The saying goes, “Once in a blue moon”, as a way of saying something that is rare or doesn’t happen very often. In this case, a blue moon is the second full moon in a month when we typically only have one. Which is the modern definition of a blue moon. There is an older definition that is kind of weird honesty. Some feel that a true blue moon is the 3rd of 4 full moons in a season. So in the summer that is 3 months you’d expect a full moon in each month but if there are 4 the 3rd is a blue moon by the older definition. That definition is indeed outdated and honesty the meteorological seasons are based on calendar months anyways. So I’m sticking with the 2nd full moon in a Month.
The second full moon of July is this Friday July 31st, and the 1st full moon was on July 1st.
Here’s a list of all the full moons in 2015:
The blue moon is in fact not the color blue at all. Even though they are rare, we usually have a blue moon ever 2 to 2 1/2 years. The last one was in August 2012, and the next one will be January 2018.
Next Blue moons by either definition: