The Harmony of the Spheres
Slide 7 of 24
The ancient Greeks were fascinated by the movement of the celestial bodies. They knew of seven such bodies (besides the stars) that moved around the Earth: the Sun, the moon and five planets [Mercury, Jupiter, Mars, Venus, Saturn]. For reasons I will discuss further on, they imagined that these seven heavenly bodies moved in circles circumscribed on seven spheres, as this picture tries to depict.
In an attempt to unify celestial motion with more earthly, but equally divine phenomena (in this case music) they associated with each sphere one of the seven notes in the musical scale (Hence the term harmony of the spheres.). This was an early example of the synthesis of apparently diverse phenomena into a single framework, in this case based on harmony.