  "Hot Jupiter" extrasolar planets are expected to be tidally locked because
they are close (<0.05 astronomical units, where 1 AU is the average Sun-Earth
distance) to their parent stars, resulting in permanent daysides and
nightsides. By observing systems where the planet and star periodically eclipse
each other, several groups have been able to estimate the temperatures of the
daysides of these planets. A key question is whether the atmosphere is able to
transport the energy incident upon the dayside to the nightside, which will
determine the temperature at different points on the planet's surface. Here we
report observations of HD 189733, the closest of these eclipsing planetary
systems, over half an orbital period, from which we can construct a 'map' of
the distribution of temperatures. We detected the increase in brightness as the
dayside of the planet rotated into view. We estimate a minimum brightness
temperature of 973 +/- 33 K and a maximum brightness temperature of 1212 +/- 11
K at a wavelength of 8 microns, indicating that energy from the irradiated
dayside is efficiently redistributed throughout the atmosphere, in contrast to
a recent claim for another hot Jupiter. Our data indicate that the peak
hemisphere-integrated brightness occurs 16$\pm$6 degrees before opposition,
corresponding to a hot spot shifted east of the substellar point. The secondary
eclipse (when the planet moves behind the star) occurs 120 +/- 24 s later than
predicted, which may indicate a slightly eccentric orbit.
