Using a mechanical clock's display to mark the state of the moon's phase is an old and common tradition among makers of clocks. One encounters moon dials on clocks from virtually every country, and in every size of clock or timepiece from huge public tower clocks right down to wristwatches. Taken to its extreme, these displays can also incorporate an orrery or planetarium, showing the present position of the known planets in the earth's solar system, but all we are concerned with here is the more commonly-seen "moon dial" on grandfather clocks from England and America.
Why a moon dial? Well, it was never intended as just a novel decoration for the amusement of the owner and his friends. There is a practical side too. Lunar displays on clocks are known in Germany from the end of the 16th century. When longcase clocks for homeowners in England became popular in the late 17th century, street and road lighting for travel at night hardly existed anywhere. If one wanted to have friends over or travel out, one had to know when moonlight would be available. Also near the end of the 17th century, the English Astronomer Royal, John Flamsteed, worked out his tables showing that the moon's phases were closely linked to the regular rise and fall of the tides. Since few coastal streams had bridges, it was useful to know when a coach could ford a stream and keep the owner and his powder dry, and a clock could provide the information.
There is also a lot of folklore around that says farmers planted by the moon's cycles, and the clock could help. Well, maybe. We'd love to see some proof of that.
Moon dials, or more properly "lunar dials", usually take the form of a disc incorporated into the main dial plate, usually in the arched top section. The ones we show here are all English or American in origin. The circumference of the disc is cut with teeth so that the machinery of the clock, usually the "motion work", which is on the front plate of the movement and is responsible for moving the hands, can engage the disc and click it forward twice every twenty four hours. Half of the disc's circumference displays one full lunar cycle, 29 and one-half days, and the days are numbered around the arch of the opening. The lunar cycle starts at the new moon (a dark night sky and no man-in-the-moon face showing on the clock), progresses to the full face showing on the 15th day, the full moon, and back to no face again as the moon wanes.
Most lunar dials are partially concealed on each side of their opening in the main dial plate by semi-circular "humps" that allow the painted face to emerge slowly just as the real moon goes out of and back into the earth's shadow. These humps on the earlier brass dials are often engraved with either scenes relating to the moon's cycle, or global maps of a very simplified form. A shining sun is a common scene on the humps of brass dials, although the sun'scycle has nothing to do with the lunar display. On painted dials, which emerged in England from about 1772 onwards, the humps are almost always fairly good transfer-printed maps of the known western and eastern hemispheres. Interestingly, the global maps produced by English dial painters for the American market are usually complete with numbered longitudinal lines around the outer edges, where the dials for domestic consumption rarely have such numbers.
Other types of lunar displays exist too, and clockmakers went to quite a bit of work sometimes to provide a unique example. Dials are seen, but rarely, with a brass sphere in the arch of the dial, with one half of the sphere silvered and the other half blackened (left). As the sphere revolves a little each day, the silvered side comes into view at the same rate as the new moon emerges out of shadow in the night sky. Such spherical moons were produced by several English makers in the northeast of the country in the early to mid-18th century, and the spiral-cut gearing needed to drive them was quite sophisticated for the time. Another type of lunar dial encountered on common 30-hour country clocks is the "penny" or "Halifax" moon, a very small disc about the size of an old English penny that rotates behind a small aperture in a square brass dial, just above the center of the hands. At the full moon, a silvered face completely fills the opening in the dial.
When a clock has been built for a customer in a seaport town, the cycle of the local tides is sometimes shown along with the moon's phase. This will usually be indicated by a set of Roman numerals painted or engraved along with the more usual Arabic numerals that indicate the moon's age. An explanatory note is usually included, such as "High Tide at Bristol Key" or some such. Rarely, a movable pointer hand will be included so that the tides, which will vary if the clock is moved up or down the coast any great distance, can be indicated separately.
SETTING A MOON DIAL
Your local newspaper, or one from a large city near you, will usually include an almanac section on the weather page. The little blackened circular graphics that show the lunar calendar will be quite obvious. To set your clock's lunar dial you need to know the date of the current month on which the full moon will occur. The trick to interpreting the data is knowing that the full moon happens on the fifteenth day of the 29 1/2 day lunar calendar, which is independent of the day it occurs in the monthly calendar. For example, take today's date as being March 18, 1999. The Washington Post shows that the full moon will occur on the 31st of March. Therefore, the 18th, today, is 13 days before the full moon. Set your moon dial so that its pointer, usually a projection out of the top of the round face, points to "2" on the numbered half circle around the arch. In 13 days, on the 31st of March, it will be the 15th day of the lunar calendar, the pointer on the round moon face of your clock will be straight up and the face fully exposed, and the full moon will be in the night sky.
Rarely, there are two full moons in a single month. The second full moon is known as the "blue moon", famed in song and fable.
Most lunar discs will rotate easily just by turning them with your fingers, but they are sometimes fragile and pushing on the disc too hard can dislodge the linkage that moves it or bend the pivot. To avoid putting too much force on the disc you may have to remove the hood of the clock, the "bonnet" as it is sometimes called, and grasp the disc front and back. As you rotate the moon disc, its positioning spring, called a "click", will very likely snap along over each tooth on the outer rim, making the characteristic clacking noise. If the disc resists an attempt to move it forward, wait a couple of hours and try again. The movement of the clock may be engaged with the linkage that moves the moon disc and pushing too hard could damage something. Be gentle, and patient. Once set, the disc shouldn't need any further adjustment, so long as the clock runs continuously.
If your lunar dial doesn't move during normal running of the clock, you are not alone. This is a common fault in longcase clocks, and especially those with painted dials. To sort it out, you will usually need the attentions of a qualified clockmaker.
Charles Probst 3/99