Working Micro Tellurium... Small Scale, Grand Design

Introduction

Just for fun, I decided to make a tiny tellurium, a mechanical model that shows how the Earth orbits the Sun and the Moon orbits the Earth. It's like an orrery, but stripped down to just the essentials: Sun, Earth, and Moon.

How tiny is it? The entire base is only 7 mm across and small enough to sit comfortably on top of a pencil (which, for comparison, has a diameter of about 7.5 mm). When you turn the decorated ring, the Earth begins its orbit around the Sun, and the Moon follows along in its own loop around the Earth.

The tellurium has a long history. Named after Tellus, the Roman goddess of the Earth, it first appeared in the early 18th century as an educational tool to illustrate the causes of day and night, the seasons, and lunar phases. Full-sized versions are often beautiful, intricate devices, and this is the smallest one I've ever seen.

In this Instructable, I’ll walk you through how you can make your own.

We are going to make the parts shown above. For that we need:

1. a pencil
2. a rubber ring, 5 mm in diameter with a thickness of 1mm.
3. a brass rod, 7mm in diameter
4. a 2.5 mm ball shaped earring (the Sun)
5. a piece of steel spring wire 0.35mm in diameter.
6. a steel ball 1.5mm in diameter (the Earth)
7. a steel ball 1 mm in diameter (the Moon)

I made this on my mini lathe, but I could probably have done it with my 3-in-1 printer that has a CNC function and a rotary module. You also need some super glue and a fine file. Optionally, I used my 3-in-1 printer with the 2 Watt Infrared laser module to engrave some decorations.

I think the video explains most of the work. So start with a 7mm brass rod and drill a 6mm wide, 3mm deep hole in it. The bottom isn't flat so later we'll cut it at 4.5mm height. You need to drill a 3mm hole that goes through the bottom of the ring. Later we'll make a pedestal that has a 3mm top, that should fit in this 3mm hole and can rotate freely.

If you have access to an IR laser, now is a good time to engrave the decorative markings as shown in the image above.

I used my CNC to cut a 3x2 mm slot in the side of the ring (half a mm from the top). If you don’t have a CNC, you can drill a 2 mm hole and carefully widen it with a small file.

Now is the time to cut it at 4.5mm from the top. Then use a ball endmill with a 0.1 mm radius to machine a groove along the full height of the ring, centered over the side hole. This groove will later hold the axle that supports both the Earth and the Moon.

The video emphasizes the axle, and while it's certainly important, it's actually a simple part: just a short length of 0.35 mm steel spring wire. (Earlier versions of the design used a more complex axle, but this streamlined version works just as well.)

The critical component in this step is the hub.

There’s a groove that runs around the hub, and its depth should be carefully adjusted so that the diameter at the bottom of the groove measures exactly 4.34 mm. This value is calculated based on the 0.35 mm wire thickness, multiplied by 12.37, the number of times the Moon orbits the Earth in a year. This ratio ensures accurate relative motion in your model.

At one end of the hub, drill a 0.7 mm diameter hole 2 or 3 mm deep. This will hold the pin of the Sun.

At the opposite end, cut an M2 thread, which will be used to secure the hub into the pedestal.

The final thing to do in this step is to cut the pin of the earring to 2 mm, add a bit of superglue and insert it into the hole in the hub.

As the video shows, making the pedestal is straightforward. The image gives the dimensions, but they don’t need to be exact. The central hole is for an M2 thread. One thing not shown in the video: I deliberately made the 3 mm section a bit too short at first. Then, after assembling the parts, I tested if the ring could rotate freely. If it couldn’t, I slightly reduced the height of the 4 mm section (thereby making the 3mm section higher) until it did.

Then take a pencil and flatten the end and glue the pedestal on top using some super glue.

Start with a 4 mm brass rod and drill a 0.4 mm hole, 1 mm deep, right at the center. Next, use a ball endmill with a 1.5 mm radius to make a shallow indentation - this will hold the Earth.

Shape the tip of the rod into an egg-like form using a file. The widest part of this shape, where the Earth will sit, should be 2.5 mm across. The full height of the egg shape should be 3 mm.

At the top of this shape, make another small indentation with a 1 mm ball endmill. The center of this one should lie exactly at the edge of the egg shape, so it only forms half an impression. This will be the spot for the Moon.

Once you're happy with the shape, cut it off the rod so the egg is about 1mm thick.

Now glue a short piece of 0.35 mm steel spring wire into the drilled hole; it should sit snugly and not go all the way through. Then glue a 1.5 mm steel ball (the Earth) into the central indentation, and a 1 mm steel ball (the Moon) into the one at the edge. Carefully remove any excess glue.

Cut the steel wire, so that 6-7 mm remains

The rubber ring is what links the hub to the Earth-Moon axle. I found it easiest to start by threading a piece of thin rope through the rubber ring before mounting it on the hub. Pull the rope through the hole in the ring, then press the hub into the ring and screw the hub onto the pedestal.

Now take the Earth-Moon part, with the axle pointing upward. Use the rope to gently pull the rubber ring through the hole and create space, then push the axle through the ring. Once it’s in place, remove the rope and twist the axle 180 degrees so the Earth and Moon end up on top. This twist is important: when viewed from above, the Earth orbits the Sun counterclockwise, and the Moon should orbit the Earth in the same direction. The axle should sit neatly in the groove.

When you turn the outer ring, you might notice the Earth-Moon system slowly rising. If that happens, undo the twist and redo it in the opposite direction. This will make it try to move downward instead, but the brass Earth-Moon part will stop it from going any further.

Congratulations, you’re now the proud owner of a working replica of what might be the smallest tellurium in the world.