I’ve wanted to build an Orrery out of LEGO for quite a while now, and a couple of months ago I finally sat down and started putting one together. I mainly focused on the movements of the objects, paying close attention to the rotation of the Sun and Earth, and the orbits of the Earth and Moon. Clearly the relative sizes of the objects, and the distances between them, are not even remotely to scale. You can see it in action in the video, and you can find instructions for building your own down below.
As far as accuracy goes, it’s not too far off the real thing. Each rotation of the crank represents one day, and the Earth rotates once with every turn. The Moon orbits the Earth every 28 days, the sun makes a full rotation every 25 days, and the Earth orbits the Sun every 375 days.
Some notes on the construction.
For the large turntable, I’m using the newest 60 tooth version. You can use the older 56 tooth version, which is slightly smaller in diameter, you just need to use a 12 tooth double bevel gear instead of the regular 8 tooth gear to drive it.
When assembling the various gears, it’s important to make sure everything rotates freely. That is, don’t push the gears onto the axles too tightly, otherwise there will be a lot of friction between the gears an the Technic liftarms beside them.
The model can be motorized by replacing the crank with a gear and attaching a Power Functions motor to it. There are many ways to do it, but I’ve posted a couple of pictures of how I did it below.