= Lunar illuminated fraction
{title2=$k$}
The <lunar illuminated fraction> is the directly sunlit projected <area> of the apparent lunar disk divided by its total projected <area>. For a <sphere> with parallel sunlight and a distant observer, outside a <lunar eclipse>, it is $k=(1+\cos\alpha)/2$, with <phase angle> $\alpha$ at the <Moon>. It measures geometry rather than the fraction of full-Moon <radiant flux>. https://aa.usno.navy.mil/data/MoonFraction[US Naval Observatory: definition of the illuminated disk fraction]{ref}.
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