  We show that a pair of thermal, antipodal hot-spots on the neutron star
surface is able to fully account for the pulsar's double blackbody spectrum and
energy-dependent pulse profile, including the observed 180 degree phase
reversal at approximately 1.2 keV. By comparing the observed pulse modulation
and phase to the model predictions, we strongly constrain the hot-spot pole
(xi) and the line-of-sight (psi) angles with respect to the spin axis. For a
nominal radius of R = 12 km and distance D = 2.2 kpc, we find (xi,psi) =
(86d,6d), with 1-sigma error ellipse of (2d,1d); this solution is degenerate in
the two angles. The best-fit spectral model for this geometry requires that the
temperatures of the two emission spots differ by a factor of 2 and their areas
by a factor of ~ 20. Including a cosine-beamed pattern for the emitted
intensity modifies the result, decreasing the angles to (84d,3d); however this
model is not statistically distinguishable from the isotropic emission case. We
also present a new upper limit on the period derivative of Pdot < 3.5E-16
(2-sigma), which limits the global dipole magnetic field to B_s < 2.0E11 G,
confirming PSR J0821-4300 as an "anti-magnetar." We discuss the results in the
context of observations and theories of nonuniform surface temperature on
isolated NSs of both weak and strong magnetic field. To explain the nonuniform
temperature of PSR J0821-4300 may require a crustal field that is much stronger
than the external, global dipole field.
