= Eddington surface boundary condition
{c}
{title2=$P_{\rm rad}(0)=2F/(3c)$}
For no incident radiation and outward intensity approximated as direction independent, the surface angular moments are $F=\pi I_0$ and $P_{\rm rad}(0)=2\pi I_0/(3c)$. Integrating the radiation momentum equation with the <Eddington closure approximation> yields the <grey atmosphere> law $T^4=3T_{\rm eff}^4(\tau+2/3)/4$. This is an approximate angular boundary condition, not isotropy across both hemispheres.
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