- The Copernican Model
- Kepler's Laws of Motion
- Galileo's contributions


[SLIDE 1]

Nicolas Copernicus was a Polish astronomer who adopted the idea of a Sun-centered universe and a moving Earth. The idea of a Sun-centered universe had first been proposed by a Greek philosopher named Aristarchus but had been discarded by the majority.

Copernicus found many problems with Ptolemy's model and decided to investigate geometric relationships between the stars. He was convinced that the Sun was the center of the universe and that planets including Earth rotated around the Sun. Despite moving to a Sun-centered model, the Copernican model was flawed because it held to the traditional belief that the planets must move in perfect circles.

Tycho Brache, usually referred to as Tycho (tie-koe), was a Danish nobleman who observed a comet and a nova, or new star (which actually turned out to be an exploding star or supernova). He provided accurate naked eye observations that backed Copernicus's theories. 

Johannes Kepler was a German astronomer hired by Tycho to provide scientific evidence to support Tycho's theory. He discovered that planetary orbits were not circles but a special oval shape called an ellipse. This discovery helped Kepler develop a Sun-centered model that predicted planetary positions very accurately.


[SLIDE 2]

Kepler developed three laws for planetary motion that he published in 1619. The first law states that the orbit of each plane about the Sun is an ellipse with the Sun at one focus. This means that the planet's distance from the Sun varies during its orbit. The closest point is the perihelion and the farthest point is the aphelion.

The 2nd law states that a planet moves faster in the part of its orbit near the Sun and slower when farther away from the Sun, sweeping out equal areas in equal times.


[SLIDE 3]

The 3rd law, also known as the law of harmonies, states that more distant planets orbit the Sun at slower average speeds, obeying the relationship of p2 = a3 where "p" is the planet's orbital period in years and "a" is its average distance from the Sun in astronomical units.

Unlike Kepler's first and second laws that describe the motion characteristics of a single planet, the third law makes a comparison between the motion characteristics of different planets. The comparison being made is that the ratio of the squares of the periods, to the cubes of their average distances from the sun is the same for every one of the planets. The third law applies to any orbiting planet that orbits the Sun or a star of similar mass.


[SLIDE 4]

Kepler's Laws matching Tycho's data helped reinforce Copernicus's ideas of a Sun-centered model of the solar system. However, many people still resisted the Copernican view due to long-held beliefs proposed by ancient Greeks about how the universe worked.

Galileo Galilei was an Italian astronomer who singlehandedly overturned popular Aristotelian beliefs about the Earth with his experiments and scientific evidence.

The first of Aristotle's ideas that he overturned was the idea that if the Earth were really in motion, then objects such as birds, falling objects, and clouds would be left behind as the Earth moved through its orbit. Galileo used experiments with rolling balls showing that a moving object remains in motion until there is a force that stops it. Isaac Newton later expounded on this idea in his first law of motion, and it explains why objects stay with the Earth instead of falling away as Aristotle had argued. This is the same reason that passengers stay inside a moving airplane even if they leave their seats.

Aristotle had also claimed that the heavens must be perfect and unchanging. Galileo invented a more powerful telescope than used before that showed imperfections in the surface of the Sun and Moon. If the heavenly bodies were not perfect, then that meant that their orbits did not have to be perfect circles.

The final objection to the Copernican model was that no one had detected stellar parallax, which should occur if the Earth orbits the Sun. While he could not directly confirm the existence of stellar parallax, using his new telescope, Galileo was able to observe the Milky Way and see that with its countless individual stars, the possibility of stellar parallax made sense since the naked eye could not detect all the billions of stars in the sky.

His observations helped confirm Copernicus's theory of a Sun-centered solar system.