Can the moon rise as a full moon and set early as a slightly gibbous moon?

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Dear Tom,

I remember a time several years ago when I saw the moon rise as a full moon and then set early the next morning as a slightly gibbous moon. Can this happen? Is it unusual?

Willy Neuman,

Dear Willy,

It happens all the time. There is a moment each month when the moon is precisely 100 percent full and that is the exact time of the full moon.

However, at that time of the lunar cycle, changes in amount of the moon’s illumination are very slow and to the naked eye the 97-99 percent illuminated moon would still look full to most people. Even two days after being full, 93-97 percent of the moon is still illuminated. It’s likely you saw what appeared to be a full moon rising, but it was actually a day or two beyond fullness. By the time the moon set the following morning, the illumination of the waning gibbous moon had decreased enough for you to notice.


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