Complication · Calendar & astronomy
Equation of Time
VerifiedA display showing the difference between mean solar time (the constant time kept by clocks) and true (apparent) solar time (as read from a sundial), which varies through the year due to the Earth's elliptical orbit and axial tilt.
Definition
A display showing the difference between mean solar time (the constant time kept by clocks) and true (apparent) solar time (as read from a sundial), which varies through the year due to the Earth's elliptical orbit and axial tilt.
Historical origin
Equation of time mechanisms appear in astronomical clocks from the seventeenth century onward, once the difference between mean and apparent solar time was well understood by astronomers such as Huygens and Flamsteed; it later became a hallmark high-complication feature of the finest pocket and wristwatches.
How it works
A specially profiled kidney-shaped (or heart-shaped) cam, cut to encode the equation of time's smoothly varying annual curve, rotates once per year, typically geared from the calendar mechanism's annual or monthly advance.
A lever or feeler follows the cam's contour and translates its varying radius into the position of a dedicated equation-of-time hand or hand-and-scale display, showing how many minutes mean time is currently ahead of or behind apparent solar time (the difference varies smoothly between roughly plus and minus a quarter of an hour across the year).
Because the cam's shape must be cut with high precision to reproduce a curve with no simple closed mechanical form, equation-of-time mechanisms are traditionally regarded as demanding to design and manufacture accurately, and are almost always found only in the highest grade of complicated watch.
Pocket watch context
A hallmark of the most elaborate nineteenth-century grande complication pocket watches, frequently paired with perpetual calendar and moon phase in the same movement.
Wristwatch context
A rare and prestigious modern wristwatch complication, valued for its historical and astronomical interest as much as any practical navigational use in the present day.
Identification
- A dedicated hand and scale (often marked in minutes, plus and minus) distinct from ordinary time and calendar hands
- Driven by a uniquely shaped annual cam, visible on the movement in open displays
Servicing notes
The equation cam and its feeler lever are precision components specific to the calibre; any adjustment or repair should be entrusted to a watchmaker experienced with astronomical complications.
General guidance only. Complicated mechanisms should be assessed and serviced by a qualified watchmaker or clockmaker specialist, not adjusted at home.
Diagram
Sources & references
- Daniels, G., Watchmaking
- Chamberlain, P. M., It's About Time
Where sources disagree, the disagreement is stated rather than resolved silently.