What do you mean I have a moon fixation? Well, I probably have - I suspect it's a feminine thing (Less of that!)
What is described here is a clock with a single hand that indicates the current moon phase. It is based on a cheap clock as can be bought for less than a couple of dollars (£1 in the UK). These clocks are powered by a single AA battery, and use a 32kHz 'tuning fork' crystal as a timing reference. To convert your prosaic low cost clock into an Orton Moon Clock requires the following steps:
Firstly, all of the hands are removed, and the clock face is substituted with the Moon Dial face shown in the graphic below. Only the hour hand is refitted to the central cluster (but retain the second for the moment - it may prove useful later).
Next, the crystal is removed. Instead of the crystal, we are going to drive the clock from a timebase of our own making, the purpose being to slow the clock down by a factor of 59. Doing this results in the hour hand completing a revolution in 29.5 days - almost precisely the time taken for the moon to do one circuit around it's phases! I have found this to be possible, at least on the clocks I have attacked (and they are nothing out of the ordinary as these clocks go). The funny little motors in these clocks don't seem to mind at all being driven many times slower than is normal.
Now, if you want a precision astronomical instrument, I recommend you build a crystal timebase, using the crystal removed from the clock. A suitable CMOS circuit is shown below.
If, however, you simply want a novelty that attracts curiosity at parties (not to mention a less than flattering reputation) then the much simpler timebase shown below will work.
Having removed the crystal, we inject our own timebase into one of the now open crystal connections. But which? I probed the open terminals with a DVM. I found one to be close to the negative supply, and the other close to mid-supply. I assumed the one at mid-supply would be the input terminal to the oscillator. This proved to be wrong! My suggestion - trial and error. If the second hand is re-attached, then you don't have to wait very long to see evidence of the clock counting our timebase frequency - the second hand will advance.
The second hand should advance every 59 seconds, and the 'Freq' preset in the simple timebase should be adjusted until this is the case (if you are lucky enough to own a frequency meter, then adjust for a frequency of 554.83 Hz). Once set, the second hand can be removed again, and the clock closed up before any of those tiny components fall out!
The alarm feature wasn't used on my clock. However, I can see the attraction of this feature to lycanthropes the world over. It should still work fine, but the tone that the clock produces is now so low that you will have to substitute a sub-woofer for the peizo sounder.
Another approach seriously considered was to drive the clock even slower - using an external timebase of 50 Hz. If you do this, then the minute hand completes a revolution in almost precisely the time the moon takes to complete it's orbital period (27.3 days). This means that the face can have the zodiac around it's edge, and the minute hand will then show where in the zodiac the moon is currently residing.
Note that this lunar 'orbital period' is not the same as the time taken for the moon to do a complete circuit of it's phases. This is because when the moon has returned to the same point on the zodiac, the sun has moved on about 30 degrees, and so the moon takes a little longer to catch up (the moon's phase is dependent on the relative position of both the sun and the moon).
It turns out that this 'catching up' effect is closely modeled by the same behavior of the minute and hour hands. Driven from a 50 Hz timebase, the hour hand completes a revolution in very roughly a year. It's only about 11% accurate, this being down to the 12:1 gearing of the clock fingers, when in fact there is a 13.38:1 ratio between the sun (earth) and moon orbital periods. Had this been more accurate, then the hour hand could have been used to indicate the sun's position in the zodiac. That would be nice. But it wasn't to be.
Despite the hour hand only roughly modeling the sun's motion through the zodiac, it turns out that, as a means of obtaining the moon phase, the moon's phase can still be indicated to an accuracy of better than 1 %! This is accomplished by attaching the 'Moon dial' graphic disc onto the hour hand. This disc rotates, thereby simulating the way the sun's movement lengthens the moon's phase period over it's orbital period.
Anyway, enough of these crazy meanderings! I'm off now to make a clock which uses a timebase with a division ratio of 3.36 x 10^12. This will create a clock which indicates which geological time epoch you are in!
It's time for my pills now.
