Both peculiar velocities and errors in the assumed redshift-distance relation ("Alcock-Paczynski effect") generate correlations between clustering amplitude and orientation with respect to the line-of-sight. In this talk we propose a novel technique to extract the Alcock-Paczynski, geometric, distortion information from the anisotropic clustering of galaxies in 3-dimensional redshift space while minimizing non-linear clustering and peculiar velocity effects. We capitalize on the recent, large dataset from the Sloan Digital Sky Survey III (SDSS-III), which provides a large comoving sample of the universe out to high redshift. We focus our analysis on the Baryon Oscillation Spectroscopic Survey (BOSS) constant mass (CMASS) sample of 549,005 bright galaxies in the redshift range 0.43<z<0.7. Careful modeling of clustering and peculiar velocities in the mildly non-linear regime together with statistical covariance from 600 realistic mock catalogues, allow us to interpret the Alcock-Paczynski effect in a cosmologically informative manner. We measure galaxy clustering in shells of comoving redshift space which provides a very clean estimate of the geometrical distortion and provides direct constraints on the combination of the diameter distance, DA, and the Hubble constant, H.