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:
The team of scientists used data gathered by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, composed of a duo of twin spacecraft that circled Earth's Moon throughout 2012, each measuring the push and pull of the other as an indicator of lunar gravity.
HST's detection of a site, which appears to show persistent, intermittent plume activity on Europa, provides a promising target for the Europa mission to investigate. Equipped with its new and sophisticated suite of science instruments, the mission can detect whatever may potentially be swimming around in the hidden global ocean sloshing around beneath its secretive crust of ice.