by Hayzelle Lababit


He's an Alexandrian Greek, and is credited to an Earth-centered(or in other words, geocentric) solar system. His solar system could be made to fit in data that he observed, but it will get very complicated.


He proposed a Sun centered(aka heliocentric) solar system in the 1500's.

Objections to Copernicus

  • How could Earth be moving at huge speeds when we don't feel it?
  • He didn't know about inertia.
  • Why can't we detect Earth's motion against the background stars (stellar parallax)?
  • His model did not fit the observational data very well.


  • He became certain that Copernicus was correct by his observations of the Sun, Venus, and the moons of Jupiter using the new telescopes.
  • Galileo was motivated to understand inertia by his desire to understand and defend Copernicus' ideas.

Tycho Brahe

  • In the late 1500's, the Danish nobleman Tycho Brahe went to make the most accurate measurements of planetary motions to date, in order to prove his own ideas of planetary motion.
  • His data was successfully understood by the German mathematician and scientist Johannes Kepler in the early 1600's.

Johannes Kepler

  • He made a firm decision that the orbits of the planets weren't perfect circles, but ellipses, with the Sun at one focus.
  • Another firm decision he made was that a planet moves faster when near the Sun, and slower when far from the Sun.
  • His laws gave a complete kinematic description of planetary motion(like planetary satellites, like the Moon)- but why did the planets move like that?

Isaac Newton

  • He discovered that the motion of a falling apple and the motion of the Moon were both actually the same motion, caused by the same force - which is the gravitational force.
  • His idea was that gravity is a universal force acting between any 2 objects.
  • He knew that the gravitational force of the apple equals the apple's weight, mg, where g=9.8 m/s²
  • He explained that the centripetal force on the moon was also supplied by the Earth's gravitational force.
  • Newton's Law of Universal Gravitation is often called an inverse square law, since force is inversely proportional to the square of the distance.
  • It also allowed extremely precise predictions of planetary orbits.


He measured gravitational forces b/t human-scale objects. Cavendish's experiments were later clarified and improved by von Jolly.

Michael Faraday

  • In the 19th century, the notion of "field" entered physics.
  • Objects with mass create an invisible disturbance in the space around them that is felt by other heavier objects - this is a gravitational field.