Analytical Description of Neutrino Oscillations in the Earth

We present an analytical description of neutrino oscillations in matter based on the Magnus expansion of the time evolution operator. This approach incorporates in a simple and, at the same time, accurate way the Earth matter effects on the flavor transition probabilities in a wide interval of the neutrino energies. As a concrete application, we examine the daily change in the observed flux of the electron neutrinos coming from the sun.