  The hot Jupiter HD 209458b is particularly amenable to detailed study as it
is among the brightest transiting exoplanet systems currently known (V-mag =
7.65; K-mag = 6.308) and has a large planet-to-star contrast ratio. HD 209458b
is predicted to be in synchronous rotation about its host star with a hot spot
that is shifted eastward of the substellar point by superrotating equatorial
winds. Here we present the first full-orbit observations of HD 209458b, in
which its 4.5 $\mu$m emission was recorded with $Spitzer$/IRAC. Our study
revises the previous 4.5 $\mu$m measurement of HD 209458b's secondary eclipse
emission downward by $\sim$35% to $0.1391%^{+0.0072%}_{-0.0069%}$, changing our
interpretation of the properties of its dayside atmosphere. We find that the
hot spot on the planet's dayside is shifted eastward of the substellar point by
$40.9^{\circ}\pm{6.0^{\circ}}$, in agreement with circulation models predicting
equatorial superrotation. HD 209458b's dayside (T$_{bright}$ = 1499 $\pm$ 15 K)
and nightside (T$_{bright}$ = 972 $\pm$ 44 K) emission indicates a day-to-night
brightness temperature contrast smaller than that observed for more highly
irradiated exoplanets, suggesting that the day-to-night temperature contrast
may be partially a function of the incident stellar radiation. The observed
phase curve shape deviates modestly from global circulation model predictions
potentially due to disequilibrium chemistry or deficiencies in the current hot
CH$_{4}$ line lists used in these models. Observations of the phase curve at
additional wavelengths are needed in order to determine the possible presence
and spatial extent of a dayside temperature inversion, as well as to improve
our overall understanding of this planet's atmospheric circulation.
