How Eratosthenes measured the size of Earth - GIF

[ToolTalkBot]

How Eratosthenes measured the size of Earth.




Previously:

"Flat Earth" photo, and why it's so hard to debunk pseudoscience
Deepest places on Earth - infographic
Distribution of Earth's water - graph
Driving a car in the coldest inhabited place on Earth - video
First image of Earth from outer space - photo
Moon transiting the earth - video
Space debris orbiting the Earth- GIF and photo
The Earth's atmosphere to scale - photo

[mklotz]

Eratosthenes was born in 276 BC. The error in his calculation of the earth's circumference was about 1 %. Today, more than two millennia later, if you gave most people the great circle distance between Alexandria and Aswan, I doubt that they would know what to calculate, much less how to perform the calculation.

[Floradawg]

Carl Sagan, the first scientist that the public could relate to in a personal way. He would appear on The Tonight Show with Johnny Carson and many other TV shows and explain scientific ideas and theories to the viewers. He had a way of making potentially dull explanations interesting. He helped to mold Neil deGrasse Tyson into the scientist that he is today.

[Ralphxyz]

How was the timing done? Both shadows had to be measured at the same time.

[Frank S]

How was the timing done? Both shadows had to be measured at the same time.

For all we know he may have done both measurements himself but a year apart

[mklotz]

How was the timing done? Both shadows had to be measured at the same time.

They probably made their measurements at high noon - when the sun was on their meridian*. High noon occurs when the obelisk shadow is shortest - easy to determine if you mark the shadow end point for an hour or so around noon.

*Alexandria and Fayum are not on the same meridian but Eratosthenes assumed they were. They're close to the same meridian and the slight error probably accounts for part of the error in his calculation.