On this program we take an English word or expression and explore it a little deeper . Today we are talking about a lunar phenomenon – a moon event . It's called a blue moon . Experts at the U.S . National Aeronautics and Space Administration (NASA ) explain this lunar event . They say a blue moon is the "extra ” moon in a season with four full moons . The NASA website says that “since the 1940s , the term ‘blue moon ’ has also been used for the second full moon in a calendar month .” This usually happens only every two-and-a-half years . The NASA experts explain that a blue moon is not actually blue in color . In fact , it looks the same as a regular monthly full moon . What makes a blue moon different is that it does not appear in the sky that often . And this is how we use it as an English expression . A “blue moon ” is a long or seemingly long period of time . If I say something happens “once in a blue moon ,” it is uncommon , infrequent , rare . It happens very seldom . These words all describe something that happens once in a blue moon . For example , I usually keep busy . So when I'm not working , I like to take sing songs , write stories or read books . But sometimes , I just like to do nothing . So , once in a blue moon , you'll find me on the couch , watching movies and eating popcorn . Here's another example . Let's say you have a friend who you do not see very much anymore . You can say you see him once in a blue moon . It feels like an eternity since you have talked . It's like he has dropped off the face of the Earth ! You can also say it has been forever since you have seen him . Forever is a long time . So , forever and a day is even longer ! And it has been forever and a day since you have seen your friend . You miss him ! You should call . And finally , here is our last example . If you have been busy , you may not have had time to practice your English . But just remember , if you only practice once in a blue moon , your English may not improve very quickly .