Watch family · Complication

Equation of Time Watches

Verified

Watches that display the equation of time — the varying difference, through the year, between mean (clock) time and apparent (sundial) solar time — usually via a supplementary hand or subdial.

Movement & mechanism

A specially shaped cam (the 'equation cam' or kidney cam), completing one rotation per year, encoding the day-by-day variation between solar and mean time
A follower reading the cam profile to drive a dedicated equation-of-time hand or indicator
Often combined with other astronomical indications (sunrise/sunset, solstices) in the most complex examples

Identification

A distinctly shaped supplementary hand, often marked '+'/'-' minutes relative to mean time
Frequently found alongside other astronomical complications on the same dial

Terminology

Equation of time — the difference between apparent solar time and mean (clock) time, varying through the year by up to about a quarter of an hour
Equation cam (kidney cam) — the annually-rotating cam encoding this variation

Known variations

Simple equation of time indication
Equation of time combined with perpetual calendar and astronomical indications

Pocket-watch ancestry

Equation of time mechanisms were established in astronomical clocks and later fine pocket watches well before the wristwatch existed; their wristwatch application is a direct, if much later, continuation of established pocket and clock practice.

Collecting notes

A rare, high-prestige complication, generally found on the most complicated and expensive watches from specialist manufactures.

Diagram

Schematic: calendar work under the dial. An outline schematic drawn to show the arrangement of the parts discussed here. It is not a dimensioned working drawing and does not depict a particular maker's calibre.

Sources & references

Daniels, G., Watchmaking

Where sources disagree, the disagreement is stated rather than resolved silently.