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# Circumference of Earth

Created by NASA.

## Want to join the conversation?

• Where was Eratosthenes born?
• He was born in Cyrene in modern day Libya.
• How did Eratosthenes send message from Syene to Alexandria about the solstice occurring in Syene to measure the shadow of the pole present at Alexandria?
• I think he knew there was no angle at summersolstice, so he only had to measure the length of the shadow at that particular moment at 12 o'clock
• How did Eratosthenes check on his pole at the same time as the other person checked his at noon i.e. how did Eratosthenes know that it was noon at Syene and that he should check his pole's angle?
Is it because of knowing the time of travel from Alexandria to Syene?
• Alexandria and Syene were close enough together (they're both in Egypt) that they used the same time. Essentially, they're in the same time zone. Also Syene is almost directly South of Alexandria meaning that they hit their solar noon (i.e. the actual time when the Sun is directly overhead) just about at the same time.
I've actually lead this experiment with my students before. The important consideration is knowing the time of your local solar noon, which is not necessarily p.m. exactly. On any given day, one point on your longitude (North-South axis) will be directly under the sun at the solar noon: the same position as Syene in the classical problem. If you measure your distance to that point, and measure your shadow at the solar noon, you can calculate the circumference of the earth. Of course, today we have modern tools that can tell you all this information, but it's not impossible to conceive how Eratosthenes gathered this information more slowly and scheduled his measurements at the right time in his age.
• Ok that gives us circumference using that equation based on assumptions. . but where can we do a hard and fast scientific experiment and actually measure the curvature. . and can I assume that the rate would be close to. . 1 mile=8 inches ×the number of miles squared? And if that is correct could you please put it into equation form. . and I'm getting nothing but straight and flat with a telescope at over 100 miles. . so how and where could I do it that would show that ratio of curvature
• Hi thanks for the information.
I have a question I was wondering if you could help me on. It seems this whole calculation is dependent on the assumption that the sun rays are arriving to Earth in a parallel manner. At the time of Eratosthenes what proof is there that this was correct? Cheers.
• 24,901.55/360 =69.1709722 x 16.39721 = 1134.21095
pi x 360 = 1131.408/69 = 16.39721
16.39721 x 22.00 = 352 + 8 = 360
intercalculate from a few decimals and same numbers occur
3.1428 X 360 = 1131.408
24,901.55/1131.408 =22.009345877
• But how can someone not living on the equator measure the circumference ??
• Neither syene nor alexandria are on the equator. Being on the equator or not is irrelevant information.
• How far is it from Alexandria to Syene, the two cities mentioned in the video
• or 500 miles. He states this in the video I believe.
• If the circumference wasnt known till modern day, how would they figure it out?