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Brightness of the lunar disk varies as a function of phase angle. This phenomenon is very well established, as is the increase in brightness at low phase angles.

Is there a simple analytical formula that describes this function? It looks like brightness could be modeled as an exponential as a function of phase angle, modulo the opposition surge.

Thanks.

EDIT: I realize that these effects depend on the wavelength of the reflected light. I'm looking for an average curve, though.

Lunar brightness curve, pointing out the opposition surge

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    $\begingroup$ Could you source that image and/or data? Presumably, curve fitting methods could find an arbitrarily good match? adsabs.harvard.edu/full/1991PASP..103.1033K may also be helpful? $\endgroup$ – user21 Mar 26 '15 at 14:12
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The magnitude of the Moon without the opposition surge is given by a simple equation:

$m = -12.73 + 1.49 \cdot |\psi| + 0.043 \cdot \psi^4,$

where $\psi$ is the phase angle in radians (Allen 1976). This can be converted to flux using $m \propto -2.5 \cdot ^{10}\!\log F$.

For the increase in flux due to the variable distance and opposition surge, I multiply F with the factor

$\left(\frac{\Delta_0}{\Delta}\right)^2 \cdot \max\left(1, 1.35 - 2.865 \cdot |\psi| \right),$

where $\Delta_0$ and $\Delta$ are the mean and current distance to the Moon, respectively. However, I can't find a reference for the last term at the moment, and I can't remember where I obtained it. I'll post that information if I find it.

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