A moon phase is a clock or watch complication that shows the current phase of the moon, from new moon through crescent and full moon and back again over the lunar month of roughly twenty-nine and a half days. On English longcase clocks it usually takes the form of a painted disc in the arch of the dial, carrying two moon faces that pass slowly across an aperture shaped by two hemispheres, so that the moon appears to wax and wane.
Moon phase indications became popular in Britain in the eighteenth century and are especially common on painted-dial longcase clocks made between about 1770 and 1850. The disc was typically advanced in small steps by the movement each day, and some dials also show the date of the month or the times of high tide at a named port. A practical motive lay behind the feature, since country people planning journeys or work after dark depended on moonlight. Regional variations include the Halifax moon, a small moon shown through a round aperture in the dial centre. Collectors value well-painted moon discs with original colours and celestial or landscape scenes, as well as working mechanisms. Moon phases are also prized on pocket and wrist watches.
