How does the reflected brightness of a Full Moon affect visibility, quantified by apparent magnitude cutoff?
Answer
It can raise the effective cutoff from 5.5 to 3 or 4
The presence of a bright Full Moon drastically impairs observation capabilities by increasing the sky background brightness, an effect that can be quantified using the magnitude scale. For instance, if an observer can typically see stars down to apparent magnitude 5.5 on a moonless night, the overwhelming light pollution caused by a Full Moon—which peaks around magnitude -12—can easily push the effective limiting magnitude up to 3 or 4. This means the fainter two-thirds of the visible stars are completely washed out and lost to observation, demonstrating why planning around the lunar cycle is absolutely essential for observing fainter celestial targets.
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