Skip to content
The Encyclopedia of Horology

Glossary · Regulation

Secular rate

The gradual, long-term drift in a timekeeper's rate as its parts settle and lubricants age.

Explanation

Secular rate describes the slow, steady change in a timekeeper's daily rate over months or years, as distinct from day-to-day scatter or changes with temperature and position. Causes include thickening oil, wear, slow changes in spring metals and, in pendulum clocks, creep in the suspension and rod.

Precision clocks and chronometers were rated over long periods to expose secular drift, and observatories and navigators corrected for it in their calculations. In marine chronometers a steady change of rate after cleaning is common for the first months, which is why newly serviced instruments were re-rated before use.

What to record

  • Plot rate against time: a straight trend is secular drift, scatter around it is ordinary variation.
  • Record service dates alongside rate records to separate settling-in from long-term drift.

Do not regulate a precision clock to chase short-term scatter; secular trends need long records.