2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/137350We present a new treatment of the Earth matter effects on the neutrino oscillations that is valid for an arbitrary density profile. When applied to the the study of the day-night effect on the solar neutrino flux it renders us a simple analytical expression, which is more accurate than those derived by using the perturbation theory and can be extended to higher energies.4 pages and 4 figures. Contribution to the 30th International Cosmic Ray Conference, Merida, Mexico, July 2007Astrophysicstext