  A new determination of the sound horizon scale in angular coordinates is
presented. It makes use of ~ 0.6 x 10^6 Luminous Red Galaxies, selected from
the Sloan Digital Sky Survey imaging data, with photometric redshifts. The
analysis covers a redshift interval that goes from z=0.5 to z=0.6. We find
evidence of the Baryon Acoustic Oscillations (BAO) signal at the ~ 2.3 sigma
confidence level, with a value of theta_{BAO} (z=0.55) = (3.90 \pm 0.38)
degrees, including systematic errors. To our understanding, this is the first
direct measurement of the angular BAO scale in the galaxy distribution, and it
is in agreement with previous BAO measurements. We also show how radial
determinations of the BAO scale can break the degeneracy in the measurement of
cosmological parameters when they are combined with BAO angular measurements.
The result is also in good agreement with the WMAP7 best-fit cosmology. We
obtain a value of w_0 = -1.03 \pm 0.16 for the equation of state parameter of
the dark energy, Omega_M = 0.26 \pm 0.04 for the matter density, when the other
parameters are fixed. We have also tested the sensitivity of current BAO
measurements to a time varying dark energy equation of state, finding w_a =
0.06 \pm 0.22 if we fix all the other parameters to the WMAP7 best-fit
cosmology.
