We experimentally observe the theoretically anticipated but so far unverified asymmetric band gap closing for the prototypical semiconductor silicon from room temperature up to the melting point. The shift of the valence band maximum contributes more than 60\% to the band gap closing in comparison to the shift of the conduction band minimum. Since we determine the temperature-dependent band edge positions with x-ray absorption and x-ray emission spectroscopy, our analysis also includes electron-phonon coupling features and processes inherent to core-level spectroscopies, like the multi-electron screening response.