Equation of Time Watches

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.