Launched aboard a Titan IVB/Centaur on October 15, 1997, Cassini was active in space for nearly 20 years, with 13 years spent orbiting Saturn and studying the planet and its system after entering orbit on July 1, 2004. The voyage to Saturn included flybys of Venus (April 1998 and July 1999), Earth (August 1999), the asteroid 2685 Masursky, and Jupiter (December 2000). The mission ended on September 15, 2017, when Cassini’s trajectory took it into Saturn’s upper atmosphere and it burned up in order to prevent any risk of contaminating Saturn’s moons, which might have offered habitable environments to stowaway terrestrial microbes on the spacecraft. The mission is widely perceived[by whom?] to have been successful beyond expectations. NASA’s Planetary Science Division Director, Jim Green, described Cassini-Huygens as a “mission of firsts”, that has revolutionized human understanding of the Saturn system, including its moons and rings, and our understanding of where life might be found in the Solar System. 
The astronomers then conducted an analysis called a Bouger correction in order to subtract the gravitational effect of topological features, such as valleys and mountains, from the total gravity field. What is then left is the gravity field hidden beneath the lunar surface, existing within its crust.
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.