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Chapter 3: Problem 37

One definition of a "blue moon" is the second full moon within the same calendar month. There is usually only one full moon within a calendar month, so the phrase "once in a blue moon" means "hardly ever." Why are blue moons so rare? Are there any months of the year in which it would be impossible to have two full moons? Explain your answer.

Short Answer

Expert verified
Blue moons are rare due to the slight excess of calendar month days over the lunar cycle days. It is only possible to encounter two full moons (a blue moon) within a calendar month if the initial full moon happens very early in the month. As February, even in a leap year, is shorter than a lunar cycle, it's impossible for a blue moon to occur.

Step by step solution

01

Understanding the Lunar Cycle

A lunar cycle, the period of time between two full moons, is approximately 29.5 days. This is known as a synodic month.
02

Comparing Lunar Cycle to Calendar Month

Our Gregorian calendar month is longer than a lunar cycle - the shortest month (February) is typically 28 or 29 days, the rest are 30 or 31 days. With that in mind, it is possible to fit two full moons within a calendar month that is longer than one lunar cycle, thus making a 'blue moon' possible.
03

Occurrence of Blue Moons

Blue moons are rare because the excess duration of calendar months over the lunar cycle isn't enough to always guarantee there'll be two full moons within a calendar month. Only if the initial full moon happens within the first couple of days of the month, will the second full moon (a 'blue moon') be possible by the end of the month.
04

Months Impossible for Blue moon

For the question about specific months where it'd be impossible to encounter two full moons, it's clearly February. Even in a leap year, February is 29 days which is shorter than the lunar cycle, meaning it's impossible for two full moons (or a 'blue moon') to occur within February.

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