  We detected the seasonal modulation of the $^7$Be neutrino interaction rate
with the Borexino detector at the Laboratori Nazionali del Gran Sasso in Italy.
The period, amplitude, and phase of the observed time evolution of the signal
are consistent with its solar origin, and the absence of an annual modulation
is rejected at 99.99\% C.L. The data are analyzed using three methods: the
sinusoidal fit, the Lomb-Scargle and the Empirical Mode Decomposition
techniques, which all yield results in excellent agreement.
