The grandfather clock moon dial began appearing on English tall clocks during the early part of the 18th century.
Tall clocks during this period were made by hand, and many a wealthy squire delighted in displaying a beautiful tall clock in their manor capable not only of telling time, but also the phase of the moon, at a glance!
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 and crops could be harvested by the light of the moon.
Likewise, the tall case clock’s moon dial was helpful to the likes of highway men, and other assorted scoundrals of the era, in the execution of their dastardly deeds during the darkness of a new moon.
The grandfather clock moon dial is actually a highly accurate lunar calendar displaying the different phases of the moon during the lunar month.
A lunar month consists of one repetition of the complete lunar phase cycle, which is equal to 29 days, 12 hours, 44 minutes, and 3 seconds. You will see 29.5 engraved above the rotating moon dial on most grandfather clocks.
The primary phases are the new moon, first quarter, full moon, and last quarter. You can find their dates and times published on calendars, in almanacs and newspapers.
You will find a picture of the current moon phase courtesy of the United States Naval Observatory displayed above.
Modern grandfather clock moon dials consist of a round disk displaying two pictures of the moon. A one half rotation of the disk occures every 29.5 days, comprising one lunation.
To set-up your moon dial correctly, first determine what phase the moon is currently in. Then press lightly against the moon dial with your fingers and at the same time rotate the dial to the right to the correct postion.
If you prefer not to get your fingerprints on the front of the moon dial, you can rotate it from inside the grandfather clock by accessing through one of the side doors.
For example, if its a new moon, you will want to rotate the moon clockwise to the right until it disappears beneath the right hemisphere, and no moon is visible.
If it is a first quarter moon, then rotate the disk clockwise until the moon starts to reveal from the left hemisphere, then stop when the moon is 1/4 displayed.
A full moon is represented when the moon dial is rotated to the right and the moon displayed at its highest position.
The last quarter of the moon is shown by rotating the moon dial to the right , so that 3/4 of the moon is behind the right hemipshere of the clock and only 14 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.