  Emission lines formed in decretion disks of Be stars often undergo long-term
cyclic variations, especially in the violet-to-red (V/R) ratio of their primary
components. From observations of the bright Be-shell star zeta Tau, the
possibly broadest and longest data set illustrating the prototype of this
behaviour was compiled from our own and archival observations. It comprises
optical and infrared spectra, broad-band polarimetry, and interferometric
observations. From 3 V/R cycles between 1997 and 2008, a mean cycle length in H
alpha of 1400-1430 days was derived. After each minimum in V/R, the shell
absorption weakens and splits into two components, leading to 3 emission peaks.
This phase makes the strongest contribution to the variability in cycle length.
V/R curves of different lines are shifted in phase. Lines formed on average
closer to the central star are ahead of the others. The shell absorption lines
fall into 2 categories differing in line width, ionization/excitation
potential, and variability of the equivalent width. The interferometry has
resolved the continuum and the line emission in Br gamma and He I 2.06. The
phasing of the Br gamma emission shows that the photocenter of the
line-emitting region lies within the plane of the disk but is offset from the
continuum source. The plane of the disk is constant throughout the observed V/R
cycles. The self-consistent, one-armed, disk-oscillation model is developed in
Paper II.
