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The Encyclopedia of Horology

Clock type · Precision

Equation Clocks

Partially verified

Clocks showing the difference between solar and mean time, or displaying both.

Movement & mechanism

  • A kidney-shaped cam turning once a year drives the equation indication
  • Some examples display both solar and mean time with two minute hands
  • Accuracy depends on how carefully the cam is cut

Identification

  • An equation indication reading plus or minus minutes
  • A kidney cam visible in the movement
  • Frequently combined with calendar work

Terminology

  • Kidney cam
  • Equation of time
  • Mean time

Known variations

  • Single-indication and dual-hand versions

Collecting notes

Originality decides value and interest: check that movement, dial and case belong together, and prefer honest wear over refinishing. Movement work belongs with a clockmaker; cleaning a dial or case yourself commonly destroys the evidence that dates the clock.

Related photographs

The Prague Astronomical Clock in Old Town - 8559
The Prague Astronomical Clock in Old Town - 8559 — Jorge Royan, CC BY-SA 3.0. Wikimedia Commons file page
Czech-2013-Prague-Astronomical clock face
Czech-2013-Prague-Astronomical clock face — Godot13, CC BY-SA 4.0. Wikimedia Commons file page

No confidently matched photograph of this subject has been found yet. These photographs show a closely related record, and are not examples of the type described on this page.

Sources & references

  1. Britten, F. J., Old Clocks and Watches and Their Makers
  2. Loomes, B., reference works on British clockmakers (edition-dependent)
  3. Museum catalogue descriptions of comparable examples

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