Merge pull request #75 from Daniel-Diaz/master

Update gravitational constant

Author: goldfirere

units-defs/Data/Constants/Mechanics.hs

@@ -25,8 +25,8 @@ import Data.Metrology.TH
 -- | Acceleration at Earth's surface due to gravity.
 declareConstant "gravity_g" 9.80665 [t| Meter :/ Second :^ Two |]
 
--- | Gravitational constant (value recommended by CODATA, see the
--- <http://arxiv.org/abs/1507.07956 arXiv> paper)
-declareConstant "gravity_G" 6.6740831e-11
+-- | Gravitational constant, taken from the
+--   <https://pml.nist.gov/cuu/Constants/Table/allascii.txt NIST table>.
+declareConstant "gravity_G" 6.67430e-11
   [t| (Meter :^ Three) :* ((Kilo :@ Gram) :^ MOne) :* (Second :^ MTwo) |]
   -- Thanks to @hesiod, PR #13