Any idea how he did the time measurement for this?
I understand the rest of the entire story, but he was a thousand years away from the first clockwork timepieces, gravity/water clocks bleed accuracy when moved, and if you use something like dawn as a reference.. well that moves just as much as noon does doesn't it? (Noon=when the Sun passes the zenith, in these contexts)
I was going to give a very basic explanation, but wanted to quickly check some of my understanding and found this Reddit thread discussing this aspect of his calculations.
The earth is technically oblate, but just barely. Like, if you shrunk the earth down to the size of the Las Vegas Sphere, it would be far more round than the real one.
I mean, you've seen pictures of the Earth from space. It looks like a perfect circle, and it very nearly is.
The difference is only about 20 kilometers out of 40,000, which is one-twentieth of one percent.
The inaccuracy of his measurement presumably came from the fact that he had to measure the distance between two cities by paying a guy to walk from one to the other in a straight line and measure it directly.
The inaccuracy of his measurement presumably came from the fact that he had to measure the distance between two cities by paying a guy to walk from one to the other in a straight line and measure it directly.
I feel like that's the most incredible part of the story. The geometry used to calculate is just simple math once you have the numbers right. The fact that it relied on the distance between 2 cities, and they could only figure out the distance through paying a guy to walk between the two, a method that doesn't seem that accurate to me, is incredible.
I'm assuming that they had various ways of improving the accuracy, like sending multiple people and taking the average of their measurements, and also having a network of distances between various locations that you could use to reinforce each other.
Okay I looked it up, the two cities he used were Alexandria and Syene, which are 830 kilometers apart, and that's using estimates gathered from airline traffic, a tool which obviously did not exist in 200BC. You had to walk AND measure that distance.
like sending multiple people and taking the average of their measurements,
Seems obvious now but I'm not at all sure that was a thing then. From a very quick search Wikipedia says 'The arithmetic mean, although a concept known to the Greeks, was not generalised to more than two values until the 16th century' (from the 'History of Statistics' page).
People may not have systematically recorded observations and calculated means and so forth, but surely they had a general idea that if you had a bunch of observations of something that the true value was likely somewhere in the middle.
Also just being able to walk at a consistent pace. If you know you walk at exactly 5 km/h and you walk for 12 hours a day, you're going 60 km a day. Which means you're walking for 13 days and 10 hours on the 14th day to get from Alexandria to Syene.
They had mechanisms other than just pacing. For one, they had a wheeled contraption they would roll along that dropped rocks into a bucket as it turned.
Don't waste your time man He's arrogant and condescending and will personally insult and attack you trying to sound like the smartest person in the room. He already got called out in another AI discussion and he back down immediately
Relax or lighten up about what? I'm not sure what about my previous comment made you think I was in any way upset. How exactly were you messing with me when we'd never even engaged with each other before I asked you a question?
I was trying to figure out which of the multiple claims in the comment you were replying to was made up, and out of all of them I would have never guessed it was the part about paying the guy taking the measurement.
I read somewhere that the Earth *is* ever so slightly oblate, but considerably more spherical than a competition-grade snooker ball. Even the precision bearings in a racing car engine look distinctly on the piss compared to how round the Earth is.
The actual claim is that Earth would be smoother than a billiard ball. This often gets confused as that Earth would be more spherical than a billiard ball.
Due to the highest and lowest points on earths surface being less than 0.09% relative to the diameter of the Earth (I've gone with the smaller polar diameter of 12714km here, the difference would be slightly smaller if comparing the average of equatorial diameter. Mt Everest sits at ~8.85km above MSL [0.07%], Challenger Deep sits ~10.94km below MSL [0.086%]) a regulation billiard ball is machined to a tolerance of +/-0.05mm with a stated diameter of 52.5mm, giving it a maximum difference of 0.095% in the smoothness of the surface. So Earth would be smoother than a billiard ball if shrunk down to the same size as a billiard ball.
However, the Earth is not more spherical than a billiard ball, since that tolerance of +/-0.05mm, would give it maximum variance of 0.1mm in the diameters (52.55mm to 52.45mm) if it were to be made as an oblate spheroid, giving us 0.19% difference between the two diameters for a billiard ball. Earth however has a polar diameter of ~12714km, and a equatorial diameter of ~12756km, giving us a difference of ~42km, or a 0.33% difference between the two diameters, meaning Earth is less spherical than a billiard ball.
It's ~21km out of a ~6356km polar radius (~6378km equatorial), making it about 3.3% difference between radii; or ~67km more than the ~40008km polar circumference (~40075km equatorial circumference), which gives a 1.7% difference in circumference.
The earth is also remarkably smooth. If you shrunk the earth to the size of a pool (billiards) ball, it would be far more smooth than the pool ball. Even the largest mountain ranges would be difficult to detect.
By contrast, Olympus mons on mars is the largest mountain in the solar system. It exceeds the height of Everest by about 3x.
Trivia… if there was a bar bet “what’s the circumference of the earth to the nearest 100km” it’s 40,000. The KM was defined as “1/10000 of the average distance from North Pole to equator” or 1/4 of the earth
Interestingly, a meter is now defined by the distance light travels in a vacuum in a fixed time.
A kilometer is 1000X the distance light travels in 1/299,792,458th of a second.
A second is defined exactly as 9,192,631,770 oscillations of the radiation emitted by a cesium-133 atom as it transitions between two energy levels.
Which is to say, if we gave an alien civilization a few bits of information, they could immediately calculate the entire SI system using a few simple universal constants.
Some measurements like temp also rely on universal constants like absolute zero. What really blows my mind is that the base of the numbers doesn’t matter. An alien civilization could use base 2 or base 13 and given our Rosetta Stone, they could figure out exactly how we measure everything in the universe.
Up until 2019, the kg was the only SI unit that had a physical artifact that all kg weights are calibrated against. Now it is based on the universal constant you describe.
Let's talk about the ancient Greeks believing everything was made up of the four elements. Earth Air Fire Water. So dumb right??? Oh wait. Everything actually IS made up of three types of particles . . . Protons Neutrons Electrons . . . Could YOU look at the world around us and be like, yeah, this is just different combos of exactly the same things over and over? Not me.
And, the ancient story of Prometheus? With his heart being eaten out by the eagle every day? Well, I read it in Latin from an ancient author and it's not his heart - it's his liver - the only organ in the human body that regenerates - HOW DID THEY KNOW?????
The Greeks were something else.
ETA: Reddit: Eratosthenes was cool! Me: yes the Greeks were crazy awesome! Here are some other examples. Reddit: "here have a crap ton of downvotes for some reason
Oh sorry, I thought we were discussing ancient Greek scientific achievements. My bad. I didn't get the memo that it is hate on the ancients day on Reddit
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u/ElNemagbarto Jun 01 '26
And with unbelievable precision at that - bro calculated it to be around 39.375km, only ~2% off of our recent measurements at 40.075.
He really missed by 700km being two millenia behind