  Many observers in the past gave detailed descriptions of the telescopic
aspect of Venus during its extremely rare transits across the Solar disk. In
particular, at the ingress and egress, the portion of the planet's disk outside
the Solar photosphere has been repeatedly perceived as outlined by a thin,
bright arc ("aureole"). Those historical visual observations allowed inferring
the existence of Venus' atmosphere, the bright arc being correctly ascribed to
the refraction of light by the outer layers of a dense atmosphere. On June 8th,
2004, fast photometry based on electronic imaging devices allowed the first
quantitative analysis of the phenomenon. Several observers used a variety of
acquisition systems to image the event -- ranging from amateur-sized to
professional telescopes and cameras -- thus collecting for the first time a
large amount of quantitative information on this atmospheric phenomenon. In
this paper, after reviewing some elements brought by the historical records, we
give a detailed report of the ground based observations of the 2004 transit.
Besides confirming the historical descriptions, we perform the first
photometric analysis of the aureole using various acquisition systems. The
spatially resolved data provide measurements of the aureole flux as a function
of the planetocentric latitude along the limb. A new differential refraction
model of solar disk through the upper atmosphere allows us to relate the
variable photometry to the latitudinal dependency of scale-height with
temperature in the South polar region, as well as the latitudinal variation of
the cloud-top layer altitude. We compare our measurements to recent analysis of
the Venus Express VIRTIS-M, VMC and SPICAV/SOIR thermal field and aerosol
distribution. Our results can be used a starting point for new, more optimized
experiments during the 2012 transit event.
