays/EarthGrav.htm 
bottom
Big G is a universal constant.  It is the worst-measured physical constant, upon which all gravitation hinges.  Even worse, the very modest accuracy values substantially diverge - and don't grow better over the years,
http://iopscience.iop.org/0957-0233/10/6/001 
1999
http://www.nature.com/news/2010/100823/full/4661030a.html 
2010
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