Blue Moon

There are many different interpretations of the the term "blue moon". The common use of it is when we refer to events that do not often occur.

Other interpretations are:

- the extra full moon in a season. Regions that have four seasons(winter, spring, summer, autum) normally have 3 full moons. one for each of the three months in every season. But in occasions an extra full moon will occur. In the summer, the first is called the early summer moon, then a midsummer moon and then the late summer moon, an extra full moon between the midsummer and late summer moon is called a blue moon.

- another interpretation is blue moon is the extra full moon in a calendar year. Commonly there are only 12 full moons in one year. One for each of the 12 months. When an extra full moon occurs, it is called a blue moon. The last one was  December 31 2009.

But the most common interpretation of a blue moon is the extra full moon in a month. like ealier mentioned, mostly each month of a calendar year has one full moon but in occasions there will be two.

This next month will have its first full moon in August 2nd and other one which we will call "blue moon" on the 31st.

In any of these cases, an extra full moon is caused due to the difference in earth's rotation and revolution around the sun versus the revolution of the moon around earth. The moon revolves around the earth in approximately 29.5 day. Since our average calender month has 30 days the out of sync time will accumalate the moon time to give us another full moon which can happen in about every 2 to 3 years. So it is not that rare of an event really.

Aside from these disambiguation there are also rare events in the past when the moon was reported to acctually appear blue in color. (BTW this was actually my understanding of the term was I was young). This is possible when there is enough smoke or dust particles in the atmosphere to filter the light seen by an earth observer. There were reports that the moons appears bluish short after the eruption on Mt. Pinatubo in 1991.