  The experimental technique of fluorescence light observation is used in
current and planned air shower experiments that aim at understanding the origin
of ultra-high energy cosmic rays. In the fluorescence technique, the geometry
of the shower is reconstructed based on the correlation between viewing angle
and arrival time of the signals detected by the telescope. The signals are
compared to those expected for different shower geometries and the best-fit
geometry is determined. The calculation of the expected signals is usually
based on a relatively simple function which is motivated by basic geometrical
considerations. This function is based on certain assumptions on the processes
of light emission and propagation through the atmosphere. For instance, the
fluorescence light is assumed to propagate with vacuum speed of light. We
investigate the validity of these assumptions and provide corrections that can
be used in the geometry reconstruction. The impact on reconstruction parameters
is studied. The results are also relevant for hybrid observations where the
shower is registered simultaneously by fluorescence and surface detectors.
