  Given an accelerator-based neutrino experiment with the beam energy E
\lesssim 1 GeV, we expand the probabilities of \nu_\mu \to \nu_e and \overline
{\nu}_\mu \to \overline {\nu}_e oscillations in matter in terms of two small
quantities \Delta_{21}/\Delta_{31} and A/\Delta_{31}, where \Delta_{21} \equiv
m^2_2 - m^2_1 and \Delta_{31} \equiv m^2_3 - m^2_1 are the neutrino
mass-squared differences, and A measures the strength of terrestrial matter
effects. Our analytical approximations are numerically more accurate than those
made by Freund in this energy region, and thus they are particularly applicable
for the study of leptonic CP violation in the low-energy MOMENT, ESS\nuSM and
T2K oscillation experiments. As a by-product, the new analytical approximations
help us to easily understand why the matter-corrected Jarlskog parameter
\widetilde{\cal J} peaks at the resonance energy E_* \simeq 0.14 GeV (or 0.12
GeV) for the normal (or inverted) neutrino mass hierarchy, and how the three
Dirac unitarity triangles are deformed due to the terrestrial matter
contamination. We also affirm that a medium-baseline neutrino oscillation
experiment with the beam energy E lying in the E_* \lesssim E \lesssim 2 E_*
range is capable of exploring leptonic CP violation with little matter-induced
suppression.
