# How did Eratosthenes knew the exact time of the day?

Eratosthenes measured the radius of the Earth with an incredibly accuracy.

To do it, you need to measure the length of the shadows from 2 different cities at the same time of the day. Then knowing the distance between the cities and a little bit of geometry he calculated the radius of the Earth.

My question is: How did he know when it was "the same time of the day"? There was no long range communications to synchronize two people measuring the shadows.

And if you measure the shadow one day, then travel to the other city to measure the shadow next day, how can you know that you will do it at the same time of the day?

Nowadays with clocks or telephones the experiment is very easy, but back then I can not figure out how he managed to do it.

• See Eratosthenes and the Mystery of the Stades: First, "It is uncertain whether he made the measurements used in the calculation, or if he relied on the information of others." Jul 13, 2017 at 13:30
• Second: "While working at the library, he learned that on the first day of summer the Egyptian town of Syene cast no shadows. This happens because at noon on the day of the summer solstice the Sun is positioned directly above the town of Syene, near the modern city of Aswan, Egypt. In contrast, on that same day in Alexandria a staff, or gnomon, did cast a shadow." Jul 13, 2017 at 13:31
• So, he needed at most one measurement at noon. This is "How did he know when it was "the same time of the day" ". Jul 13, 2017 at 13:32
• The town of Syene cast no shadows (at some specific moment of the day). On the same day (and of course at the same time) in Alexandria, a staff did. But the question is still: How can you know back then that you are talking about the same exact moment of the day? Jul 13, 2017 at 13:36
• You must take care of the fact that ancient Greeks scientists were quite smart: they were not Neandertal men. It can be enough to measure the shadow in different point in time "around noon" and consider the shortest one. Jul 13, 2017 at 13:39