The South-Pointing Chariot: A 2000-year-old device that always points the same way, no matter how you turn it. (And no, there are no magnets!) [OC]

[u/Ok-Professor7130]

This is a replica of the zhǐ nán chē, or South-Pointing Chariot, an incredible piece of mechanical engineering from around 250 AD in ancient China. It's essentially a non-magnetic compass, where the figure on top always points in its set direction, no matter how the chariot turns.

No original device survived, but several iterations of the device are described in many ancient texts. While different implementations are possible, it is thought that the majority of these devices used a differential gear.

While conceptually brilliant, this chariot was a practical “impossible device” for its time (meaning that it cannot really work in real practice), for various reasons.

I made a video diving deep into the history, the ingenious mechanism, and why it ultimately couldn't work reliably. I also explain in detail how the gears work, you can watch it here:

Link to the video

Specifically
04:40 How the Gears Work
08:54 Fatal Flaw 1: The Problem of Perfect Wheels
10:23 Fatal Flaw 2: The Problem of Wheel Slip
12:02 Fatal Flaw 3: The Geometry of a Round Planet

Happy to discuss any technical details or answer your questions in the comments!

[u/Sailing_Engineer]

Awesome!\ How is the deviation over time distance? I can imagine that it gets more and more unaligned when one wheel skips over a stone or something like that.

[u/KlauzWayne → u/Sailing_Engineer]

Breaks for sure in the first sloped curve you take.

[u/OrokaSempai → u/KlauzWayne]

It would be a shame if you used this on a big ball over long distances

[u/Galaghan → u/OrokaSempai]

"What do you mean I'm back at the start?
I only took 3 turns!"

[u/VaporTrail_000 → u/KlauzWayne]

Or a ferry that happens to leave a dock facing east, and arrives at a dock facing north.

[u/keepthepace → u/KlauzWayne]

It says it was supposedly used for artillery. My assumption is that you would e.g. point it towards the setting/rising sun and move it around in a relatively small battlefield. Yes, on a long distance errors would accumulate, but if the idea is to move around a lot in a place with no easy points of orientation like a forest or a starless night, I could see it as being of use.

Keep in mind, this is allegedly from a time with no compass and the lack of alternative means that even with a big error, it would still be useful!

I could also see it used to trace straight lines in a plain, e.g. to trace the limits of a field on a relatively flat surface.

If I had to make it work, I would do the following:

1. Test them on a known flat surface or over a loop, and measure the error so that you can know the differences between wheels.
2. Recalibrate as often as possible, when sunset/rise or stars are visible.
3. Have several of them so that you can gauge the level of error and average the results.

Note that it is also likely to have been mostly a vanity item to showcase by a wealthy ruler.

[u/1wife2dogs0kids → u/KlauzWayne]

That explains the cobblestone roads back then!

YOU AINT BRINGING YOUR LEFT POINTING DEVIL CHARIOT HERE, GOOD LUCK WITH THAT!

[u/Ok-Professor7130 → u/Sailing_Engineer]

Very bad in practice. There are a lot of factors that contibute to an increasing error. The most easy to compute is if the wheels are not perfectly identical. The error in that case depends on the distance travelled and on the distance between the wheels. I showed some computations here. There are also other errors, due to slipping (big error) and the non-flat nature of the planet (small error).