
Newton's
laws of motion and the law of universal gravitation provided a theoretical
explanation for Kepler's empirical laws of planetary motion. Just as Kepler
modified Copernicus's model by introducing ellipses rather than circles, so
too did Newton make corrections to Kepler's first and third laws. It turns
out that a planet does not orbit the exact center of the Sun. Instead, both
the planet and the Sun orbit their common center of mass.
Because the Sun and the planet feel equal and opposite gravitational forces
(by Newton's third law), the Sun must also move (by Newton's first law),
driven by the gravitational influence of the planet. The Sun is so much more
massive than any planet that the center of mass of the planet—Sun system is
very close to the center of the Sun, which is why Kepler's laws are so
accurate.
