Have you ever had one of those days fishing where it seemed like you could do no wrong? Like everything you tried resulted in fish being caught? And not only fish being caught, but large fish being caught? This was all probably due to the moon and what phase the moon was in. That's right, something as simple as what phase the moon is in can have an effect on your fishing success.
So here you have two definitions of a blue moon but the one for a calendar blue moon does not describe the true meaning of a blue moon. Here's why:
Ganymede, and four other moons dwelling in our Sun's family, possess liquid water beneath their frigid crusts of ice. The others are Saturn's moons, Titan and Enceladus, and two other Galilean moons of Jupiter--Europa and Callisto. Planetary scientists think the oceans of Europa and Enceladus are in contact with rock--thus making these two moons high-priority targets for future astrobiology missions.