[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".
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.)
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.
So the mechanism of a uniaxial gimbal (as pictured in the examples at end of article, and presumably manually adjusted) holding the magnifying glass would seem to introduce some error over the course of a year. But I like the concept, having some weird affinity for automation based on low-tech / earlier-tech principles.
This is purely for fun, without any practical use, but I wonder if you could build a "fully automated" sundial cannon by using the cannon's recoil as a power source.
The recoil both gives you kinetic energy and an impulse that is timed at noon. You could use it to drive some mechanism that prepares the cannon for the next day, e.g. refills gunpowder and adjusts the magnifying glass.
The latter could be done by having a gear with a cam that is driven 1/365 forward with each firing of the cannon and that encodes the position of the magnifying glass for each day of the year.
That only leaves you to design the exact shape of the cam. And deal with cloudy days and leap years...
So at the Greenwich observatory in UK they have a red ball that drops or rises, and has done so for centuries, at the appointed time for thr benefit of maritime traffic in the vicinity. If one does away with the cannon (benefits, drawbacks), presumably such a mechanism cojld be made to accomplish a similar function with the ball?
You'd need an external energy source (no gunpowder) and a way to translate the beam of sunlight into a mechanical impulse (boiling water or heating a bimetallic strip maybe), but generally, yes of course.
Only when used in conjunction with a standardised 'mean-time' 24 hour clock. If you plot the position of the sun at the same mean clock time, then sometimes the sun will be ahead, sometimes behind.
Before they introduced Daylight Saving Time, it was probably at local noon. It would be at local noon if it were fired between October and March, but it's only May to September, so...
Friendly reminder that if you are syncing your clock from marslight instead then remember that Mars, unlike Sol, isn’t always the same distance from us. Those light minute errors could mean you’ll miss the bus home!
I’m joking but this is also a real thing. For an example, see this code in the astrolib port used by SensorWatch’s “movement” project:
The sound is hard to mistake for anything else and travels better. Such cannons were used for synchronization of clocks on ships for navigation, among other uses.
Because it is way more fun, unique and create a much better cloud of smoke. A great way to show off your wealth at the time it was built. Things does not always have a technical reason to be chosen. People are silly.
Same azimuth, different altitude. You could adjust the altitude angle daily, or just set the fuse along the north-south axis, and it will be lit in a slightly different place each day.
