The grandfather moon dial clock began appearing during the early part of the 18th century. Tall clocks, during this period, were made by hand. Many wealthy squires took delight in owning a beautiful tall clock that not only told time, but also should the moon's phase. Knowing the correct moon phase was important to those living an agrarian lifestyle. Night trips on horseback, or by coach, were much safer during the light of a full moon. Additionally, farmers could harvest crops under a full moon.
Unfortunately, moon dials also aided the Highwaymen and other criminals of the era. Under the cover of darkness that a new moon offered, criminals could perform dastardly deeds without being identified by their victims. The Grandfather Moon Dial clock is actually a highly accurate lunar calendar and displays the different phases of the moon during a given lunar month. A lunar month consists of one complete lunar phase cycle, which is equal to 29 days, 12 hours, 44 minutes, and 3 seconds. You will see the number 29.5 engraved, above the rotating moon dial, on most Grandfather clocks.
The primary lunar phases are the new moon, first quarter, full moon, and last quarter. You can find their dates and times published in calendars, almanacs, and newspapers. Modern grandfather moon dials consist of a round disk displaying two pictures of the moon. A one half rotation of the disk occurs every 29.5 days completing one lunation. To set-up your moon dial correctly, first determine what phase the moon currently is in. Then, pressing lightly against the moon dial, with your fingers, rotate the dial to the right until it is in the correct position.
If you don't want your fingerprints on the front of the moon dial you can rotate via one of the side doors, or by removing the hood. For example, if it's a new moon, rotate the disk clockwise until the moon disappears beneath the right hemisphere, and the moon is not visible. If it is a first-quarter moon, rotate the disk clockwise until the moon is revealed one-quarter on the left hemisphere. A full moon is represented when the moon dial is rotated to the right and the moon is displayed at its highest position. The last-quarter moon is displayed by rotating the dial to the right so that three-quarters of the moon is behind the right hemisphere and only one-quarter of the moon is displayed. Of course, you can set your moon dial to any moon phase variation in between, by using the principles above.