How long is the full moon?
There are multiple ways of approaching this question. If we look at it from a purely theoretical standpoint, we might be tempted to say that the full moon is infinitely short, since the phases of the moon are changing continuously. The moon is not yet quite full shortly before the full moon, and is already waning shortly afterwards.
However, there is a practical aspect that lets us quantify the full moon as a finite and measurable span of time: Since the Sun is significantly bigger than the Moon, its rays are able to reach just a little over half of the Moon’s surface. This means that the timespan in which the visible side of the Moon’s surface is irradiated (as seen from Earth) is longer than infinitely short.
It is not our intention to make this too astronomical, because in general linguistic terms, one extends this duration instinctively either forward or backward. For example: full moon day is the day in the calendar when full moon takes place. This would explain the colloquial statement “today is the full moon” (as you can see in our full moon memo).
We can also add and subtract 24 hours around the time of the full moon, since it is generally already or still 98–99% full, and thus appears like a circle to our eyes.
The categorical definition is slightly more complicated, individually diverse for each month and only a delight for real astronomy fans. Let’s just agree there is always one specific time for full moon. Take a look at our full moon calendar …