The moon clock shows you the image of the moon as it would look like if you actually could see the moon!
In this way you always know in which phase of the 29,5305882 days lunar cycle the moon is in! The moon clock is designed as a normal clock where the minute hand has been removed.
The hour hand points out the actual moon phase and is 29,5305882 days to make a "12 hour" rotation. The second hand moves "one secound" every 59 seconds. So this is really a slowed down clock keeping track on the moon phase!