The moon phase clock on a grandfather clock is very accurate if set correctly. In addition, the clock must be functioning properly. Once a grandfather clock stops running, the moon phase clock also shuts down.
To set the moon phase clock, the user first determines the precise moon phase for the date. Almanacs and calendars often have this information. In addition, the U.S. Naval Observatory features an interactive table for moon phases at AA.USNO.Navy.mil. An easy and accurate way to set the moon phase clock is to begin with the most recent full moon. For each day past the full moon, the user revolves the moon dial one click in a clockwise direction.