Overview on Keplers three Laws of Planetary Motion

For countless ages, man has marveled at the stars.  From Aristotle to Einstein, humans have attempted to bring order and understanding to the very nature of the planets, moons and stars; however, it wasn’t until Johannes Kepler, a German born scientist presented his theories on planetary motion that man’s study of the heavens seemed clear.

Johannes Kepler

Kepler was born in Swabia, Gernany in 1571.  As a young child, he worked in his grandfather’s inn, but showed a competence in mathematics.  Kepler’s formal education was received at University of Tübingen, where he studied astronomy and mathematics. 

During his lifetime, Kepler challenged the old views on planetary movement, and remastered refractive telescopes for more accurate study of the stars.  By the time of his death, in 1630, Kepler had laid the foundation for astronomy and influenced the works of other notable scientists, including Sir Isaac Newton.

Early Influences and Theories

While Kepler studied at Tübingen, the general theories of planetary motion were based on the Ptolemaic system. Under this system, it was believed that the seven known planets, Mercury, Venus, Mars, Jupiter, Saturn, Sun and Moon, circled the Earth.  The terra-centric view was based on Aristole’s early interpretation of the Universe. 

However, prior to Kepler’s birth, Italian astronomer Nicolas Copernicus has proposed a theory that the Sun and not the Earth was the center of the solar system.  This heliocentric theory seemed to offer a new explanation for planetary motion, but had not been widely regarded. 

Another influential mind of the day was Tycho Brahe, a Danish nobleman and astronomer.  While studying the night skies, Brahe made numerous observations that could not be explained by the Ptolemaic theories of the planets.  Brahe’s study of the 1572 Supernova, accurate observations of celestial bodies, and a more acceptable theory of geo-heliocentric universe brought him recognition; however, Brahe died in 1601 before the true importance of his observations could be realized. It was also Brahe for whom Kepler had served as an assistant at the University of Prague.

Kepler’s Laws of Planetary Motion

During Kepler’s time, observations about planetary movements were in conflict with the theories of the day.  In particular, the standing Ptolemaic theory did not explain the usual behavior of Mars.  It was inspired by these dilemma that Kepler devised his theories of celestial movement.

Kepler’s First Law

According to the first law of planetary motion, the orbits of the planets are elliptical and the Sun is the center of the solar system.  Distinguished from the prior views of the alignment of planets, Kepler’s law set the Sun at the focus of the planetary orbits.   Kepler, viewing Brahe’s data, further outlined that the planets’ distance from the sun was not constant, but instead constantly changing. 

Kepler’s Second Law

Based on the planetary observations, Kepler confirmed that the imaginary line joining the Sun with each planet sweeps out equal areas in equal times as the planet traveled along its orbit.  For practical understanding, Kepler’s second law indicates that when the planet was moving slower when further away from the Sun (aphelion) and faster along its orbit when nearest the Sun (perihelion.)  Basically stated, the area covered by a planet’s orbit will be equal for equal periods of time.

Kepler’s Third Law

While confusing at first reading, Kepler’s third law explains that the ratio of the squares of the revolutionary periods for two planets are equal to the ratio of the cubes of their semi-major axes.  Translated into more understandable terms, this law explains that orbital radii have a mathematical relationship with the time a planet takes to orbit the sun.

Kepler’s first two laws of planetary motion were published in 1609 and the last law was published in 1619.  His theories transformed scientific views about the planets and their relationship to one another.  His work influenced revolutions in scientific thought and earned him as one of the fathers of modern astronomy.  

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