  We have carefully examined, in both analytical and numerical ways, how small
the terrestrial matter effects can be in a given medium-baseline reactor
antineutrino oscillation experiment like JUNO or RENO-50. Taking the ongoing
JUNO experiment for example, we show that the inclusion of terrestrial matter
effects may reduce the sensitivity of the neutrino mass ordering measurement by
\Delta \chi^2_{\rm MO} \simeq 0.6, and a neglect of such effects may shift the
best-fit values of the flavor mixing angle \theta_{12} and the neutrino
mass-squared difference \Delta_{21} by about 1\sigma to 2\sigma in the future
data analysis. In addition, a preliminary estimate indicates that a 2\sigma
sensitivity of establishing the terrestrial matter effects can be achieved for
about 10 years of data taking at JUNO with the help of a proper near detector
implementation.
