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:
After a long and dangerous journey through the space between planets, the Cassini/Huygens Spacecraft reached Saturn on July 1, 2004. On December 25, 2004, the Huygens Probe was purposely liberated from the Cassini Orbiter. Huygens then began its historic descent through the dense blanket of golden-orange fog to at last lift the veil hiding Titan's long-hidden face.
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.