The moon in its ever waxing and waning suspension above earth had been the subject of much speculation until 1969 when man stepped onto its still dusty surface and lifted the shroud of mystery. Outside bedroom windows the moon ushers in sleep to the weary and keeps company to those who remain awake. No wonder the moon phase is such a vastly popular complication.
A Perpetual Calender provides the date, day, month while automatically making allowances for the different lengths of each month as well as leap years. A timepiece of this type may also display the moon phase; however, the moon phase can stand alone making for a eye catching complication.
The phases of the moon are displayed on a disc visible via an aperture, or more rarely by a hand. A typical moon phase watch a disc portraying two moons is driven by a wheel with 59 teeth. Every 24 hours a finger turns the wheel by one notch. In reality a lunation lasts 29 days, 12 hours, 44 minutes and 2.8 seconds or 29.53 days; however, the lunation displayed by an ordinary moon phase watch lasts 29.5 days thus the moon phase depicted on the watch is out one day every 2 years, 7 months and approximately 20 days. Astronomical moon phase watches compensate for this difference by utilizing a wheel with 135 teeth, thus achieving a lunation of 29 days, 12 hours and 45 minutes. This reduces the difference to just one day every 122 years.
Many watch companies invent new ways of depicting the moon phase. The new De Buthune DB25 Moon Phase Watch reveals a spherical moon phase below the 12 'o' clock. Consisting of two hemispheres, one in steel and the other in platinum, rotates on its own axis.
Some watches indicate the "age of the moon", displaying the number of days since the last new moon.