Time & Measurement

The Byzantine Sundial-Calendar as a Portable Geared Lunisolar State Computer

Period and origin

Byzantium, conventionally dated 400-600 CE; the most important surviving geared device between Antikythera and the medieval traditions

What it was built to do

To carry a predicted calendrical state and an observed local time in one portable instrument, for any of sixteen named places.

How the mechanism works

A sundial plate, a suspension element, a lunar disc with gear and an arbor carrying a ratchet and gears survive. Gear trains map calendrical state to annual solar longitude and lunar phase; the sundial independently maps solar illumination to local time for a chosen latitude.

Input

A manual calendrical advance, and the Sun itself for the dial.

Transformation

Geared mapping of calendar state to solar longitude and lunar phase, alongside an independent optical mapping of illumination to local hour.

Output

Lunisolar state and local time, together, for a selected geographic setting.

What is evidenced

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

What it demonstrates

A prediction and a measurement of the same world, carried side by side - and an honest gap, because the exact coupling between the two functions does not survive.

All mechanisms