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:
Envision that astrology is made up of the study of all of the planets and the sun. The astrological birth chart and the study of how planetary alignments affect sun signs are based on these aspects of astrology. The moon actually influences this overall picture, and can cause certain aspects of astrological phenomena to influence our lives differently than was otherwise predicted.
"For the smaller craters, it's like if you're filling a bucket, eventually your bucket gets full, but if you keep pouring cups of water into the bucket, you can't tell how many cups of water beyond full you've gone. Looking at the larger craters at the subsurface might give us insight, because that 'bucket' isn't full yet," Dr. Soderblom added.