Enter the length of the Shadow (cm):

Enter the Height of the Gnomon (cm):

A couple thousand years ago, Eratosthenes measured the circumference of the Earth by measuring the altitude of the Sun in two locations in Egypt (Alexandria and Syene). On the day of the measurement the Sun had an altitude of 90 degrees in Syene. Eratostgenes measured the altitude of the Sun in Alexandria and the distance between Syene and Alexandria. By knowing the difference in altitude of the Sun at two locations that are due North-South of each other and the distance between them we can use trigonometry to calculate the circumference of the Earth. We can recreate the experiment. At the equinoxes (Sept 22 and March 20th) the Sun's altitude on the Equator at solar noon is 90 degrees (for our purposes the altitude is close enough to 90 degrees a week before and a week after the equinox). You can use Google Maps to measure the distance from your location the equator. Measure the altitude of the Sun at solar noon between Sept 15th and Sept 29th. The altitude calculated above will automatically be filled in the altitude entry box, or you can type it in. Enter the distance to the equator. Press "Calculate the circumference of the Earth" to calculate the circumference. You will also be given the known value of the circumference and the percent error, which is a measure of how well the value you found matches the known value.

Enter the Altitude of the Sun (between Sept 14 and Sept 29) (degrees):