Ferguson’s Orrery gives a lot of information about the earth, moon, season, and eclipses but nothing of the rest of our Solar System. The Compernican Planetarium sort of rounds out the picture by showing how the rest (out to Saturn at least) of the planets orbit the sun.
Since the orbit time vary so greatly the challenge is to cut gears to satisfy Saturn with its very long orbit and also be able to have Mercury demonstrate its orbit on the same system.
Reduced to the mechanism’s simplest concept there are two arbors or shafts in the mechanism. The driving shaft is powered by a hand crank and all six gears on the driving shaft are turned in unison. The planets are all driven on the second shaft and each planet is free to rotate independently. Through their specific gear ratios each celestial body rotates around the sun in correct relationship to the other celestial bodies.
A special thanks to Clayton for another great project - you can purchase plans at
The design overview shows the stack of gears and the six planets that are being model in the Orrery.
The calendar dial is printed on four sheet that are joined on a light table. I try and make each sheet have a section of the centering hole, in that way the joins are radial.
All the gear data is transfered to a CNC program, which in my case is Mach III which ultimately sends the message to the CNC
The first step is to cut out the “spokes” of the wheel. This is done from a dxf file and is a separate process from cutting the gear teeth.
The wheels for this model are all 1/2” Baltic Birch ply. The spoke design is only created once and then scaled for every wheel.
The teeth are cut after the spokes. Both patterns are aligned by the center pilot so that the teeth can be centered on the spoke pattern.
I thought I should start with the “hardest” wheel first and then the rest should go easily by comparison. The Saturn wheel with 147 teeth looked to be then hardest.
The spokes were easily cut with 1/8” cutter and the teeth with a 1/16” cutter and the wheel is 1/2” thick
Moving to the Jupiter wheel with its 83 teeth had a lot more room to use a 3/32” cutter
Mars spokes were cut with 1/8” cutter and the spokes are noticeably thinner. I will have to watch for strength.
The earth wheel is driven at a 1:1 ratio
Venus is “sped up” which means its particular driving wheel will have more teeth than this driven planet one
The largest “speed up” ratio is Mercury with its 40 day cycle around the sun.
Here are all six “Driven” planet wheels from Mercury, Venus, Earth, Mars, Jupiter, and Saturn. These wheels will all be on the same arbor. Next is the driving wheels.
The Mercury Drive wheel is a dual wheel with the wheel driving Mercury and it is also a crown wheel that is driven by the mechanism’s crank.
The Venus pair on the depthing guage. The gears all run on two arbors so I pre-set the depthing guage and test all paris on the same setting.
The Earth set is a one to one so the gears are identical
Mars starts the reduction gearing sets
Jupiter is the last gear set.
Saturn is driven by a 5 leaf lantern pinion. Its Saturn that test the depthing since the 147 teeth are very fine and very close to the drive arbor.
This side view shows the stack - and they all turn easily with no binding. Things will be even better with the proper spacers installed.
Time to start working on the frame. The frame is built from 3/4” melamine covered MDF. The unit rests on 5 feet that are glued to the bottom
The top is slightly larger than the base and is made of the same material. I drilled two hole at the back by mistake and they get filled.
Although the original design had a solid top - I couldn’t resist duplicating the spoke design, which will allow a better view of the gears. The drive arbor hole is in one of the spokes.
Theses little guys attach to the arbors coming from the planet driven gears and will house the support wires for the little planets
This is the drive pinion for the crown pinion set up that “powers” the Planetarium. The crown is built into the Mercury drive wheel.
3/16” dowels are the pinion leaves
The crank has two 1/8” washers to ensure clearance from the frame.
One inch dowels make the spreader part of the frame at the front and a solid piece at the back so that the crank can be installed
The mechanism can be take apart by removing four screws on the bottom and separating the top and bottom
To insure free movement spacers of different thicknesses are put between the gears
The Mercury drive is also the crown and 1/4” dowels are driven into 36 holes around this gear.
The dowels are chamfered or “eased” slightly on both sides so that the model will turn freely in both directions.
This is the first time the gear set is mounted in its frame.
A trial fit shows that everything works
As expected the most difficult match is on the Saturn gear - and although it would not work in clock works - it functions fine here.
The Sun and all the planets are fashioned out of FINA which is an artists modeling clay that is baked hard in the oven.
The suspension wires are formed and push easily in the unbaked modeling clay. they are mounted on a board ready for the oven.
Assembly of the final model is now complete and the planets are in their correct relative position.
This site shows the relative planetary position for any date and location on earth
Once again thanks to Clayton Boyer for putting the plans together for this
Ole Romer, instrument builder to King Christian V of Denmark, this is Romer’s Copernican ceiling planetarium 1735 using Johannes Kepler’s elliptical orbits of 1605
Each "driven" gear is on a differnt diameter brass tube such that each succeeding gear turns a smaller and smaller brass tube inside the previous one.