Everything You Wanted To Know About Moon Dials

Including How to Set Them

History and Use

Before we tackle how to set the moon dial it might help if you are aware of its purpose, tradition and history.  After reading the wealth of information below you will be able to impress your friends and neighbors with how knowledgeable you are when they ask you what that thing is on the top of your clock.  If you are not interested in this information you can skip down to “INSTRUCTIONS FOR SETTING” below.

The moon dial or lunar phase dial is not of any practical use to most of us in our modern times.  Its place on current production grandfather clocks is more out of a sense of tradition than anything else.  From a manufacturer’s standpoint it is not much more than a talking point when selling a clock.  In other words, it is another feature like raised brass numerals, beveled glass or a cable driven movement that enhances a clock’s value.

Mechanically speaking, the moon dial exists in conjunction with timekeeping devices (grandfather clocks most frequently) simply because the passing of the moon is easily  relatable mathematically and mechanically to the passing of time; minutes, hours and days.  Our concept of dividing time is based on the length of time it takes for the earth to travel around the sun.  The month, week, day, hour and minute are simply smaller and smaller divisions of that amount of time.  The work of most clocks is to simply keep track of the hour and minute.  Others add a second hand and still others will tell us the day of the month.  Those that do tell us the day of the month rarely take into account the differing lengths of our months (some 31 days, some 30, one 28 which is sometimes 29) and have to be reset manually most months to compensate for this. 

If you want a clock that does compensate for the inconsistent number of days in our months, they are available.  These clocks had what were called “perpetual calendars” and were very complex.  An example is pictured below on the left.  The top dial tells us the time of course and the lower dial (close up below right) is the perpetual calendar.  This complexity was necessary to account for the different length of our months and also, believe it or not, to account for leap year when every fourth year, February has 29 days.  If a perpetual calendar on your clock is not enough for you they get more complex still.  There are some time pieces called Astronomical Clocks that were designed to show us the movement of all the planets in relationship to the sun.  This takes us back to our original subject; since it is possible for a clock to give us all this information why not have the clock also tell us what the moon is doing? 

You might be thinking, “Who cares, why bother?”  First of all, it is a fairly easy procedure to add this function to a clock, even easier than designing a clock to tell us the day of the month.  The lunar month is always the same length, 29 ½ days, whereas the calendar month, as mentioned earlier, is not always the same.  Secondly, in times past the moon dial did provide needed information.  There was a time when knowing the phase of the moon was very important, especially to farmers.  Before the industrial revolution when almost all economies were agriculturally based big farmers were some of the wealthiest of individuals and able to afford clocks which were very expensive at the time.  The farmer knew that when the moon was full or nearly full there would be more light in the evening to allow working the fields later than normal.  Also there was a particular tradition based on the movement of the moon around the earth and its effect on the earth about when to plant certain crops.  Crops that grew above ground like corn and wheat were to be planted in the light of the moon where as crops that grew below ground like potatoes and carrots were to be planted in the dark of the moon.  In summation, farmers could afford clocks and they wanted to know what the moon was doing.  Back in those days you did not go to the local clock retail store and buy a clock, you went to the local clockmaker and paid him to make one for you.  It is reasonable to assume that a farmer, knowing it was possible, might ask the clockmaker to be sure to put a moon dial on his clock.  It is in looking back on the history of the moon dial that we find the purpose of its presence on our modern clocks; that purpose is tradition.

EXAMINING THE MOON DIAL

It is helpful to know what you are looking at when viewing a moon dial.  The first things we typically notice are the pictures of the moon on the dial itself.  First of all there are two moons (see pictures at bottom of this page).  This is necessary because the dial only rotates half the way around it’s circumference to simulate one set of moon phases.  When one moon disappears on the right side on day 29 ½ there has to be the other moon ready to pop up on day one on the left side of the scale.  The moon dial has other pictures on it as well.  Traditionally there are two; a seascape and a landscape.  These pictures simply give us something to look at when the moon is either just rising or almost disappearing.   Some people mistakenly think the pictures are predicting the weather.  I had a customer tell me this one day and I said, “What?”  She said, “Well, when the picture of the sea is visible that means it is going to rain!”  I said, “If that is true then it is going to be raining for quite a few days as that picture will be visible for a couple weeks!”   Other people think that the moon is really a sun and that the “sun” should be visible during the day and disappear at night.  This is wrong as well.  The picture of the moon only advances a small distance each day, it does not rotate constantly.  Remember that a moon month takes 29 ½ days, therefore it takes the moon picture the same length of time to travel across the simulated “sky” of our moon dial.

Next thing we notice is the scale along the top with the numbers on it.  Usually we see a 5, 10, 15, 20, 25 and 29 ½.  These numbers are not calendar dates as is often supposed but are lunar dates instead.  What is the difference?  The difference is that lunar dates and calendar dates do not coincide except by happenstance.  Occasionally they do fall directly in line with each other but not for long as the length of our calendar months vary from month to month.    The numbers on the scale allow us to tell the lunar date.  On most modern floor clocks we do this by drawing an imaginary line from the supporting nut, bolt or rivet at the dial’s center, through the center of the moon to point at the scale.  In the picture above, which is from a very old bell striking tall case clock, you can see a small “pointer” on the top of the moons “head”.  This pointer points at the lunar date on the scale above.

The last thing to examine are the two half circles that look like portions of the globe.  These are called hemispheres.  More than anything else these are here to hide portions of the moon on the rotating dial in order to give us the crescent effect.  Usually these hemispheres will picture Europe and Asia on one side and the Americas on the other. 

INSTRUCTIONS FOR SETTING

Simple Answer

The easiest way is to simply go outside on a clear night and look at the moon.  The best time is when the moon is full.  Have you ever been driving home at night when someone in the car says, “Look, it’s a full moon tonight”?  Next time you hear someone say it or you notice it yourself, run in the house and set your dial so that the moon is in the center and completely visible.

Slightly More Complicated (but still easy) Answer

The Weather page of almost any newspaper will give you moon phase information.  In our paper it looks like the picture below.

Using the above picture for an example let us say that today is February 25.  Moving backwards in time from the 25th we look for the date on the moon table (shown above) that most recently passed.  The table tells us that on February 20th the moon was full.  With this information the first thing we want to do is maneuver the moon dial on the clock into the full moon position.  (See pictures below for examples of what the four different phases will look like on your clock.)  The full moon on our clock’s moon dial now represents where the moon was on the 20th.  Since in our example today is the 25th which  was five days ago, we must move the moon dial forward or clockwise 5 days to put us where we should be on the 25th.  To do this we simply observe the scale above the moon picture counting each space as we go.  Most modern clocks will make a clicking sound as you advance and you can count that as well.  However, some moon dials advance every twelve hours instead of every twenty-four.  In this case, each two clicks you hear signify one day not two!   In addition, some of the newest moon dials have no index spring at all and therefore you will hear no clicking sound.  Because of these two exceptions it is best to use the lunar scale spaces as your indicator.  In this example, using the information in our newspaper, we ignore the numbers on the scale as these are lunar dates, not calendar dates.  Only use the numbers if you happen to know what today’s lunar date is.

You follow this same procedure no matter what the date is.  Once again, using the same picture from my newspaper, if today was February 10th, we can see that the moon was in the new moon phase four days previously.  So we would set the moon dial to show the new moon phase (See pictures above for moon phase examples) and then move forward four days from there; from the 6th when the moon was new to the 10th, today’s current day (in our example).

Another way to do this is to use a common household wall calendar.  On each month’s page of some calendars you will notice pictures of four different little moons, one picture of each of the four different phases.  Sometimes these moons appear together at the top or bottom of the page (with little numbers below or beside them) and other times they appear on specific dates of the calendar.  If there are individual moons on specific days the calendar is telling you that on that date, this is what the moon will look like.  If they are grouped together (much like what would appear in a newspaper) the number below or beside them is telling you what day of the month that particular moon phase will fall on.  With this information you can set the moon dial just like I previously described.

And still one more way to set a moon dial is to start with the lunar date.  If you have the lunar date that corresponds to today’s date you will then be able to use the numbered lunar scale on the arch above the moon pictures.  A Farmer’s Almanac will tell you this information or you could search the web for a lunar calendar converter.

MOVING THE MOON DIAL

Now we have the question, “How do I actually move or advance the dial”.  In most cases you can simply touch the exposed part of the dial with your finger and with a little pressure you are able to move the dial forward.  If you want to avoid touching the dial directly with your finger (especially if the dial is brass) you can use something else to touch the dial instead of your finger.  Ideally something that is not slick like perhaps a pencil eraser or one of those rubber finger tip “thingies” people use when they are flipping through many pages of paper.  Another way is to remove the side panel from your clock, if you have one, or remove the hood if it is removable, and advance the dial from the rear by simply putting your finger on the dial teeth and rotating it forward.  Use the picture above as a reference.  From this position you do not have to touch the portion of the dial that is visible from the front so there is no fear of mussing the dial up or causing it to tarnish.

Sometimes when you attempt to move the moon dial it will appear stuck.  Usually when this occurs it means the clock is in the process of advancing the dial itself.  In this case it is important not to force it.  You will either need to move the hands several hours or wait several hours until the advancing pin is clear of the dial teeth.

In closing it is necessary to be aware of the index spring that lies up against the side of the moon dial teeth.  From looking at the dial examples in the pictures above and below you can see the moon dial looks like a circular saw blade.  The index spring lies against these teeth and helps to control how far the dial will advance.  Sometimes when setting the dial the moon dial will get pushed back in a way that enables the index spring to get wedged between the clock dial and the moon dial itself.  When this occurs you will no longer hear the clicking sound as you advance it and you will feel a significant drag or scraping sound.  When this occurs you need to flex the index spring back into proper position so that it lies against the teeth of the moon dial.    This index spring is visible in the picture above.  It is the brass piece just to the right of the lower half of the actual moon dial itself.