Kepler's laws of planetary motion were based on empirical observations and described how planets moved, but they didn't explain *why* they moved that way. Newton's law of universal gravitation provided the underlying cause for Kepler's observations.

Here's a breakdown:

* Kepler's Laws:

* Law of Ellipses: Planets move in elliptical orbits with the sun at one focus.

* Law of Areas: A line connecting a planet to the sun sweeps out equal areas in equal times.

* Law of Periods: The square of a planet's orbital period is proportional to the cube of its average distance from the sun.

* Newton's Law of Universal Gravitation:

* Every particle in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

Newton's work explained why Kepler's laws worked:

* Elliptical Orbits: The force of gravity causes planets to move in elliptical orbits, not perfect circles.

* Equal Areas: The force of gravity changes the speed of a planet as it orbits, causing it to move faster when it's closer to the sun and slower when it's farther away. This results in the equal areas swept out by the line connecting the planet and the sun.

* Periods and Distance: The relationship between a planet's orbital period and its distance from the sun is a consequence of the gravitational force acting between the planet and the sun.

In essence, Newton's law of universal gravitation provided the theoretical framework that explained Kepler's empirical observations.