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Let's assume some things:

  • The ground under the object is flat land.
  • The object always remains in the same position.
  • The object is clearly visible from the ground.
  • It is a high priority for the people to reach the object.

The method used can be anything, such as building a tower or just piling up a huge mound.

How high would you have to place this object to make it credible that people have never reached it in hundreds of years?

Bonus question: How high does it have to float to avoid being hit by flying objects thrown by trebuchets, ballistas or any machine specifically built for hitting the object?

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    $\begingroup$ It would look adorable at the start, but they'd eventually make it. It's all about time. After all they are humans. They learn and adapt. To me, it's completely plausible that they'd get the resources to build a rocket in over a hundred years, if the right people get brought together to lead the project. please edit your question and add a time limit. $\endgroup$ – justthisonequestion Oct 23 at 14:28
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    $\begingroup$ Is your medieval society capable of creating hot-air balloons or blimps? The first hot air balloon flew in 1783 which is quite a while ago. Also, kites have been in use for a long time. Provided it's windy enough, a kite with a really long rope could get quite high. $\endgroup$ – Dragongeek Oct 23 at 16:47
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There is a well-researched limit on how high a tower (Tower of Babel) can be built without a metal frame (steel or bronze): stone, baked brick, or fully vitrified brick all have their limits, based on the strength of the material. I recall this figure as being about 4000 feet with the strongest of these (fully vitrified brick), using a profile somewhat like a very squat-looking Eiffel Tower (slope increasing with height).

The volume of these bricks required is problematic, given that the base of a tower that high would be significantly more than a mile across (I don't recall the exact figure), as is the very technology of making fully vitrified brick, which (in our world) wasn't invented until the 19th century -- but that's your figure. Without the mid-20th century technology of steel frame building, it's impossible to build taller than approximately 4000 feet (1200+ meters) with any real world material.

No trebuchet ever built threw as high as this -- they were built for lobbing heavy objects distances of a fraction of a mile, and for maximum range (within distances where the Earth may be treated as flat), the highest point in the trajectory is just half the range. Even for a torsion catapult of the spear-throwing sort, 4000 feet is awfully high, though I won't claim it would be impossible to reach.

Gunpowder weapons, using a large powder charge to launch a lightweight ball, can easily reach this altitude, so you may want to place a limit of pre-gunpowder technology (practical guns that could do this date to the early 15th century).

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  • $\begingroup$ I'm interested in the Tower-of-Babel height limit research! Would you mind commenting/editing the link to the sources you were looking at? Thanks :) $\endgroup$ – Qami Oct 23 at 14:52
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    $\begingroup$ I'd be looking more at rockets than cannons, when considering the maximum altitude of gunpowder weapons, though the range differences probably won't be that much different, at least initially. Still, it'd be an entertaining thing for a civilisation to discover multistage rocketry before the steam engine.= $\endgroup$ – Starfish Prime Oct 23 at 14:56
  • $\begingroup$ I read it in an article that was written in the general style of xkcd, but I can't find it again with Google (lots of stuff on how tall brick buildings can get, but nothing I could find relative to zigurrats or similar construction limits. $\endgroup$ – Zeiss Ikon Oct 23 at 15:01
  • $\begingroup$ @StarfishPrime Modern Vietnamese/Cambodian villagers build black powder rockets for celebrations, using pretty primitive methods. Don't recall how high they go, though. $\endgroup$ – Zeiss Ikon Oct 23 at 15:02
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    $\begingroup$ @StarfishPrime Seems to me I recall Estes models can go about 2000 feet with three stages (if you could get it to fly vertical). That's probably close to the limit for black powder rockets -- very hard to get them to stage straight, and the bigger the engines the more likely they are to just blow up instead of launch. $\endgroup$ – Zeiss Ikon Oct 23 at 17:50
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There is no definite limit. Given sufficient time a medieval civilization could build a very tall artificial mountain out of stone blocks. The height depends on how determined the people are to get to this floating object, how big and industrious the nation is that is trying to reach it and most importantly how long they have to do it. Years, decades, centuries, millennia or more? As the time span increases the height potentially reached increases although the plausibility of the scenario decreases.

As a guestimate I suggest that decades are a reasonable best estimate. In that case 1000m would be inaccessible even if they were to build a pyramid, put a tower on top and a ballista on top of that.

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    $\begingroup$ Quite literally moving a mountain! In which case the answer is height of the tallest possible mountain + height of the tallest possible building + height of the longest possible ladder + range of a rope cast by the strongest bow $\endgroup$ – elemtilas Oct 23 at 22:59
  • $\begingroup$ The stamina to continue building it through the centuries would probably need to come from a religious cult. The great medieval cathedrals were the result of such a cult and required significant resources. The pyramids are another example, on a larger scale in a more centralized (I think) society. $\endgroup$ – Peter - Reinstate Monica Oct 23 at 23:38
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    $\begingroup$ @elemtilas I suspect the definition of "the tallest mountain" would preclude sticking a building on top. After all, you could always imagine a mountain that was the tallest mountain with a building shaped rock on top. Ladder allows more flexibility as it is less permanent. Although I suspect a modified siege weapon would beat a ladder + bow. $\endgroup$ – Zwuwdz Oct 24 at 4:02
  • $\begingroup$ @PeterA.Schneider -- Cathedrals weren't the result of a cult. $\endgroup$ – elemtilas Oct 24 at 13:44
  • $\begingroup$ @Zwuwdz -- Perhaps not a very heavy building? $\endgroup$ – elemtilas Oct 24 at 13:45
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"The object always remains in the same position." + "(people have never reached it in) hundreds of years"

Assuming the object is placed within the atmosphere, these two constraints sort of make the height irrelevant. Any technology that can manufacture an object capable of staying afloat, undamaged, without moving relative to the earth (ie unaffected by wind currents, storms, potentially even more extreme events like volcanic eruptions etc) for hundreds of years can probably make the same object completely untouchable by any means a medieval-level civilization could possess or develop.

Assuming the object can be in space, the cheap answer is to put it in geostationary orbit. It would have to be pretty damn big to remain "clearly visible from the ground", but if you accept that it can only be visible, say, during the nighttime, it is feasible to keep such an object there for many hundreds of years, and a medieval-level civilization would have no way to reach it.

Another cheap answer is holographic projection. Bury a projection method in a nearby mountain and just have it project an image. That way you can ensure that no matter what, it cannot be reached. Bonus points if you program the projector to always shift the image slightly upwards when it detects some mass closing in.

But if you don't like any of the above: a medieval civilization can make a hot air balloon. The highest a hot air balloon has reached is about 21 km, comfortably over twice as high as Mount Everest. If you were to get creative and mount a ballista (should be better than a trebuchet if you want height, when pointed straight up) on some kind of hot-air-balloon-grid (hey, you said reaching the damn thing is a priority, right?), you might edge in another half kilometer or so. To make it safe I'd say that somewhere in the area of 25 km would make it pretty safe from the hands of your medieval civ.

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  • $\begingroup$ FWIW, the OP didn't ask for mechanisms to suspend the object, just how high it should be. Medieval hot air balloons are somewhat anachronistic, and early hot air balloons certainly won't reach 21000m (and even if they could, the chances of being able to navigate towards the object of interest are slim to none til you can make combustion engines). $\endgroup$ – Starfish Prime Oct 23 at 14:54
  • $\begingroup$ The OP didn't specify any increase of technology level here, so no, a medieval society probably couldn't make a hot air balloon even if they had the idea for one. At least, not one that could get very far off the ground. $\endgroup$ – K. Morgan Oct 23 at 14:58
  • $\begingroup$ @StarfishPrime without a mechanism to suspend the object, the question "how high should I place it" becomes meaningless, any height lower than orbit will result in the object falling down to earth. I am aware that hot air balloons weren't actually invented until the late 1700's but there is nothing to indicate that with some clever substitutions, something similar could not have been developed a few centuries earlier (especially if you don't care about the thing or you surviving the trip or returning to the ground). $\endgroup$ – Nightmayre Oct 23 at 15:17
  • $\begingroup$ To me, the implication seemed to be "this has been taken care of", at least "this isn't what the question is about", but I take your point. $\endgroup$ – Starfish Prime Oct 23 at 16:00
  • $\begingroup$ You may wish to refer to the recent discussion about whether the Romans could have invented hot-air balloons (worldbuilding.stackexchange.com/questions/158162/…) - it's not as easy as you might think. $\endgroup$ – K. Morgan Oct 23 at 16:17
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It seems to me that if the object floats a few kilometers or miles high it will be out of reach of any method ancient or medieval society would have.

They could try building a tower of Babel or ladder or ramp up to the object or shooting catapult bolts or cannon balls up once they discover gunpowder in the late middle ages.

But I doubt that they could reach kilometers or miles high.

Here are some links to questions about building tall structures:

Tallest building possible by the Roman Empire1

Could a supertall building have been built in the 18th century?2

Practical height of towers without elevators3

But possibly there is a relatively narrow gorge thousands of feet deep. And the object might float thousands of feet above the river at the center of the gorge, and about level with the top of the gorge thousands of feet away to the side.

In that case humans might try to build a structure on top of the wall of the gorge hundreds or thousands of feet tall, and try out out catapults, trebuchets, gunpowder rockets and cannons in other places to try increasing their range.

And when the humans finally have weapons with sufficient range they might take them up to the top of their tower that`is hundreds or thousands of feet tall and is far enough above the object, and try shooting at the object after calculating trajectories that should intercept the object.

Or the humans might try filling up the gorge and raising its level. If the object doesn't rise higher with each rise in the level of the the gorge, the humans might hope that eventually, after centuries or millennia, they might be able to raise the ground level high enough to built a tower on top to reach the object.

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So answering your question, I think, a trebuchet would be the highest firing weapon. The largest one at Warwick castle throws a stone 300m, so let's assume a perfect arc, 150m up. (https://en.wikipedia.org/wiki/Trebuchet)

The highest medieval tower was Chateau de Coucy donjon, which was 55m high (https://www.guinnessworldrecords.com/world-records/512991-tallest-castle-tower-%E2%80%93-ever/)

The Great Pyramid of Cholula was 450m high (https://en.wikipedia.org/wiki/Great_Pyramid_of_Cholula)

The largest man made mountain is Sophienhöhe, which is 301.8 m. (I'm assuming it would be technically feasible to build this in any era) (https://en.wikipedia.org /wiki/Sophienh%C3%B6he)

So 150 + 55 + 450 + 301.8 = 956.8m

Round it up to 1km.

As others have said though, eventually, they'd probably find a way to get up there, especially if annoyed enough...

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  • $\begingroup$ You're assuming that the object they are trying to hit also uses medieval tech. It could be alien tech hovering there like the USS Enterprise. $\endgroup$ – K. Morgan Oct 23 at 16:28
  • $\begingroup$ @Riddles The donjon of Chateau de Coucy was the highest medieval castle tower built in Europe. Other medieval structures were significantly taller. Continued. $\endgroup$ – M. A. Golding Oct 23 at 23:49
  • $\begingroup$ @ Riddles The Great Pyramid of Giza was built 146.7 meters or 481 feet tall, and is now reduced to "merely" 138.8 meters or 455 feet tall after losing the top. It is often said to have been the tallest structure for thousands of years until the middle ages. $\endgroup$ – M. A. Golding Oct 24 at 2:30
  • $\begingroup$ @Riddles the Pharos of Alexandria was reported to be about 103 to 118 meters or 338 to 387 feet tall. Or possibly 120 to 137 meters, or 394 to 449 feet. Emperor Claudius built a great light house at Portus, the port of Rome, and it has been suggested that for reasons of prestige he would have built it taller than the Pharos and thus possibly rivaling the Great Pyramid. And during the middle ages really powerful states could have built similarly tall structures if they wished. $\endgroup$ – M. A. Golding Oct 24 at 2:40
  • $\begingroup$ @Riddles. The great Pyramid of Cholula is 450 meters by 450 meters, or 1,480 by 1,480 feet, and is 55 meters (180.446 feet) or 66 meters (215.535 feet) tall. google.com/… It was completed during the middle ages, so counts as a medieval structure. $\endgroup$ – M. A. Golding Oct 24 at 2:53

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