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.
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