Thank you for giving a real, concrete answer, unlike the people going, "uh they just walked back and forth, or they just wrote down what time they did it" not understanding why this alone wouldn't work. No, they need to have a reference datum.
The idea is that at a given time, both places should have the same shadow.
That is the opposite of the idea. The idea is that one place would have no shadow (it is directly below the sun) and the other place would have some shadow (it is at some angle to the sun).
the answer is time zones, noon would likely be measured after the sun… so even if noon happens at different times between the obelisks overall, the local time will always be 12:00 when it happens, we just invented time zones
I struggle to grasp how you missunderstand one of the most basic experimental setups ever created. Must be all that brainspace used to calculate psi :P
Just to add details: Aswan, in Upper (southern) Egypt, is only about half a degree north of the Tropic (from Greek τροπικός, adj. of τροπή "a turning") of Cancer, which is a latitude line (at 23°26" N) denoting the farthest north the sun makes it all year, that is to say, the farthest north one could be and experience the sun directly overhead, which usually happens on June 21.
Now in actual fact, there will only be one point on the tropic where the sun does precisely this, but a considerable swath of the band from 23°N to 24°N will have the sun at a declination of at least 89° (less than 1° off from directly overhead) at at least some day near the summer solstice.
Alexandria, on the other hand, is almost eight degrees of latitude away from the tropic, and so the closest the sun will come to the zenith is about 8° off, which also happens on or near the summer solstice.
It is easy to see (vertical angles) that the acute angle adjacent to a vertical gnomon or obelisk in the triangle formed with the shadow is congruent to the angle the sun is down from the zenith.
So in fact they realized that in Aswan it's possible to have no shadow but in Alexandria there is no way to have no shadow? In that case, the "same time" idea is actually not very important to conclude that the Earth isn't flat or even know its radius. The shortest shadow lengths observed in Aswan and in Alexandria over a year, plus the Alexandria-Aswan distance, were enough to conclude on the curvature and radius of the Earth.
But how did they communicate "OK! My obelisk isn't casting a shadow! Check YOUR shadow now?" The distance on his map is approx 500 miles between obelisks.
they didnt need to check both obelisks at the same time. They knew one obelisk did not cast a shadow at a certain date (the solstice) so, on that date, they went and measured the shadow on the other obelisk. Whatever length was measured there was the difference between the obelisks' shadows.
They knew one obelisk did not cast a shadow at a certain date (the solstice) so, on that date
Surely it's also about the time of day, not just the date? You need to compare shadow lengths at the same time on the same date. How could they accurately measure time back then?
I'm guessing they measured the shadow when it was shortest.
On the southern obelisk the sun was directly overhead so they measured no shadow at its shortest.
On the northern obelisk they measured the shadow at its shortest which had to be at the same time the other obelisk had no shadow. So no need to synchronize clocks. Just measure the shadow at its shortest which must be at the same time for both.
Pretty shitty attitude to lash out at someone asking legitimate questions. This is how people learn. It sucks that if they were to take you seriously, they would be discouraged from asking questions the next time around. Just pretending like they understand when they don't.
I am most certainly not a flat-earther, but I also was curious how they managed to ensure that they measured at the same time. They did not have watches, maybe they had sun-dials (if they did, would they be accurate enough? They are based on shadows, obviously)?
The answer is not obvious; and as expected, the people back then were clearly pretty smart to come up with it.
I'm happy understanding more clearly how the experiment was conducted. And if I ever run into a flat-earth loon, and they ask the same questions I had, I'll have the answer ready for them... And then they'll deflect and probably go on about some ancient Egyptian conspiracy, or just ignore me and start talking about the ice wall.
You measure the shadow when the sun its at its peak. Since both places are (roughly) on the same longitudinal line (i.e Alexandria is to the north of Syene), it will happen at (roughly) the same moment of the day.
or, as the other commenter said, you measure when the shadow is at its shortest (which is another way of saying you measure when the sun its at its peak, for places that are on the same longitude)
the method is very dependent on the two cities being on the same longitude. if the cities were Ecuador and Alexandria, the measured angle would be the same, but the distance is much greater.
You are a scholar in alexandria and always walk by this nice obelisk and maybe even sit down in it‘s shade to relax for a while. You read in a book, that there is another obelisk wich doesn’t cast a shadow ob a specific date. It strikes you as odd, because you can’t remember your obelisk not casting a shadow. So you set out on that day to look at your obelisk and there it is: a long shadow. You must find out why that is.
This. My smooth brain still doesnt understand this and I read every comment chain here...
The only way I could deduce is some sort of sand piece. So on day 1 then roll the sand piece at 10pm and when the sand piece is empty, its 12pm and measure that point. So then day 2 then roll the sand piece at 10pm based on the obelisk time at position 1, move to position 2 and at 12pm (when the sand piece is empty) they measure the line at the new position. (The timings are just an example, I know they cant travel 500miles in 2hrs lol)
Oh your right, that skipped my smooth mind. As long as the 2 places where veritical to the sun then the sundial would produce the same time frame at both places to compare to. The length of the shadow is irrelevant for that.
This is something that has bothered me for years for the reason stated before about instant communication. I'm not a flat earther, I just wish your explanation was included whenever this idea is demonstrated.
Ironically, it's this idea that a lot of flat earthers use to justify their beliefs. Fortunately, there is other supportive evidence that we can use to conclude the earth is round.
There is a post somewhere on /r/AskHistorians about if they knew how far the sun is. They did some calculations, those were wrong actually. But they did understand that the sun was very very far away from the earth, astronomically far. I think they got the moon wrong as well.
Ok, so I know that obelisk A will not have a shadow at 10.55 am today. How do I ensure that someone measures shadow of obelisk B at 10.55 am today and not say 11.45 am by mistake?
Solar noon is the moment where shadows are shortest.
At position A they knew the shadow length was zero (which can only happen at solar noon) on a specific day of the year, so they didn't need to measure anything at this site.
At position B they marked the shadow length continuously on that same day as the shadows grew shorter and shorter as solar noon approached and then got longer as past solar noon. The shortest shadow length measured must had been at exact solar noon, the same moment the shadow was measured at zero length at position A.
How did they know the incoming sunlight was very parallel instead of radiating radially from a much closer point source?
In other words, why didn't they concluded that the earth was flat and that the different shadow lengths were explained by the sun being much much closer than it really is?
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