Original vintage 1982 print advertisement for his and hers two-tone Heuer moonphase watches – references 280.001, 281.001, and 819.002 – featuring the “Moon Phases Story.”
Dimensions: roughly 8 inches wide by 10.25 inches high
As told by Heuer in this advert, The Moon Phases Story recounts: “Once upon a time, there was a skilled watchmaker. One night, he saw the moon reflected in the glass of a watch face and got a stylish inspiration…TAG Heuer’s moon phase watches were born from this anecdote. Each one has a different expression depending on the year and circumstances in which it was made. Look closely and you will hear the story of the distant past engraved on the face of the moon.”
Moonphase watches display the sunlit portion of the moon as observed from Earth at any given point during each 29.5-day lunar month, the time it takes to get from a new moon when it's not visible to a full moon and back again. First developed for a wristwatch by Patek Philippe in 1925, their moonphase complication debuted before Rolex introduced its own interpretation of the complication in 1949 with the Reference 8171.
Most commonly, the moonphase is displayed through a crescent-shaped aperture to identify the waxing and waning phases of the Moon. This is known as a ‘bosom’ moonphase watch, where two identical moons sit on a disc under the watch dial. The disc is controlled by a 59-tooth driving wheel, which uses a mechanical finger to advance the disc one notch every 24 hours. When a lunar cycle is complete, the second moon appears in the aperture, therefore the total of 59 teeth (29.5 x 2).
Given the origin of timekeeping, namely to understand the movement of the sun, the moon and the constellations, when early horologists evolved from the sundial to gears and hands, one of the important events they tracked was the lunar cycle.
The concept of the moonphase dates back far before the wristwatch – in 205BC, the ancient Greeks developed the Antikythera mechanism, an astronomical calculator comprising 30 bronze gears inside a wooden case. It was able to display the position of the Moon, dates, and even the timing for the ancient Olympic Games. It would be another 11 centuries before the Chinese scientist Su Song invented the first astronomical clock.
These clocks were especially popular during the Renaissance. Usually built inside cathedrals, they displayed the orbit of the known planets and the phases of the Moon, along with the time and month of the year. Astronomical clocks depicted the belief that God’s creation of Earth sat at the center of the universe with the Sun orbiting around it, a fact that scientists later disproved, much to the chagrin of the clergy.
Due to the imperfect number of days in a lunar month (the true cycle lasts 29 days, 12 hours and 44 minutes, or 29.53 days), a moonphase watch will actually be off by 0.3 days per month. This means that the entire cycle is off by one full day every two years, seven-and-a-half months.
This discrepancy has been overcome by the inclusion of a more sophisticated mechanism that was developed to incorporate a 135-tooth gear to drive the moon disc. This improvement increases the accuracy of the movement so the complication will only be off once every 122 years. However, for a watch with the standard 59-tooth gear, you will need to manually adjust it roughly every two-and-a-half years.