  The size of the horizon at the matter-radiation equality is a key scale of
the Big Bang cosmology that is directly related to the energy-matter content of
the Universe. In this letter, we argue that this scale can be accurately
measured from the observed clustering of galaxies in new large scale surveys.
We demonstrate that the zero-crossing, r_c, of the 2-point galaxy correlation
function is closely related to the horizon size at matter-radiation equality
for a large variety of flat LCDM models. Using large-volume cosmological
simulations, we also show that the pristine zero-crossing is unaltered by
non-linear evolution of density fluctuations, redshift distortions and galaxy
biases. This makes r_c a very powerful standard ruler that can be accurately
measured, at a percent level, in upcoming experiments that will collect
redshifts of millions of galaxies and quasars.
