The center of the dial is a rotating disc with a small window.
The disc makes a single full rotation every 12 hours, accurately depicting the angle of illumination of the moon as it turns.
As the moon makes its entire 29.5 day cycle, the moon disk (beneath the dial disc) via the window changes, increasing or decreasing in size as the moon waxes or wanes.
The moon-phase is set by means of a recessed button located on the case edge between 5 & 6 o’clock position.