  Noether's theorem identifies fundamental conserved quantities, called Noether
charges, from a Hamiltonian. To-date Noether charges remain largely elusive
within theories of gravity: We do not know how to directly measure them, and
their physical interpretation remains unsettled in general spacetimes. Here we
show that the surface gravity as naturally defined for a family of observers in
arbitrarily dynamical spacetimes is a directly measurable Noether charge. This
Noether charge reduces to the accepted value on stationary horizons, and, when
integrated over a closed surface, yields an energy with the characteristics of
gravitating mass. Stokes' theorem then identifies the gravitating energy
density as the time-component of a locally conserved Noether current in general
spacetimes. Our conclusion, that this Noether charge is extractable from
astronomical observations, holds the potential for determining the detailed
distribution of the gravitating mass in galaxies, galaxy clusters and beyond.
