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The Encyclopedia of Horology

Complication · Calendar & astronomy

Sidereal Time Display

Partially verified

A display of time measured against the stars rather than the sun.

Definition

A display of time measured against the stars rather than the sun.

Historical origin

Required for astronomical observation, where star positions are tracked in sidereal rather than solar time.

How it works

An additional gear train, or a separately geared dial, runs at the sidereal rate, about four minutes per day faster than mean solar time.

In some designs a second movement or train is fitted rather than deriving the rate from the main train.

Pocket watch context

Found in astronomical pocket watches and observatory clocks.

Wristwatch context

Very rare in wristwatches.

Clock context

Observatory clocks commonly kept sidereal time for telescope setting.

Identification

  • A second time display running slightly fast relative to civil time

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

date ring, thirty-one positionsdate aperturedriving wheel and jumper
Schematic: calendar work under the dial. An outline schematic drawn to show the arrangement of the parts discussed here. It is not a dimensioned working drawing and does not depict a particular maker's calibre.

Sources & references

  1. de Carle, D., Watch and Clock Encyclopedia
  2. Britten, F. J., Old Clocks and Watches and Their Makers
  3. Horological Journal (British Horological Institute), technical articles

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