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Sun and Moon rise/set dial and development of second-order Moon anomaly function - November 2016

Representing accurately the motion of the Moon as it appears in the sky in a mechanical fashion upon a dial is a very difficult effort.  The combined gravitational influence of the Sun and Earth cause many perturbations. Together these are known as anomalies. In practice there are five major anomalies associated with the Moon's orbital movement (see illustrations below). And there are two anomolies associated with the Earth's tilt and orbit. In theory there are dozens but the main seven will account for 98+% of these. The five anomalies are arranged in their order of the greatest to least orbital perturbations. The degree is the change in measurement from an idealized orbit. These five corrections will give an accurate time of Moon rise and set.


1. Great Anomaly: This is the effect of the Moon’s elliptical orbit around the Earth and has a ±.6.58° equaling 26.322 minutes effect every anomalistic month which is defined as the time between the Moon's successive perigees and is approximately 27.55 days.


2. Evection: This is the change in the Moon's ecliptic longitude: This is caused by the gravitational pull of the Sun and Earth which causes the Moon to accelerate as it moves toward and decelerate as it moves away from the Sun. The period is 31.81 days. This is ± 1.274° equaling 5.097 minutes.


3. Variation: The combined effect of the Sun and Earth on the Moon's orbit  at lunar conjunction (when the Earth, Moon and Sun, in that order, are in alignment) and at lunar opposition (when the Moon, Earth and Sun, in that order, are in alignment). The Variation is ±0.658° equaling 2.632 minutes and has a period of half a synodic month or 14.77 days commonly known as a lunar month which is 29.531 days.


4. Annual Equation: This is ±0.186° equaling 0.856 minutes and has the period of one anomalistic year or 365.26 days. It is the combined influence of the Sun and Earth on the Moon owing to the Earth’s elliptical orbit. 


5. Reduction: This is ±0.214° equaling 0.8569 minutes and has a period of one-half the anomalistic month or 13.77 days and is due to the tilt of the Moon's orbit of 5.8° to the ecliptic.

However to truly show not only when the Moon will rise and set  but where the Moon will rise and set, two additional corrections are needed, these are associated with the Earth and its orbit, and to a very minor extent the Sun itself.


6. Projection: Two factors are needed to account for the Earth's 23.5° tilt from the ecliptic as well as it's elliptical orbit around the Sun. These factors are the same as those needed to compute the equation of time. The projection is ±2.464° which translates into about ±9.857 minutes in time. It has a period of one-half the tropical month or 13.661 days.


7. Solar Equation: This encompasses a further two very minor anomalies associated with the Sun

Very few mechanical clocks have incorporated all nine corrections for these anomalies to allow for an accurate representation of the movement of the Moon. A few examples are Jean-Baptiste Schwilgué in his famous astronomical cathedral clock in Strasbourg, France, 1838-1843; Jens Olsen, Copenhagen, Denmark 1945-1955; Rasmus Sørnes, Moss, Norway, 1958-1966; Hans Lang, Essen, Germany, 1982-1986; Festo Corporation, Esslingen, Germany, 1995-2001 (this movement employs ten corrections). There are surely many others I have overlooked, but one interesting item that pops out is the fact that other than Schwigué's all are of recent design and fabrication; all less than 62 years old. This is probably due to the complexity of calculations and fabrication needed to make these corrections.

  To help with these calculations is a very useful on-line tool shown by the screen shot to the left. This not only allows one to correctly calculate a desired gear ratio to extraordinary accuracy but also lets the creator 'play around' with various gear sets, numbers of wheels and sizes. This is particularly useful for our purposes when the most efficient result is not always what we are looking for, but perhaps a more visually pleasing set of wheels that gets us the same result. The calculator can be found here:  



These four illustrations depict the Moon's orbit around the Earth for one, five, ten and sixty years respectively. The last illustration shows a rather thick doughnut around the Earth. One can readily see why it is so difficult to accurately describe the Moon's orbit!

We will correct only for the three greatest anomalies. These are the Great Anomaly at ±.6.58° equaling 26.322 minutes and the Projection (which contains two corrections) at ±2.464° which translates into about ±9.857 minutes in time. So the total maximum error involved is 36.179 minutes. All of the remaining anomalies amount to a combined 9.442 minutes. It is no accident that the dials on the Schwilgué and Festo clocks are large to take advantage of this information. The combined total of all anomalies comes to ±45.621 minutes from a simple rolling moon dial. Our Moon rise and set dial is fairly small at just over 3½" or 10cm in diameter so any further corrections would be barely noticeable at this scale. Even thirty six minutes will be hard to discern. This is really more of an exercise to incorporate a classic complication rather than important additional accuracy in the dial.



These four photos show the first concrete example of what Buchanan is proposing for the Moon corrections. They show an alternative way of applying a differential gear system and are based on Antide Janvier's method to simulate the erratic motions of celestial bodies and that were later employed by Jean-Baptiste Schwilgué. I have always been fascinated by this variable differential gear arrangement and wanted to have it incorporated into the panoply of mechanical contrivances within the machine.

The first two photos show the slant wheel in its two extreme positions. In this model Buchanan is mimicking the Great Anomaly in connection with the 23.50 tilt of the earth’s axis. The next two photos show the same wheel set with a pair of dials denoted in degrees mounted to the slant wheel as well as the input wheel. In this way one can see how the slant wheel moves in relation to the input wheel. Since we did a Breguet style differential for the equation work I suggested we use this design for the Moon’s motion.


That relationship as represented by the Great Anomaly is plotted on two graphs. The first one above shows the differential changes as a change in radial degrees over one complete rotation. It plots the difference between the pointer on the drive wheel which turns at a constant rate and the readout of the slant wheel, Buchanan calls this the “error from the true pointer”. That true pointer is the one mounted to the drive wheel as seen in the fourth photo above of the differential mockup.

The second graph is the same information, but plotted as the mathematical function. This is not unlike the graphs one would get plotting the equation of time which is the difference between the mean solar time, or the time we read off our clocks, and the position of the Sun in the sky. The physical cam used to represent this function in a clock has a similar kidney shape as well as the graphical illustration that looks like a sine wave for the mathematical function. This similarity to the equation of time function is no accident as the Earth’s tilt is one of the two components used to determine the equation of time, the second is the eccentricity of its orbit, which is also contained within the fourth anomaly known as the Great Anomaly, 2nd term.

Illustrated above is the initial design layout for the Moon complication. Here we have made manifest the two largest anomalies represented by the two differential wheel cages 3 and 4. These represent the First or Great Anomaly and the second the Moon’s evection, the acceleration and deceleration as it approaches and move away from the Sun. Note that the angle in the first differential is in error at 9.29° but should be 6.29°.

The interesting thing in Deryck’s design is that he takes Janvier’s slant wheel differential one step further. In Janvier’s application the areas represented by each square box remains stationary, that is the wheels ‘G’ and ‘L’ rotate and move from left to right, but the entire structure including the rest of the wheel works rotate on their axis but do not otherwise move. In this design, the boxes should be seen as rotating cages, not unlike a tourbillion and each rotate around arbors ‘J’ and ‘T’. The first differential represented by box ‘4’ supplies its output to the next differential, box ‘3’. These all rotate along with the Moon once every 29.53 days. Each cage will be approximately 3 inches or 7.5 cm in diameter. I did discover later that Schwigué did use this rotating cage design. As the saying goes “There’s nothing new under the sun”, see photo below. Although we have employed a few novel mechanical ideas in other areas of the clock.

This is a photo of the drive to the Moon on Schwilgué's astronomical cathedral clock in Strasbourg. The two silver horizontal structures in the back and foreground is his rotating cage. Within the cage is one slant wheel differential combined with a second pair of interactive differentials in the middle portion of the device. Note the sector gear just to the left of the center nested differentials just before the last two large output wheels. The gears are being driven from right to left. Just this one mechanical unit displays a constellation of complex and visually stunning mechanical achievements a tour de force in wheel work design. We owe much to the masters of the past.


Here the complex mathematical formulas are depicted.

This drawing shows one of the two anomaly correction, slant wheel differentials. The two slightly lenticular, biconvex circles on the front elevation section of the drawing, left, are showing the two wheels that are slanted in relation to that elevation and the rest of the wheel work.

In this photo are shown the areas where the wheel assemblies will be located. The twin anomaly differentials are anticipated to each be just over 3” in diameter and 2 ¼” long (7.5 cm, 6 cm). The pair with their associated drive gearing will be 5” (12.5 cm) wide. This will nicely balance the density seen in the calendar mechanism on the left hand side of the machine.


The first drawing depicts the gear trains for the moon’s phases as well as its cycling around the dial. Next is an overlay with paper disks that depict the wheel train controlling the sun rise and set shutters.


Here we have a completed drawing with both the moon and sun gearing represented respectively drive wheels all revolve around the dial's axis according to the input of the two anomaly differentials. Their input is the first small red wheel at 12 o’clock (1). That wheel is fixed and moves the second large red wheel (2), upon which is mounted a nested pair of wheels (3, 4), which are fixed together. The largest of the pair (3), meshes with the center wheel (5), which is fixed. The smaller of the pair (4), meshes with wheel (6) and then to (7) and (8). Wheel (8) delivers rotation to the Moon globe through a pair of bevel wheels.

The rotation of the input wheel feed from the anomaly differentials, by rotating the large red wheel upon which the rest of the moon mechanism is mounted induces rotation of the nested pair of wheels engaged with the fixed center wheel; thus causing rotation throughout the rest of the wheel train through to the moon globe.

These gears are represented by the calculator shown above for a four stage wheel set to obtain the accuracy of the moon’s rotation to seven decimal places. The other thing the calculator does besides giving extraordinary accurate results is that it allows Buchanan to perform a trial and error exercise with various numbers and tooth counts almost instantly. Thus he could try out a five stage to see how it might look vs. a four but still retaining the correct results. It not only saves time but allows for a much better design both mathematically as well as esthetically.

The sun itself revolves around the dial once per day and is mounted to the center green wheel (a). That wheel drives the next four wheels (b, c, d, e) which then turn a pair of cams (f, f’). These cams each in turn have roller follower arms that rotate upon their edges and are attached to set of sector gears. Those in turn control the two shutters for the sun rise and set horizon. Only one set of sector gears is shown as they are superimposed upon each other in this view.

Front and side elevation of the Sun and Moon rise/set dial 

This is how the pair of differentials will fill out behind the dial work. They are drawn to the same scale as the dial work, These will reach almost to the first strike hammer mechanism as shown by the diagramed box in a prior photo above.

Buchanan sent the drawing I requested to see how far the slant wheels as designed would project beyond the dials. In this front elevation the anomaly differential is about 2” in diameter. We later decided that it would be esthetically more advantageous to expand that wheel to 3” (7.75 cm). This will allow more space to be taken up by this device, yes there still is a bit of limited real estate left in this machine! But more importantly, the larger wheels combined with a delicately thin cage design will make the entire pair of differential wheel presentation look more delicate and appear to ‘float’ as they rotate within their turning cages.


A schematic showing the full assemblage for the Sun and Moon rise/set dial complication. Note how close the actual accuracy of each system comes to the ideal target.

The proposed design for the Sun and Moon rise/set dial. The following information and thus complications can be read from this dial and thus add 16 complications to the astronomical clock:

bullet1. time of sunset
bullet2. time of sunrise
bullet3. visual position of the sun in the sky
bullet4.  visual indication of the phase of the moon
bullet5. age of the moon.
bullet6. degrees of ascension of the moon (Height in the sky)
bullet7. degrees to moon set.
bullet8. hours until moon rise   (we have a double hour scale on the rotating degree scale but, stretched slightly compared to a real hour dial), the moon rotates in the dial in 24 hours and 55 minutes) Zero hours is at the moon and the hours count away from the moon, so, the hour on east the horizon marker gives you: hours until moon rise
bullet9. hours since moon rise
bullet10. hours until moon set
bullet11. hours since moon set
bullet12. length of day
bullet13. length of night
bullet14. visual position of the moon in the sky
bullet15. age of the Great Anomaly
bullet16. age of the Tropical month.

We also have the time of the year on the cam pack setting dial the same as the equation of time dial.

I say "Pretty nifty work!" Again, a well done to Buchanan. For those of you that have tried their hand at clock making you are the people who can really appreciate what is being accomplished here.

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