Have you ever seen a blue moon? Have you ever seen a blue moon? I found an interesting articles which is about the blue moon., Once in a blue moon means a very rare thing. There were two occasions when was full moon in this year. The first was on he 2nd of July and the second was ont he 31th of July in 2015. According to modern folklore if there are two full moons in the month the second one must be turn blue. But the case is not so simple. The particles which injected by volcanoes usually scatter the red light and we can see the rest, the blue light. This particles are one micron wide The smaller particles such as aerosols scatter the blue light and the result is the red light. We often can see red moon or red sun when they are low for the same reason.