Measure the Earth's Circumference yourself

For those of you who want to do this experiment yourself, here is a step by step guide.

1. Find a willing partner on the same longitude as you. It is also very important that the place the sun is shining directly down on the earth is not between the two points you are using. If both your points are above the Tropic of Cancer or below the Tropic of Capricorn, this will not be a problem.
2. Find a spire of a known height, such as a flagpole, at each location. The spire has to be at a location where it casts a shadow on flat ground. If no existing spire is at the location, you can create one using a meter stick.
3. At the same time, have the people at both locations measure the length of the shadow that their spire is casting. This time should be predetermined. Both spires must be casting a shadow at this time. When both people have collected the data, share the data with each other.
4. Now that you have the necessary data, you can calculate the circumference of the earth. First you need to find the difference between the angles that the sun was hitting the spires. First, divide the length of your spires shadow by the height of your spire. Then find the arctangent of this number. Repeat this process for the other spire. These two numbers are the angles that the sun is hitting the two spires. Then calculate the absolute value of the difference between the two angles. Next, divide 360 by this number. Finally, multiply this number by the distance between the two spires. When you have completed this, you have calculated the earths circumference.