Johannes Kepler Biography
Johannes Kepler biography, outline: born 1571, Germany. Suggested planets move in elliptical orbits. Died 1630.
Johannes Kepler biography, deails: Johannes Kepler was born into poverty, at a time of war and religious conflict, to a wayward criminal father and a mother who was exiled after accusations of witchcraft. He grew to be a hypochondriac with little self-esteem. He benefited from a ducal scholarship and went to school, and then university, at the University of Tübingen. There, Kepler studied under Michael Maestlin, a talented German astronomer who taught him the heliocentric ideas of Copernicus. Kepler dedicated himself to resolving the problems with the Copernican system in the early 1590s.
In the year 1600 Johannes Kepler set off for Prague where he met Tycho Brahe. Brahe's observations and Kepler's calculations would destroy the Church's view of cosmology. This was fitting in the year the Inquisition burned Giordano Bruno.
Johannes Kepler modified Nicholas Copernicus’ heliocentric theory by suggesting the planets move in elliptical (not circular) orbits about the Sun. This, together with his other discoveries, are used to calculate planetary motions to this day. They have immortalized their discoverer, being called Kepler's three laws of planetary motion.
Kepler's three laws of planetary motion are still considered valid for the motions of planets in our solar system - except in very rare situations. They only break down if the masses and motions of a star and its planets approach the realm of Einstein's general theory of relativity, or if the two-body approximation breaks down.
Kepler's three laws of planetary motion are:
- The orbit of each planet about the sun is an ellipse with the sun at one focus.
- The line joining the Sun to a planet (the radius) sweeps out equal areas in equal times.
- The square of the time taken for a planet to complete an orbit is proportional to the mean radius cubed.
Planetary orbits are very nearly circular. It needed the observational prowess of Tycho Brahe, and the calculating ability of Johannes Kepler, to determine their elliptical nature. The predictions made by Kepler's three laws of planetary motion were more accurate than those made by Ptolemy's geocentric model. However, the Church only accepted that it was a good model for making calculations. They did not accept it as the "true" model of the solar system. "How can the Earth and the other planets be held in orbit around the Sun when everything that we see around us is attracted to the Earth?" Singh p.58.
- Kepler's three laws of planetary motion were used by Sir Isaac Newton, when formulating his universal law of gravitation. Newton showed that an object subject to the gravitational force from the Sun can follow an elliptical orbit, as specified by Kepler's first law of planetary motion. But it can also follow a parabolic or hyperbolic path, depending on the energy of the object. Therefore, a comet might enter the solar system and leave without returning.
Kepler's three laws of planetary motion do not take into account the perturbing gravitational effects of the planets on each other. But even the motions of just three bodies under mutual attraction have only a few special-case analytical solutions. Fortunately, the Sun dramatically outweighs the planets and their motions become a two-body problem where one body is fixed (unless extreme accuracy is sought).
The world, for the first time, had a geocentric model that produced better predictions than Ptolemy's heliocentric cosmology.
Although forced by his calculations to accept them, the idea of elliptical orbits was repugnant to Kepler because ellipses are less perfect than circles. Also, he could not find an explanation as to why the planets moved this way. This was provided by Sir Isaac Newton.
|Before Johannes Kepler||Contemporary||After Johannes Kepler|
|Aristotle biography||Tycho Brahe biography||Sir Isaac Newton biography|
|Ptolemy biography||Johannes Kepler biography|
|Copernicus biography||Galileo biography|