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.
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. 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 s 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 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.
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 |