Complication · Calendar & astronomy
Equation of Time
Partially verifiedAn indication of the difference between solar and mean time.
Definition
An indication of the difference between solar and mean time.
Historical origin
A response to the historical transition from sundial time to mean time, and a display of astronomical learning in fine watches.
How it works
A kidney-shaped cam rotating once per year is followed by a lever, which drives a hand or an additional minute hand showing the equation.
The cam's profile encodes the annual variation, so its accuracy depends on how carefully it is cut.
Pocket watch context
Found in the most elaborate pocket watches and precision clocks.
Wristwatch context
A rare high complication in wristwatches.
Clock context
Precision regulators sometimes carried equation tables or mechanisms so sundial readings could be reconciled with mean time.
Identification
- An indication reading plus or minus minutes
- A kidney-shaped cam visible in the movement
Servicing notes
This is specialist work. Levers, springs and cam profiles in complicated work are calibre-specific, and incorrect reassembly commonly damages parts that cannot be bought. Refer the movement to a watchmaker experienced with this mechanism.
General guidance only. Complicated mechanisms should be assessed and serviced by a qualified watchmaker or clockmaker specialist, not adjusted at home.
Diagram
Sources & references
- de Carle, D., Watch and Clock Encyclopedia
- Britten, F. J., Old Clocks and Watches and Their Makers
- Horological Journal (British Horological Institute), technical articles
Where sources disagree, the disagreement is stated rather than resolved silently.