A form of auxiliary lunar dial of circular shape which was adapted for use on square-dial clocks. Although halfpenny moons were occasionally used by London makers as early as 1735, the vast majority of examples are to be found on the clocks of Lancashire and Yorkshire makers later in the 18th century. An alternative to the halfpenny moon on a square-dial clock was the segmented moon opening, like the usual moon movement in an arch dial but here inverted, the moon's disc descending to full moon then ascending, instead of the climbing and descending path followed in an arch dial.