  We present the time drift of the cosmological redshift in a general
spherically symmetric spacetime. We demonstrate that its observation would
allow us to test the Copernican principle and so determine if our universe is
radially inhomogeneous, an important issue in our understanding of dark energy.
In particular, when combined with distance data, this extra observable allows
one to fully reconstruct the geometry of a spacetime describing a spherically
symmetric under-dense region around us, purely from background observations.
