Our SAWTA 4 prep gets a slight reprieve today with the ETA 7751, a watch that requires almost all of the same adjustments as a 7750, yet has some fun extra features as well.
The difference between the 7750 and the 7751 is all in the calendar. While the 7750 uses a day/date calendar, the 7751 incorporates an entire traditional triple calendar with moonphase. That means it displays the month, day, date and phase of the moon—but makes no special corrections for which months have fewer than 31 days.
The 7751 didn't debut until the 1980s, but the layout of the calendar strongly echoes vintage triple calendar watches from the 1940s.
Since it's a simple calendar—as opposed to annual calendars, which keep track of the 30/31 day months, and perpetual calendars, which account for February's weirdness and leap years—it's an extremely simple jumper chain that runs all of the indicators. The date wheel, just up and left from center, drives the date pointer (center) and day wheel (top left, removed), as well as the moonphase, via the long yoke reaching down and to the right. The month is indexed by the date pointer wheel's finger, which comes around once every 31 days.
Simple jumper-actuated moonphases like this use a 59-tooth disc with two moons. The lunar cycle takes 29.53 days, which is pretty close to 29.5 days. The jumper indexes the wheel by 1/59 of a rotation every day, and since the ratio is so close, it takes a couple of years for the phase to be off by a day. Geared moonphases, which advance continuously and don't use a jumper, are typically accurate to 122.6 years or greater, well beyond the watch's expected service interval.
Regardless of how it's implemented, the moonphase is my favorite complication. It's almost completely useless, but classic, beautiful and utterly romantic. A vintage triple calendar watch is on my wish list, but unfortunately they tend to be a little out of the price range of a thrifty watchmaking student.
The rest of the watch is bog-standard 7750, which is to say good practice for the SAWTA 4 exam. That's coming up next week!
Watchmaking student at the Lititz Watch Technicum, formerly a radio and TV newswriter in Chicago.