have you ever heard someone use the phrase once in blue moon people use this expression to describe something that they do not do very often for example someone might say that he tries to avoid eating sweets because they are unhealthy but will eat chocolate once in a blue moon or some one who does not usually like to go to the beach might say i visit the shore once in a blue moon while many people use this phrase not everyone knows the meaning behind it
the first thing to knows is that the moon itself is never actually blue this is just an expression the phrase blue moon actually has to do with the shape of the moon not the color
as the moon travels around the earth it appears to change shape we associate certain names with certain shapes of the moon for example when we can see a small part of the moon it is called a crescent moon a crescent is a shape that looks like the tip of a fingernail when we cannot see the moon at all it is called a new moon when we can see the entire moon it is called a full moon usually there is only one full moon every month sometimes however there will be two full moons in one month when this happens the second full moon is called a blue moon
over the next 20 years there will only be 15 blue moons as you can see ablue moon is a very rare event this fact has led people to use the expression once in a blue moon to describe other very rare events in their lives