This is a number ranging between 0 and 1 based on the log of the illuminance of
the moon outside the atmosphere. It is calculated as
factor = (log(I) - max_loglux) / (max_loglux - min_loglux) + 1.0,
where I is the illuminance of the moon outside the atmosphere and min_loglux and
max_loglux are hardcoded to -0.504030345621 and -4.39964634562 respectively.
Although the value should never be outside of [0, 1], for safety it is clipped
to be between these values. For more background, see
http://forum.flightgear.org/viewtopic.php?f=47&t=28201&start=60#p270516 .
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