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Discussion Starter #1
I have seen a number of watches with Valjoux 7734 movements that include a "moonphase" complication but have "6-9-15-18-21-24" indicators instead of the usual 0–29.5 indicators. (See image of Tissot example, below.)

Can anyone explain the 6-9-15-18-21-24 indicators? I can't make heads or tails of it but would like to understand before purchasing such a watch. What could those numbers indicate?

Thanks!

15442936
 

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Discussion Starter #5
Yes, but what about when the 0-5 and 25-29.5 are cut off, as in the example above?

So far, the first explanation (cutoff by the other subdials) is the most persuasive explanation.
 

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Again, my educated guess is that there are principally two dates (numbers) that matter; and, they are the number "6" and the number "21" — everything before "6" and after "21" is the waxing gibbous/crescent. The dates in between the "6" and the "21" are the waning gibbous/crescent. Note the "29" date of a moon phase watch is the odd one out as it acts as a "bridge" between the waxing gibbous/crescent moon phases.

Crown and Caliber's article below may help with a better understanding of setting a moon phase what to the phases of the moon.
Screen-Shot-2020-03-18-at-3.34.36-PM.png


 

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Again, my educated guess is that there are principally two dates (numbers) that matter; and, they are the number "6" and the number "21" — everything before "6" and after "21" is the waxing gibbous/crescent. The dates in between the "6" and the "21" are the waning gibbous/crescent. Note the "29" date of a moon phase watch is the odd one out as it acts as a "bridge" between the waxing gibbous/crescent moon phases.

Crown and Caliber's article below may help with a better understanding of setting a moon phase what to the phases of the moon.
View attachment 15443108

Why wouldn't the waxing gibbous and waxing crescent phases "matter"? And even if you had those, on a 6–24 "moonphase" dial, you'd still be shy 2.5 days. Not trying to be facetious, I just don't understand...
 

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My best guess is that as I said before the two numbers that are important are the "6" and "21"; which are separated by 14 numbers and correspond to the Full and New moons. The two moons on an ETA\Valjoux 7751 calibre wheel disk (as shown below) are characterized and represented on opposite sides of the wheel disk. And, everything between these two moon characters are "noise" or in this case are represented as "stars".

Also remember, the ETA\Valjoux 7751 calibre (and those built on the ETA\Valjoux 7751 plateform) will require readjusted every few months to account for the ~29.5-day variance between the date calendar and moon phase calendar.

Clear as mud, right?

s-l1600.jpg
 

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It might help to look at a "complete" moonphase indicator as an example:
15443446

In its present position, the "top" or "apex" of the moon on the above watch is pointing to approximately 13 on the 0-29.5 scale. That means it has been 13 days since the last new moon. In about 1 or 2 more days, when the moon is pointing to 14/15, or when it is in the exact center, it will be a full moon. Then, from 15 on the scale down to 29.5, as the moon continues to move to the right, it is a waning moon, and when the moon gets back to 29.5, you have another new moon.

The watch you posted is no different than the one above, but whereas the above watch has indicator numbers at 0, 5, 10, etc., yours has them at 6, 9, 12, 15, etc. And, yours has the two edges of the scale cut off. But it's the same scale. On the watch you posted, the moon indicator is pointing to approximately 19, meaning it has been 19 days since the last new moon.

I hope this helps and I am not misunderstanding your question.
 

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Discussion Starter #10
Again, my educated guess is that there are principally two dates (numbers) that matter; and, they are the number "6" and the number "21" — everything before "6" and after "21" is the waxing gibbous/crescent. The dates in between the "6" and the "21" are the waning gibbous/crescent. Note the "29" date of a moon phase watch is the odd one out as it acts as a "bridge" between the waxing gibbous/crescent moon phases.

Crown and Caliber's article below may help with a better understanding of setting a moon phase what to the phases of the moon.
View attachment 15443108

I don't think that chart makes sense because it shows a 31-day cycle when the actual moon cycle is 29.5 days.

15448244


This chart—on which I've taken the liberty of crossing out the days that do not appear on the example watch I first posted—shows 30 days (presumably, the 30th day is to cover the last 1/2 day of the cycle) and is, I think, a bit more helpful. This chart shows the 15th day of the cycle with a full moon, which is where it should be because the full moon appears 15 days after the last new moon. It also aligns with the 15 marker as shown on the example watch I first posted.
 

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On the chart that you have shown, it looks like the new moon is on day 30 (C&C's chart day 21) and the full moon is on day 15 (C&C's chart day 6). My only question to you is: how do you account for the second moon the disk, which would be the new moon phase?
 

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Discussion Starter #12
On the chart that you have shown, it looks like the new moon is on day 30 (C&C's chart day 21) and the full moon is on day 15 (C&C's chart day 6). My only question to you is: how do you account for the second moon the disk, which would be the new moon phase?
Because the second moon is directly opposite the first moon, they both end up exactly where they should be. To wit, when one moon is aligned with the "15" indicator, showing a full moon, the other moon is directly below, hidden behind the dial just as a new moon would be hidden from the eye insofar as it would not be illuminated.

For what it's worth, I'm pretty sure that the new moon phase would never be shown on a moon phase watch—it would always be hidden behind those half disks.

The "issue" with the example watch is that it "loses" approximately ten days of the moon cycle (or more precisely, it loses 9 1/2 days of the visible moon cycle) due to the location of the other subdials.
 

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@BMPB is right about the numbers being cut off.

Here’s the VC moonphase he posted, superimposed onto the OP’s. You’ll see the VC’s numbers line up correctly on the CT&F’s scale.

15448632
 

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Discussion Starter #15
The superimposition is quite helpful, @BMPB. Key takeaways here:
  1. The example watch from CTF is indeed a moon phase watch (and not a day-date indicator).
  2. The example watch's moon phase implementation omits days 2-5 and 25–29.5 of the lunar cycle (day 1, i.e., new moon, would never be presented in a bosom moon phase complication).
  3. The reason for point 2 is likely size constraints.
  4. The result is that (whatever one thinks of the CTF's aesthetics) this is a compromised moon phase implementation.
 

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@michael4826-p2, earlier I was one of Marc's "Watch and Learn" videos pertaining to the Moonphase watch, which you may find informative. In this video, he gives a demonstration of his digital YES watch and how it handles several functions including the Moonphase, again, you may find it interesting as well. Below are the links for your reference:


 

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Agree with @BMPB - it is just the end numbers cut off.
 
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Now, if this was Stack Exchange, the best answer would’ve floated to the top already.
 

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The superimposition is quite helpful, @BMPB. Key takeaways here:
  1. The example watch from CTF is indeed a moon phase watch (and not a day-date indicator).
  2. The example watch's moon phase implementation omits days 2-5 and 25–29.5 of the lunar cycle (day 1, i.e., new moon, would never be presented in a bosom moon phase complication).
  3. The reason for point 2 is likely size constraints.
  4. The result is that (whatever one thinks of the CTF's aesthetics) this is a compromised moon phase implementation.
Pretty much. It looks like it was shoehorned into the dial. It doesn't help that the hour and minute hands happen to cover half of the moonphase window in the pic, either.

Personally, I've thought of these "bosom" moonphases (new term to me, lol) as curiosities; because although I think they're classically entertaining, like a grandfather clock, the actual display never sits right with me as a functional device. The Moon itself doesn't change phases like a cookie being eaten. Unfortunately, spherical moonphase displays are super rare — I can think of only one brand off the top of my head, and they're still out of my budget.

Anyway, now we know why it's printed the way it is. Whether you should buy it or not, that's up to you, man. It could be a fun piece of a collection. See what you can find out about the company, too — is it at all related to the Tissot we know today?
 
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