What is 'Blue Moon'?

Second full moon of the two full moons in a month

The correct answer is option 3. Second full moon of the two full moons in a month.

The term "Blue Moon" refers to the occurrence of an additional full moon within a specific period, typically a calendar month. A typical lunar cycle lasts approximately 29.5 days, which means that most months contain only one full moon. However, because calendar months are either 30 or 31 days long (except for February, which can be 28 or 29 days), there is occasionally an opportunity for two full moons to occur within the same month.

The most common definition of a Blue Moon is the occurrence of the second full moon within a calendar month. When this happens, the second full moon is referred to as the "Blue Moon." It's important to note that the term "Blue Moon" doesn't actually refer to the moon's color; instead, it's a designation based on the occurrence of two full moons in a single month.

Blue Moons are relatively rare, happening roughly once every 2 to 3 years on average. They have captured people's imaginations and are sometimes associated with folklore and superstitions, although their significance varies across different cultures.

In summary, a Blue Moon is the occurrence of a second full moon within a calendar month, an event that happens infrequently and is named not for the moon's color but for its timing within the calendar.