This was my question too. How did they synch the clocks??
Discussion
It’s called knowing when noon was on the summer solstice when the sun was directly overhead (June 21). So at this time of day there was no shadow cast by the obelisk at Syrene. At the next summer solstice at noon you do the same thing at Alexandria, but this time a shadow is cast at 7 degrees. The rest is simple maths.
Correct. Time wasn’t super a factor as those 2 places are close enough in longitude.
There is a fascinating book about John Harrison called Longitude. Before we could tell time precisely longitude (at sea mainly) was SUPER difficult to determine. Worth a read.
The FE proponent of course says of the above that it just proves how close the sun is to the earth such that the light rays are not parallel.
I’ve given up trying to reason with them.