The Harmony of the Spheres
Source
Breviari d'Amour — British Library, Ms Yates Thompson 31, f.45 (late 13th century)
The trail
Reading Boethius → Song IX references Plato's Timaeus → read Cicero's Dream of Scipio → searched for medieval cosmological diagrams
The passage
That is produced by the onward rush and motion of the spheres themselves; the intervals between them, though unequal, being exactly arranged in a fixed proportion, by an agreeable blending of high and low tones various harmonies are produced.
Concentric celestial spheres with distances in miles, two angelic figures observing, gold stars on deep blue. The monk who drew this was trying to make visible what Cicero said was audible — the music the spheres make as they turn. You cannot hear it because you have never not heard it. The most present thing is the most invisible.