Antique Clocks

How to set the moon dial on your clock

Some clocks have a dial to show the phases and age of the moon. There are several types of moon dial, the commonest being what is called a 'rolling' moon, which is a disc set on the very top of a clock dial, called the arch. The moon dial is designed to show the shape of the moon as it appears in the sky, the number alongside it being the lunar date. This is not the same thing at all as the calendar date. A lunar month has consistently twenty nine and a half days in every month and these are usually lettered in Arabic (English) numerals. Some moon dials also incorporate a tidal dial showing high water times at a certain port, sometimes several ports at once. Tide times are normally lettered in Roman numerals.

The current lunar date can be established by reference to a calendar or diary - many show a white circle to mean a full moon and a black circle to mean a new moon (i.e. no moon). Look up the last nearest full or new moon setting and add the number of days since then to ascertain today's correct lunar date. Once set, the moon dial stays accurate for the year, and does not need constant re-setting as a calendar dial does.

The disc of a rolling moon turns clockwise; most will not allow turning anti-clockwise. It will probably be 'in drive' (i.e. changing) during a period of about three hours in every twelve, and cannot be re-set when in drive. At other times the moon disc will be free to turn clockwise until the appropriate lunar date is showing. Most moon dials have a ratchet to prevent the dial slipping back again. This will be visible behind the dial, usually at the top. If re-setting the moon dial, be careful not to inadvertently disengage the ratchet.

If the moon does not progress as the clock runs, then the moon drive mechanism is probably faulty or lacking, and needs the attention of a restorer.

Copyright © 2013 Brian Loomes