  We explore the infrared spectrum of a three-dimensional dynamical model of
planet HD209458b as a function of orbital phase. The dynamical model predicts
day-side atmospheric pressure-temperature profiles that are much more
isothermal at pressures less than 1 bar than one-dimensional
radiative-convective models have found. The resulting day-side thermal spectra
are very similar to a blackbody, and only weak water absorption features are
seen at short wavelengths. The dayside emission is consequently in
significantly better agreement with ground-based and space-based secondary
eclipse data than any previous models, which predict strong flux peaks and deep
absorption features. At other orbital phases, absorption due to carbon monoxide
and methane is also predicted. We compute the spectra under two treatments of
atmospheric chemistry: one uses the predictions of equilibrium chemistry, and
the other uses non-equilibrium chemistry, which ties the timescales of methane
and carbon monoxide chemistry to dynamical timescales. As a function of orbital
phase, we predict planet-to-star flux ratios for standard infrared bands and
all Spitzer Space Telescope bands. In Spitzer bands, we predict 2-fold to
15-fold variation in planetary flux as a function of orbital phase with
equilibrium chemistry, and 2-fold to 4-fold variation with non-equilibrium
chemistry. Variation is generally more pronounced in bands from 3-10 $\mu$m
than at longer wavelengths. The orbital phase of maximum thermal emission in
infrared bands is 15--45 orbital degrees before the time of secondary eclipse.
Changes in flux as a function of orbital phase for HD209458b should be
observable with Spitzer, given the previously acheived observational error
bars.
