The moon, for the most part, influences our emotions. In certain phases of the moon, the predictions made through the study of astrological phenomena that would otherwise occur fail to happen, because our emotions do not produce the reactions to the astrological phenomena that would normally be expected. In other phases of the moon, astrological phenomena of planetary alignments and their effect on the Zodiac sun signs are not altered from their original reading.
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:
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.