10 - Confirming Copernicus: evidence for Earth’s motions

Summary

Although Newtonian physics provided a sensible explanation for why the Earth should rotate on its axis and orbit the Sun, there was still no direct evidence for Earth’s motion. The first such evidence was provided by James Bradley, who attempted to reproduce Hooke’s parallax measurements and instead discovered the aberration of starlight. This slight displacement of a star’s apparent position occurs because of the Earth’s orbital motion and the finite speed of light. It was not until the late 1830s that astronomers finally detected annual stellar parallax, again confirming Earth’s orbital motion. Astronomers also sought direct evidence for Earth’s rotation. French astronomers confirmed that the Earth bulges out slightly at the equator, an effect that Newton had predicted as a result of Earth’s rotation. Experiments on the deflection of falling bodies also seemed to confirm Earth’s rotation, but the results were clouded in uncertainty. It was Foucault’s famous pendulum that provided the best direct evidence for the rotation of the Earth. These and other successes helped to establish the validity of Newtonian physics and brought about the successful conclusion of the Copernican Revolution.