What is a “Moondial”?

16 October 2014

The Short Answer (TSA)

            A “moondial” is just like a sundial.  Only, instead of working to tell time with the light of the sun, the moondial is designed to tell time by the light of the moon.

            A sundial was an early type of clock.  The sundial is a circle-shaped disk fixed to the ground.  Then, rising from the disk is a fixed wedge-shaped object called a “style.”  When the sun shines, the style casts a shadow, which points to various positions on the circular “face” of the sundial during the course of the day.

Sundial

            To someone who understood how to interpret the movements of the shadow, the sundial was an effective clock – at least -- on cloudless days.  The moondial was a good "idea."  But, to me, stretched the principle of the sundial a bit too far.  No matter.  In its day, the moondial was taken quite seriously as a timepiece.

            The problem with the moondial was that it was only really accurate on the night of the full moon.  And, of course, it had to be a clear night to allow the moonlight to shine directly on the moondial.   

            After the night of the full moon, the moondial “lost” time -- running 48 minutes slower every night.  48 minutes!  One week after the full moon, the moondial was running 5 hours and 36 minutes slow!  The sunlit part of the face of the moon becomes smaller after the full moon.   So, the light necessary to allow the moondial to tell time runs out a little over a week after the full moon. 

            During the new moon phase, the sunlit part of the moon is facing entirely away from the earth, so there is no moonlight at all.  After the new moon, it takes about a week before the moon’s face reflects enough light to get the moondial “running” again.  Of course, when it starts, about a week before the full moon, the moondial is running 5 hours and 36 minutes fast.

            The moondial fans of the past weren’t discouraged by any of this and made rather complicated charts to “correct” for the gains and losses in the displayed time on the moondial’s face. 

Simple (?!) Moondial Correction Chart

            Sadly, even with these charts, the moondial had one other drawback.  Because the moon’s orbit around the earth isn’t a perfect circle, the moondial doesn’t gain or lose exactly 48 minutes a night before or after the full moon.  So, even with the correction charts, the moondial is almost never accurate.

            The moondial was a great idea, but a “less than great” clock.