= Photon-number half-cone under relativistic aberration
{title2=$\cos\alpha=v/c$}
For isotropic planar photon emission in a rest frame, <aberration of light> maps its forward half of the angular directions into a lab cone of semi-angle $\alpha$ satisfying the displayed equation. This follows from $\cos\theta=(\cos\theta'+\beta)/(1+\beta\cos\theta')$ at $\theta'=\pm\pi/2$. For a fast positive <Lorentz boost>, $\alpha\sim1/\gamma$. The half fraction concerns <photon> count or directions: the <relativistic Doppler effect> changes the angular energy weights, so the cone need not contain half the radiant energy.
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