Final answer:
A blue moon, or a second full moon in one month, occurs every 2.7 years due to the difference in the orbital period of the Moon and the synodic period.
Step-by-step explanation:
The occurrence of a blue moon, or a second full moon in one month, happens approximately every 2.7 years due to the difference in the actual orbital period of the Moon and the synodic period. The synodic period, which is the time between new moon phases, is about 29.5 days, while the actual orbital period of the Moon is 27.3217 days. This difference means that it takes the Moon about 2 more days to catch up with the Sun and reach the position of the next new moon.