Jason Hodlers ♾️/2099999997690000🏴 on Nostr: They didn't need to. They only needed to sync the days (which is easy to do), and ...
They didn't need to. They only needed to sync the days (which is easy to do), and then measure each shadow on that day when the shadows are at their shortest (when the sun is at its highest for each location). If they did this a couple hours apart (due to time zones), it didn't matter, since they weren't measuring the longitudinal difference (i.e., their distance east and west), but their latitudinal difference (i.e., their distance north and south).
Now, Alexandria and Syene weren't perfectly north and south of each other, which might account for some of Eratosthenes' errors in calculation, along with the imprecise measurements they had back then. But it still proved demonstrably that the Earth is more or less a sphere, and what its approximate size was. And you can still do this today, without any clocks or modern forms of communication, which is pretty cool!
Now, Alexandria and Syene weren't perfectly north and south of each other, which might account for some of Eratosthenes' errors in calculation, along with the imprecise measurements they had back then. But it still proved demonstrably that the Earth is more or less a sphere, and what its approximate size was. And you can still do this today, without any clocks or modern forms of communication, which is pretty cool!