The Sundial Cannon of Åtvidaberg (2017)

[Warning: bad joke ahead] Every day at noon a soldier fired a cannon to signal it was noon. A guy was curious as to how he knew when to fire the cannon. So he asked the soldier, who told him "the guy in the guard station gives me a signal, and I fire the canon". He asks the guy in the guard station how he knows when to signal, "I use the clock on the wall, a guy comes and sets it occasionally". He finds the guy who sets the clock and asks him how he knows what time it is, "I sync my watch to the clock in the town square, then set that clock from my watch". So he finds the guy who sets the town square clock and asks how he knows what time to set it to. "Oh, I just sync it to the noon cannon".

Haha I didn't realize this was a joke. It was, nearly word for word, a problem on my control systems final exam a few semesters ago!

I don’t know control theory but I think it wouldn’t work

That may be precisely the problem.

[Warning: overanalysis of bad joke ahead]

Wouldn't this setup actually work though? At least if it's a proper sundial cannon.

Most components of that system are mechanical clocks, which do keep the time on their own, they just drift. The only exception is the gunner who fully relies on an external time signal.

So if that were all, the system would work for a while, but slowly drift and become ever more inaccurate, until it basically has nothing do do with the actual time anymore.

However, the gunner does not always operate the cannon: He only does on cloudy days. On sunny days, the cannon operates itself, using the time signal of the actual sun - which is then passed through the chain and let's the mechanical clocks resync to "sun time".

So in effect, the system's time signal comes from the sun. The rest is just an overly complicated way to "interpolate" the time if the sun is not visible or not at noon.

(Now I also wonder if someone ever build a "self-syncing" mechanical clock - e.g. for a clocktower - using this principle: Use a standard clockwork, but add some mechanism that resets the clock to noon when a focused beam of sunlight hits a certain point.)

It would work as long as someone actually looks at the sun every so often.

The first line says that the gunner operates the cannon every day at noon

> However, the gunner does not always operate the cannon: He only does on cloudy days. On sunny days, the cannon operates itself, using the time signal of the actual sun

That's why I don't understand how you came to this conclusion, what's your thinking behind it?

You're right. The OP article contains a line, that they fire the sundial cannon manually as a "fallback":

> The 6-pound cannon is fired everyday at 1 PM, from May to September. On sunny days the sun automatically sets it to light, but on days when clouds obscure the sun, the sun gunner on duty fires the midday salute with a match.

I imagine that was what reminded the GP of the joke and I kinda used it as context - but yeah, on rereading, it's not part of the joke. I admit, I jumped to conclusions there.