I chose a ten planet model, incorporating Pluto and Eris, with a modified scale (see step 1). Distances, for reasons that you know doubt understand, are not to scale. My model is not mechanical, although the planets can be rotated individually.
For more information and plans to build a true mechanical orrery, check out Clayton Boyer's beautiful designs here.
Edit: Thank you to everyone who voted for me in the Celestron Space Contest! As you might be able to guess, this is still a work in progress. I will post the final pictures after it is completely finished. Thanks!
Step 1: Calculations
I set Jupiter's size at 7 inches (diameter) and scaled the other planets relative to it. Be aware that seven inches is a lot bigger than it sounds and a lot heavier than you think (Jupiter weights ~3 lbs). Many of my later design constraints came from Jupiter and Saturn's weight.
The terrestrial planets and earth's moon are magnified by a factor of 5 in order for them to be visible (In the photo below, Pluto at ~0.6 inches in diameter is approximately the correct size for Earth without the 5x magnification). The sun is essentially not to scale, for reasons that are unlikely to become clear at the moment.
The calculations below do not include Eris. Reports vary as to whether it is in fact significantly larger than Pluto or only slightly larger. I chose the smaller value for the radius as it is the most recent. All the data is from Nasa. The scale was calculated in excel and modeled in photoshop.
Step 2: A Note Regarding Dwarf Planets
Step 3: Part 1: The Planets - Stuff You Need
Wood for the spheres: For several different colors I started out with 4-5 feet each of red oak, mahogany, walnut, hickory, and maple.
Note: I did not use all of this for building the spheres, but I used all of it and more in building the orrery structure.
Wood for the patterns: I used plywood. Cardboard or mat board can also be used.
Copper for Saturn's rings
Brass Rod: 1/8" thick for Saturn's rings
Wood Glue (I used Titebond)
Epoxy and inlay materials (optional)
Dremel with sanding and carving attachments
Wood Burner (optional)
*Not all of this equipment is absolutely necessary. Chisels, a hammer, and indefinite patience can be substituted for a band saw. A dremel is incredibly useful for touching up the sanding process, but it is not necessary. A lathe, and the ability to use it, would substitute almost all of the equipment listed above.
Step 4: Gluing the Blocks
Thin layers cannot be glued width-wise as they will bow in the clamps. Glue thin layers onto thicker layers, rinse and repeat. Wait at least 30 minutes between gluing layers, and at least 24 hours before doing any work on the block.
Step 5: Cutting the pattern
The stronger the pattern material, the less likely you are to break it as you are trying to fit the planet through it.
Step 6: Cutting the Blocks
Make sure that you are cutting with a flat surface on the platform. Under no circumstances should you try to cut a curved surface on the band saw. It is possible, and it will save a great deal of time, but it could also cause the blade to bind, screwing up your piece and possibly your hand and definitely your blade.
Step 7: Shaping the spheres
Begin on the corners. Use your judgment until the block is pretty much spherical. Then start using the pattern, trying to fit the circle over the sphere and marking where the block is too big. Sand, repeat. Using a 40 grit sand paper will make this process go by much easier.
After the block fits the pattern, it is time to use the dremel. Feel where the block is still angular and use the sanding attachment to smooth it out.
In general the smaller the planet, the easier it is to shape. Plan on spending a lot of time on any sphere over 5 inches in diameter.
Step 8: Adding Something Extra - Venus, Earth, Uranus, and Neptune
For more detail on how to inlay, check out supersoftdrink's excellent instructable here.
Step 9: Saturn's Rings
Note: Saturn's rings are not to scale
Step 10: Finishing the Planets
For a finish I used a mixture of 1/2 tung oil 100% pure and 1/2 mineral spirits. Mixing them together in a mason jar, I applied it to the planet, rubbing it in with my hands. Then I wiped it off with a rag and waited for it to dry. Then I rubbed the planet with a piece of 0000 steel wool, just enough to roughen the surface. Then I wiped away the little woolly bits and repeated the process. Tung oil, rag, dry, steel wool, tung oil, rag for seven coats.
Step 11: Part 2: The Structure - Stuff you need
~14" x 4" x 3/4" piece of hardwood (I used mahogany)
1/4" aluminum rods
For the structure:
1 hollow steel pipe (7/8" diameter, 6' long)
Wood for the rotating blocks (I used white oak)
Wood for the counterweights (I used white and red oak)
Wood for the base (I used Walnut)
Additional Weights (I used lead shot, available at any sporting good store, and concrete to fill in the gaps)
3 2.5 lb bar weights (for saturn and Jupiter's counter-weights)
8 (1/2" by 1/2"), 2 (0.75" x 0.75") brass elbow joints (available in the plumbing section of any hardware store)
1/2" wooden dowel for the elbow joint plugs
7/8" wooden dowel for the lamp, counter-weight plugs
0.75" wooden dowel for the large elbow joint plugs
Several each of:
1/4" diameter steel rod (used for horizontal rods)
3/16" diameter steel rod (used for vertical rods)
0.5" hollow diameter steel rod (used for horizontal rods for the heavier planets)
1/8" diameter brass rod (used for vertical rods and pins)*
*A brass vertical rod indicates that the planet has been magnified by a factor of 5, while a steel vertical rod indicates it has not been magnified.
Drill Press (with various size bits, including Forstner bits)
Dremel with metal cutting wheel
Step 12: Rotating Pins
Every metal rod had to be sanded to fit. Sanding also transforms a grungy metal rod into a shiny metal rod. I used a power sander at 100 to 150 grit for sanding metal and a belt sander (80 grit) to sharpen the tips. For these pins I did not sand one end to help prevent the pins coming loose and falling into the hollow rod. Each pin is slightly less than 1.5" long.
Step 13: Rotating Blocks
Step 14: Elbow Joints
Step 15: Adding horizontal and vertical rods
For obvious reasons, distances are not at all, in any way, to scale. I tried to conserve horizontal distance as much as possible and chose the length of the vertical rods based on esthetics.
Each rod was cut using a dremel and sanded down with a belt and power sander.
Step 16: Earth's Moon
For simplicity's sake, Earth's moon is the only moon I included in this system.
Step 17: The Counterweights
The counterweights are wooden blocks drilled and weighted as necessary to compensate for the weight of the planet and metal bars. I did this by trial and error rather than calculation - if anyone could instruct me on how I could have calculated the appropriate weight to compensate for the weight and distance of the planet, I would be much obliged.
For most of the counterweights I was able to drill 7/8" holes with a forstner bit 3-5 inches deep and fill the holes with lead and concrete to achieve a sufficient weight. The size of the counterweight block depended entirely on how heavy it needed to be, with a self imposed limit of 9" from the bar to the end of the counterweight. I used 5/16" metal bars to connect the counterweight to the rotating block (for a few weights the metal bar was sufficient to add weight). Please let me know if you are interested in more exact dimensions and weights.
After the weights were put in, I plugged the holes with an appropriately sized wooden dowel and epoxy and sanded the surfaces smooth.
Jupiter and Saturn, however, needed something extra...
Step 18: Counterweighting Jupiter and Saturn
First I prepared wooden boards (3/4" x 5" x 7") out of red oak. I needed 3 for Jupiter and 2 for Saturn. I marked the size of the weight and used the drill press (Forstner bit, to the rescue!) to create a recessed area for the weight. VERY CAREFULLY I used chisels to finish hollowing out a recessed area (I lost more than one board by cracking it with a chisel).
Because Jupiter's block houses 2 weights, I also used a scroll saw to cut out a hole to house the weight. Jupiter now has 3 boards: 1 with a recess and 1 with a hole and 1 untouched. Saturn has two boards, both with recesses.
Time to glue! I put the weights in and added lead shot and concrete in the empty areas to maximize the weight. Then I glued the boards together with wood glue and clamps.
For all of the blocks, after gluing I used a table saw to square the blocks before finishing them.
Step 19: The Base
The design of the base comes from a lamp my mom designed. She also did most of the leg work in squaring, cutting, gluing the blocks for my base, as well as preparing the mortise and tenon and lap joints. Props to moms everywhere, and especially mine!
As you can see from the pictures above, the base consists of two crossed wooden blocks and four upright blocks. The upright blocks are connected and glued via mortise and tenon joints into the horizontal crossed blocks. Two walnut blocks were cut and glued to approximately 3" x 3" x 20". Using a dado blade on a table saw we created a lap joint to form a cross.
The upright pieces are connected to the base by mortise and tenon joints. Using a drill press, we drilled as much of the mortise as we could and used chisels to fine tune the fit. Using the table saw we created the tenon on each upright. After creating the tenon, we used a band saw to prettify the uprights.
It just so happened that the 6' steel pole I bought was threaded on both ends. My mom found a threaded metal bar that my pole could screw into. Using a drill press we drilled a hole for the pole, and inserted the threaded metal bar, gluing with epoxy.
Sand, glue, finish.
Step 20: Part 4 - The Sun
The final sun design will use a lap joint. See step 20 for more instructions.
Step 21: Finishing Touches
I sanded all the metal rods with 220 grit and finished the base and blocks using the same method described in step 10.