How to Find the Radius of the Earth

1). Record the distance between your location and your partner's location. As an example, we will use Eratosthenes' situation. The distance between Syene and Alexandria is 787 kilometers.
2). Drive one of the meter sticks into the ground in your location in a sunny spot. Tack one end of a piece of string to the top of the stick. Have your partner do the same in her location. Make sure both sticks are perpendicular to the earth and that the same length of stick protrudes from the ground.
3). Measure the angle of the shadow of your meter stick when the sun is overhead and the shadow is smallest. Place the loose end of the string at the end of the cast shadow and hold it taut. Use the protractor to measure the angle where the string meets the stick at the top. Have your partner do the same in her location at the exact same time. Record the measurements.
4). Subtract the angle measurements to determine the difference in the angle of shadows between the two locations. For Eratosthenes, at midday on the summer solstice where the sun's angle was directly overhead, the angle was zero. Though he did not have instant communications as we do now, he was able to determine the angle of the sun's rays in Alexandria at the same time, which was about 7.2 degrees. Therefore, the difference was 7.2 degrees.
5). Compute the circumference of the earth using the distance and angle measurements you have.
Since the locations can be thought of as points on a circle that goes around the earth, the distance between the locations can be thought of as an arc measurement on a 360-degree circle. For Eratosthenes, the arc was 7.2 degrees.
Also, the distance between locations can be thought of as a part of the total circumference of the earth. In Erastothenes' case, it was 787 kilometers.
So the proportion for Erastothenes would be 7.2 / 360 = 787 / x where x=circumference of the earth in kilometers, or 39,350 kilometers.
6). Compute the radius of the earth using the formula C (circumference) = 2 x pi x r (radius). Erastosthenes' formula would look like this: 39,350 = 2 x 3.14 x r, or 6,267 kilometers.