# Timeline

### by Hayzelle Lababit

## Copernicus

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.

## Tycho Brahe

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

## Johannes Kepler

- He made a firm decision that the orbits of the planets weren't perfect circles, but
, with the Sun at one focus.*ellipses* - 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
description of planetary motion(like planetary satellites, like the Moon)- but*kinematic*did the planets move like that?*why*

## Isaac Newton

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