Time & Measurement

Giovanni de’ Dondi’s Astrarium as a Continuous Cosmic Clock

Period and origin

Padua, completed 1364; known from de’ Dondi’s own treatise rather than from a surviving machine

What it was built to do

To drive the manually set geometry of the equatorium continuously, so that a clock, a calendar, a planetarium and a mechanical evaluator of medieval astronomy were one instrument.

How the mechanism works

A weight-driven clock and escapement supply a regulated temporal input. Branching gear trains convert it into the separate mean and anomalistic motions of the Sun, Moon and five planets, with non-circular gears in the Mercury and lunar mechanisms turning uniform motion into prescribed non-uniform motion.

Input

One regulated time base from a weight-driven escapement.

Transformation

Branching gear trains, each carrying its own period and epoch, with non-circular gears synthesising the required non-uniform motions.

Output

Several astronomical phases and calendar indices advancing concurrently, each on its own display.

What is evidenced

Deduction: mathematically forced, or nearly forced, by the evidence.

What it demonstrates

What it costs to make a model autonomous: once the instrument runs itself, every relation it holds must be embodied rather than set by hand each time.

All mechanisms