There is a time each month where the moon is precisely 100 percent full, and that is the exact time of the full moon. However, at the time of the lunar cycle, changes in the amount of the moon's illumination are very slow, only 97-99 percent-illuminated moon would still appear to be full. Even two days before or after the moon is full, 93-97 percent is still lit. What appeared to be a full moon, was actually a day or so before fullness and is called a waxing gibbous. After complete fullness, though still appearing full, as the lit portion of the moon decreases it is called a waning gibbous.
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