The concept of poetic time, experienced through poetry, music and plastic arts, has firstly been individuated by Gaston Bachelard in the 1930s. The spectator's sensation of time rupture and ecstasy has been defined as time rearrangement into a vertical structure, linking to eternity. Although this particular perception of time has been described as triggered by spectator's experience of sublime art forms, this paper finds earlier evidence of the phenomenon in Giacomo Leopardi's poem \emph{L'Infinito} (1819), as result of nature contemplation. Analysis of the sound that induces the vision of vertical time in Leopardi's poetry is performed by signal processing techniques. Relations between time, frequency and lag representations of the signal are established, finding correspondence of the signal power spectral density and time verticality in the lag domain.