  The average Rayleigh capillary breakup length of a cylindrical Newtonian
viscous liquid jet moving with homogeneous velocity $\hat{U}$ (negligible
external forces) must be determined by the selection of normal modes with
time-independent amplitude and wavelength (invariant modes, IMs). Both positive
and negative group velocity IMs exist in ample ranges of the parameter domain
(Weber and Ohnesorge numbers), which explains (i) the average breakup length
independence on ambient conditions (long-term resonance), and (ii) its
proportionality to the inverse of the spatial growth rate of the dominant
positive group velocity IM. Published experimental results since Grace (1965,
PhD Thesis) confirm our proposal.
