It is a mechanical device that show the sun and the planets and it shows how the planets revolve around the sun. You probably have seen one but did you know what it is called?

I knew about these things but had no idea they had a name.

I talked to a couple of friends about them. "You know, it is a mechanical device where the planets revolve around the sun". Sure they said. They had seen them.

Well, it was much to my delight to discover a new word. "Orrery".

And this took me down a beautiful rabbit hole because there are more words and names depending on what it actually displays.

For example there is something called a tellurion which shows just the sun, earth and moon. And it is used to show the location and phases of the moon along with eclipses.

And technically that is what I made in this project: A tellurion.

The tutorial on how to make this project is on my website right here

This project taught me a lesson in not making assumptions without testing them.
I started out assuming I couldn't use foam board to make gears. And after testing a lot of other stuff I eventually made the foam board gears.... and they worked reasonably well.

SO... how did I get to this finished project starting from nothing?

I looked up various gear mechanisms on the web and found all kinds of interesting things. But... a basic assumption I made was that if I wanted gears I would have to make them out of metal or out of wood.

I didn't like that at all. I couldn't ask my web visitors to make gears.

Stage 1 was rubber bands. I made a box and put two dowels in it. I ran a rubber band from one dowel to the other and attempted to turn one of the dowels. No luck at all with that. No matter how I adjusted things I couldn't get any turning motion. The pulling pressure of the rubber band couldn't be overcome by the turning. The band would slip.

Then I tried pulleys with strings but that didn't work either. I once again ran into the problem of tightness versus the limitations of foam board. The string would dig or slip before turning.

So... I finally reverted back to the original idea I dismissed. Gears cut out of foam board.

And it worked reasonably well.

From here it was a matter of working out the gears. How would I do this? I needed some kind of system of gears that would give me a rotation then a rotation within that rotation.

I did a whole lot of sketches trying to figure out the gear system but I couldn't get anything I was confident in - particularly because of the limitations of foam board.

So... I took a different approach: Build it on the fly. I would just start with one planetary object, get it to work then add another object. Until I had them all.

And that's what I did.

I started first with a drive gear as a hand crank. This powers a second gear that the sun turns on.

Yup! Works great!

Sun is definitely too big though, or the orrery is too small. Easy enough to fix that with the final version.

Ok, step one is done. Now to simply move on to the next step. How to add the Earth revolving around it.

I tinkered around with this a lot. And made a lot of sketches. I could have added an arm that sticks out from the sun shaft. But then the earth wouldn't rotate on it's own axis. It would only spin around the sun. .

I continued to tinker and try different things. And the simplest thing is what I ended up doing.

I added a second gear on top of the first one. This one is however not attached to the drive shaft dowel. It is stationary.

And this solved two problems for me. It allows me to rotate something around the sun ( the earth) and it also give it a rotation of its own.

Let me show you.

And this is how I did it. There is an arm that is affixed to that center sun shaft. So, when that shaft rotates the arm revolves around the sun. Great!

And now we can see how I get the earth to rotate on its own axis. There is a gear under the earth arm. As the arm spins around the big gear that smaller gear turns.

Turns out to be a decent design. I like it! Minimal gears to accomplish the task.

Want to see this orrery in action? Check out the youtube video right here

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