Time & Measurement

The Equatorium as a Planetary Position Engine

Period and origin

Islamic and Latin astronomy, eleventh to sixteenth century

What it was built to do

To evaluate a geometrical planetary theory for a chosen date without repeating the trigonometric and tabular work the model would otherwise demand.

How the mechanism works

The user sets mean motions on graduated circles, and the instrument constructs the deferent, eccentric, epicycle and equant geometry directly. The geometry of the model is embodied as the geometry of the instrument.

Input

A date, entered as the mean motions of the planet at that date.

Transformation

Direct geometric construction of the theory’s deferent, eccentric, epicycle and equant, performed by setting and reading graduated circles.

Output

The geocentric longitude the model predicts for that date.

What is evidenced

Established: the artefact or the documentary record carries this directly.

What it demonstrates

That a model can be executed rather than computed, and that the instrument then inherits both the model’s reach and its errors exactly.

All mechanisms