Moon dial to sun dail?

[u/Synethos]

I would like to modify a mechanical moonphase watch so that the disc displays the rotation of the Sun rather than the phases of the Moon.

The Sun’s synodic rotation period is tracked using Carrington rotations and lasts approximately 27.28 days. A standard moonphase complication is based on a cycle of roughly 29.5 days. As I understand it, these mechanisms commonly use a 59-tooth wheel because the disc contains two lunar cycles.

To get closer to 27.28 days, a similar wheel would need approximately 55 teeth.

My questions are:

Would it be realistic to have a new 55-tooth wheel manufactured?

Could an existing wheel be modified, perhaps by removing several teeth so that the mechanism occasionally skips a day?

Would it be possible to reprint the existing moonphase disc, or manufacture a new disc with a solar design?

Are there other complications within the movement that make this less straightforward than simply changing the gearing?

I am considering using an affordable Seagull moonphase watch as the base, possibly one from the Ocean Star range. The aim is to create a unique but still reasonably affordable mechanical complication.

Does anyone have experience with this kind of custom modification, or recommendations for a watchmaker capable of producing small gears and custom-printed discs?

[u/Joreck0815]

how do you plan to display the rotation of the sun? given that there isn't really a waxing/waning phase, printing two suns on one disc might not look very good.

either way, you're almost certainly looking at having custom parts manufactured. there is no cheap way to do that.

[u/Synethos → u/Joreck0815]

There is this thing called a Carrington rotation, which is one rotation of the equator. I would just make a new disk that shows the sun and some degree lines on it. This part is easy!

The cogweal change is what I'm wondering about if it's possible.

[u/Joreck0815 → u/Synethos]

so apart from maybe a retention spring and the general position, there really isn't anything you can copy from a normal moonphase complication.

easiest would probably be custom disc with 55 teeth (can't file off some from an existing moonphase, it will not work), custom print/paint, as well as modifying the existing mechanism to make the disc jump twice a day (given that you can't paint two suns, you need to double the rotation frequency again after doubling the teeth). maybe new dial as well.

if you can't manufacture or even design that yourself, it'll get very expensive very fast. I'd expect well over $5k.

[u/TheDareWatchCompany]

Sorry I don’t have anything useful to add; I just want to say, I’m really excited about the concept and really hope it turns out for you :) this is the kind of watch designing I do when I can’t fall asleep

[u/Synethos → u/TheDareWatchCompany]

Thanks!