  The most fundamental premise to the standard model of the universe, the
Cosmological Principle (CP), states that the large-scale properties of the
universe are the same in all directions and at all comoving positions.
Demonstrating this theoretical hypothesis has proven to be a formidable
challenge. The cross-over scale R_{iso} above which the galaxy distribution
becomes statistically isotropic is vaguely defined and poorly (if not at all)
quantified. Here we report on a formalism that allows us to provide an
unambiguous operational definition and an estimate of R_{iso}. We apply the
method to galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7,
finding that R_{iso}\sim 150h^{-1} Mpc. Besides providing a consistency test of
the Copernican principle, this result is in agreement with predictions based on
numerical simulations of the spatial distribution of galaxies in cold dark
matter dominated cosmological models.
