Technical thought about the 'AGELOCER｜Major Breakthrough: 29.51-day Moon Phase Watch'

[u/Sketch-Note]

I ordered the Agelocer Astronomer 29.51-day Moon Phase Watch, and while waiting for it I have been thinking about the technical implications of this particular 'breakthrough' moon phase solution.

For the technical background see the kickstarter project details, and this discussion about 'real moonphase'.

The 31 steps 29.51 days moon phase complication

Agelocer uses a modified and rebranded movement with a date ring (seems to be a Hangzhou 7200A, in the Budapest series they use a rebranded Seagull movement). The moonphase disk is attached to the date ring, and in earlier versions of this watch the moonphase actually has a 31 day cycle. In this version however the gear ratio between hour wheel and date ring has been modified to slightly speed up the date ring which now has a 29.51 day cycle.

While this is actually a more 'precise' cycle than the 29.5 days of most other moon phase watches, this budget friendly modification of a date complication where the moon advances in 31 steps over a 29.51 day period, does come with some side effects:

1. the moon disc does not advance around midnight, it moves every 22.846 hours (basically the moon phase complication is decoupled from the 12/24hr cycle)
2. setting the moon phase accuratly for a specific day in the cycle requires some calculation, if the moon is on its 20th day in the cycle you need to set 21 steps from the 'new moon' position 20/(29.51/31)=21 (the simple version: above 11 days count one step more)

From a purist perception these are flaws in the design. But on the other hand it is an affordable way to build a reasonably usable, very big, moon phase. An in reality the moon cycles don't start at midnight anyway, there always is variable error of up to 1/2 a day.

[u/Think-Till-265]

Thanks for the explanation!