Complication · Calendar & astronomy

Moon Phase

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A display, usually a rotating disc showing two moons behind an aperture shaped like the sky, indicating the current phase of the moon across the synodic (lunar) month.

Definition

A display, usually a rotating disc showing two moons behind an aperture shaped like the sky, indicating the current phase of the moon across the synodic (lunar) month.

Historical origin

Moon-phase indications on clocks and watches date back to at least the sixteenth and seventeenth centuries, closely tied to earlier astronomical clock traditions such as church and astronomical clocks that modelled lunar motion.

How it works

A disc bearing two painted or engraved moons (180 degrees apart) sits beneath the dial, visible through a shaped aperture; a jumper wheel with 59 teeth advances the disc by one tooth every 24 hours, so each moon takes 29.5 days to cross the aperture — matching the synodic month of approximately 29 days, 12 hours and 44 minutes.

Because 59 teeth divided across two full lunar cycles (59 days) only approximates the true synodic period, a standard moon-phase mechanism drifts by about one day roughly every two and a half years and requires periodic manual correction.

Higher-grade 'precision' or 'astronomical' moon-phase mechanisms use more accurate gearing (some running for 122 years or longer between one day of drift) at the cost of additional wheel work.

Age of the Moon: A display showing the number of days elapsed since the last new moon.

A scale from 1 to 29.5 days is printed on the dial, with a hand or a pointer showing the current day in the lunar cycle.

Spherical Moon Phase: A three-dimensional representation of the moon's phase using a rotating ball.

A small sphere, half dark and half light (or gold), rotates on its axis to show the lunar phase.

Often uses high-precision gearing for accuracy over hundreds of years.

Pocket watch context

A fixture of complicated nineteenth-century pocket watches, very often combined with a calendar dial and, in the most elaborate examples, with equation of time and other astronomical indications.

Wristwatch context

A popular dress-watch complication today, frequently paired with an annual or perpetual calendar, prized as much for its display as its timekeeping utility.

Clock context

A long-standing feature of longcase clock arches and bracket clock dials, typically driven from the clock's own 24-hour motion work via an equivalent 59-tooth (or more accurate) wheel.

Identification

Aperture shaped like a stylised night sky with a rotating disc of two moons beneath
Standard-grade mechanisms will show visible drift from the true lunar phase after a few years without correction
Correction is usually via a pusher advancing the disc one day at a time
A scale numbered up to 29 1/2
Often located around the moon phase window
A 3D ball on the dial that rotates
No flat moon phase window

Servicing notes

The disc and its jumper are generally robust, but forcing the corrector against the jumper's resistance point can bend it; specialist correction and disc replacement (especially for painted moon discs) should go to a watchmaker or restorer.

General guidance only. Complicated mechanisms should be assessed and serviced by a qualified watchmaker or clockmaker specialist, not adjusted at home.

Background

The Moon's phase can be approximated from the number of days elapsed since a known new moon. The elapsed time is reduced modulo 29.53059 days, the mean length of the synodic month. Such a calculation assumes a perfectly circular lunar orbit and ignores the time of day at which the new moon fell.

The result is accurate enough for a novelty clock. More precise results must account for the Moon's apogee and perigee.

Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.