A tale of two bridges and a round Earth

Abstract

Columbus concluded that Earth is round while watching a distant ship sink beneath the horizon. You too can observe the curvature of Earth on the San Francisco Bay (and other places); not only that: you can determine Earth’s radius from such observations. I present here a method to use the vertical shift between two bridges over the Bay due to Earth’s curvature as seen in a photograph, where the more distant bridge is apparently lower than the proximate one (as shown in Fig. 1). Starting with each bridge’s known dimensions and location and applying atmospheric refraction corrections and then measuring the vertical shift between the two bridges on the image allows a calculation of Earth’s radius within a surprising 1% of the accepted value.