## Al-Biruni’s Method to Determine the Radius of the Earth

In the 11th century, Al-Biruni successfully determined the radius of the Earth. He accomplished that by measuring the dip of the horizon from the top of a hill. From the measurements, he was able to calculate the radius of the Earth.

In the 21st century, we can easily repeat the same experiment with practically no effort. We just need a smartphone and an opportunity to observe the horizon from a high altitude, like during a flight.