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.
Thanks Gail for that question. Forward any furniture or related questions to me and I will see what I can do about answering them. -Ken Champagne