Heinrich Cornelius Agrippa (1486-1535), the polymath born near Cologne in Germany, dedicated the sacred number to the Muses: Calliope, Urania, Polymnia, Terpsichore, Clio, Melpomene, Erato, Euterpe and Thalia.
Agrippa associated each of the nine Muses to an appropriate the Spheres, by noting that the first resembles the supreme Sphere (the Primum mobile), and descending in order to the Sphere of the Moon, he determined that Calliope is appropriated to the Primum mobile; Urania to the Starry Heaven, Polymnia to Saturn, Terpsichore, to Jupiter, Cleo to Mars, Melpomene to the Sun, Erato to Venus, Euterpe to Mercury, and Thalia to the Moon.1
"We all come from the Goddess…and to her we shall return."
There is also some interesting bit about the number nine and John Lennon which you can read about at the Beatles Bible blog here.
Painting by Edward Burne-Jones titled The Garden of the Hesperides
Agrippa image via Wikipedia
1 Esoterica Archives