  We report the detection of transits by the 3.1 M_Jup companion to the V=8.17
G0V star HD 17156. The transit was observed by three independant observers on
Sept. 9/10, 2007 (two in central Italy and one in the Canary Islands), who
obtained detections at confidence levels of 3.0 sigma, 5.3 sigma, and 7.9
sigma, respectively. The observations were carried out under the auspices of
the Transitsearch.org network, which organizes follow-up photometric transit
searches of known planet-bearing stars during the time intervals when transits
are expected to possibly occur. Analyses of the 7.9 sigma data set indicates a
transit depth d=0.0062+/-0.0004, and a transit duration t=186+/-5 min. These
values are consistent with the transit of a Jupiter-sized planet with an impact
parameter b=a*cos(i)/R_star ~ 0.8. This planet occupies a unique regime among
known transiting extrasolar planets, both as a result of its large orbital
eccentricity (e=0.67) and long orbital period (P=21.2 d). The planet receives a
26-fold variation in insolation during the course of its orbit, which will make
it a useful object for characterization of exoplanetary atmospheric dynamics.
