Sphaera Mundi.

The astrolabe encoded the sky as a flat projection. The orrery encodes it as a mechanism — bodies on brass rings, each at its correct relative orbital period. The model isn't a picture of the heavens; it is the heavens, rendered to a tabletop scale where you can hold them in your hand and watch them turn.

Eratosthenes of Cyrene ran the Library of Alexandria in the third century BCE. He measured Earth's circumference within one percent — a deep well in Syene that lit to the bottom at noon on the summer solstice, a stick in Alexandria casting a shadow at the same hour, the angle of the shadow, and walked-off distance between the cities. The first quantitative model of the cosmos at scale. Every orrery since has been a descendant of that measurement.

George Graham was a Quaker watchmaker on Fleet Street in early-eighteenth-century London — the same Greenwich-adjacent clockmaking tradition that produced John Harrison (Plate XVIII). He built the first geared mechanical solar system for Charles Boyle, 4th Earl of Orrery, who gave the object its name. Graham's gearing made the bodies revolve at their correct relative periods; the mathematics of the heavens transferred into the geometry of the gears.

James Ferguson grew up tending sheep in Banffshire. He taught himself astronomy lying on his back with a string of beads strung at the proper celestial distances, and went on to give public lectures on the planetary system in Edinburgh and London, illustrated by orreries he built himself. His engraved diagrams — published in Astronomy Explained Upon Sir Isaac Newton's Principles, 1756 — are how most people for the next century first encountered the solar system. The orrery escaped the noble's library.

The orrery became the planetarium, then the textbook diagram, then the simulation. NASA's JPL ephemeris is an orrery in software — the same problem, the same answer-by-mechanism. What didn't change is the proposition: the solar system can be modelled by gears at the correct ratios. The astrolabe is the original handheld; the orrery is the original simulation.