Historical Astronomy

The Changinging View of our Solar System


The early View of our Solar System  (Ptolemy Model)

Aristotle was a Greek philosopher who lived from 384 to 322 B.C.  Aristotle tried to understand and explain the world around him through philosophy rather than science.  According to Aristotle, the earth stood still at the center of the universe.  The moon, and stars circled the earth in celestial spheres.  Around 140 A.D. Plolemy developed a mathematical model to support the teachings of Aristotle.  The Plolemic model put the earth at the center and the heavenly bodies moved perfectly in spheres around the earth.  To explain the sometimes backward retrograde motion of the planets, Plolemy's model contained wheels within wheels known as epicycles. 


As the centuries passed, there were more errors with the Plolemy model when it came to predicting the location of the planets and eclipses.


Nicholas Copernicus  1473-1543

Copernicus was born in what is now Poland and at the age of 34 wrote a pamphlet that contained the idea that the sun not the earth was the center of the universe.  The earth rotated on its axis and revolved around the sun.  While the Copernicus model was not totally accurate, this heliocentric (sun centered) view was correct. 



Johannes Kepler 571-1630

Kepler was born in Germany where he studied to become a Lutheran pastor.  As a college student, Kepler had become a believer in the Copernican model of the solar system.  Kepler took the Copernican model one more step by speculating that the reasons for the spacing of the planetary orbits was due to the architecture of the heavens.  Kepler thought the planets orbit the sun in the five regular solids: the cube, tetrahedron, dodecahedron, icosahedron, and octahedron.  Unfortunately for Kepler, these orbital shapes did not match the detailed planetary observations of Tycho Brahe.  By studying Tycho's data on the location of the planets, Kepler determined the planets must follow elliptical orbits around the sun.  This led to Kepler's three laws.

Keplers Laws

1.  The planets orbit the sun in elliptical orbits with the sun at one focus. 

2.  A line from the planets to the sun sweeps out equal areas in equal times. This means that when a planet is closer to the sun, it orbits faster.

3. The planet's orbital period squared is proportional to its average distance from the sun cubed. 



Galileo Galilei 1564-1642

Copernicus, Kepler, Tycho, and Galileo were all important mathematicians and astronomers but Galileo was the only one to use a telescope in his study of the heavens.  Born in Italy, Galileo lived during the same time as Kepler and Tycho.  While Galileo did not invent the telescope, he was the first to use this new instrument to study the night sky.  Some of his discoveries include; Venus going through phases, four satellites orbiting the planet Jupiter, and spots on the sun's surface.  These discoveries led Galileo to the Copernican solar system with the sun at the center.  Because of his teachings, Galileo was sentenced to life imprisonment and was held in confinement at his villa. 



Isaac Newton 1642-1726

Newton was born in the English village of Woolsthorpe on Christmas day, eleven months after the death of Galileo.  Many consider Newton to be the greatest scientist of all time.  Some of his contributions include the invention of differential calculus, three laws of motion, and the reflector telescope.  Newton's laws of motion and gravity can be used to calculate the velocity to put a satellite in motion around another moon or planet. 
























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