Grandfather Clock Moon Dial

The arch of the moon dial is calibrated into 29-1/2 days, corresponding to the lunar month. There are 59 teeth cut around the edge of the moon disc. A small gear is mounted on the hand shaft to mesh with the larger wheel near the hand shaft. This train of wheels moves the disc forward at a speed of 1/2 a revolution in 29-1/2 days, completing the lunar month.

The grandfather clock moon dial is as accurate as it is practical to make it. Actually, it is a trifle off, and if you watch the moon phase, you will have to adjust it every few months. The moon actually circles the earth in an average of 27 days, 7 hours, 43.2 minutes, and the period of time from new moon is 29 days, 12 hours, and 44.05 minutes, not the exact 29-1/2 days measured by your clock.

The moon disk can be moved by hand in the back of the dial. Place the moon exactly beneath No. 15 on the moon dial. Using a calendar, determine the number of days since the last full moon. Move the moon disk, one click in a clockwise direction for every day since full moon, .e.g., the moon is five days after full moon, move the moon disk five clicks. Should the moon gears be active, the disk cannot be turned. Move the minute hand clockwise (as many hours as needed) to take the gears out of action, then set the moon disk and readjust the time.

Full Moon Dates