Read Chapter 5 from Discovering Astronomy.
WebAssign -- Chapter 5. Answer homework questions covering Chapter 5.
In 1609, Kepler's laws of planetary motion were published. Kepler was a Copernican who believed that the planets orbited Sun. He found harmony in the heavens when he discovered his three laws. But, these laws were merely descriptive. They were based on his observations. They had no theoretical underpinning. That is, he knew that the planets obeyed these laws but didn't know why they obeyed these laws.
The why was provided by Isaac Newton about 80 years later. Newton was the first to figure out how the force on an object was connected to the motion of the object. Then, he figured out that the force that causes an apple to fall from a tree--gravity--is the same force that keeps Moon in orbit.
Imagine the genius! The force that make an apple fall is the same force that makes Moon travel in a circular orbit around Earth. None of us would see that both of these types of motion are caused by the same physical law, but Newton figure it out and ushered in the first major scientific revolution.
The next scientific revolution would not occur until 1905 with Albert Einstein's great publications on relativity and the photon. We are still riding the wave of this second revolution.
Newton and Einstein--they are the great giants of physics whose understanding of the universe form the foundation of modern astronomy.
In the simulation below, Newton's second law of motion and Newton's law of gravitation are used to calculate the position of a planet, one day at a time.