Once in a blue moon – have you ever heard this expression before?  Do you know what it means?  The expression ‘once in a blue moon’ means ‘rarely.’  It refers to something that does not happen very often.  Here are two examples of how to use this expression in context.

Examples:
I only go to visit my parents once in a blue moon.
I really wish my husband would wash the dishes more often than once in a blue moon.

This may seem like a strange expression, but there is an interesting story behind where this expression comes from.  A ‘blue moon’ is a term used outside of this expression.  It is a term used to describe the second full moon in a calendar month.

The last blue moon that occurred happened on August 31st of this year.  In August of this year there was a full moon on the 1st of the month and on the 31st.  The full moon on the 31st was called a ‘blue moon’ in English.  The occurrence of a full moon twice in one calendar month is very rare (thus the origin of this expression).  The next blue moon won’t occur until the year 2015!  I hope you can begin to see where this expression originates.

Very rarely, the moon actually does appear to be blue too. This sometimes occurs after a volcanic eruption. Dust particles* in the atmosphere are normally of a size to diffract** blue light, making the moon appear reddish at sunset.  Larger volcanic dust particles on the other hand diffract red light, making the moon appear bluish.  So, there are two explanations for where this expression comes from and both point to occurrences that happen rarely.

I hope you are reading this blog and practicing your English more than once in a blue moon.  Remember to check out www.transparent.com for resources to help you learn English.

*particles = a small portion of matter
**diffract = the process in which light waves are spread out as a result of passing through a narrow space or hitting a certain matter