Historical Eclipses and Earth's Rotation



The Earth and Moon are a dynamic, closely coupled system. What happens to one is mutually reflected in the motions of the other through an exchange of angular momentum and energy which, even over historical times, has been observed and attracted notable attention. In 1695, Edmund Halley, a contemporary of Newton, discovered a discrepancy between predicted and observed times of solar eclipses. From this, Halley concluded that the Moon's motion in its orbit was accelerated, that is, the Moon would appear ahead in orbit from the expected time, and the Earth's rotation slowed. Later, in 1754, Immanuel Kant, the immortal German philosopher, suggested tidal friction slowing the Earth as the cause. Because of the lack of observational evidence of the expected acceleration in the motion of the Sun and other planets as seen from the rotating platform of the Earth, it was not until 1905 and the discovery of the solar acceleration by P. Cowell that closure of the theory took place.