Johannes Kepler developer
of three laws of planetary motion
Johannes Kepler was born on December 27,
1571, in Weil (near Stuttgart), Germany. It was
while studying theology and the classics at the
University of Tübingen that he learned about the
heliocentric theory of planetary motion first
developed by Nicolaus Copernicus. From 1594 to 1600, he held the chair
of astronomy and mathematics at the University of
Graz, Austria.
While at Graz, Kepler worked
out a complex geometric hypothesis to account for
distances between the planetary orbits. He then
proposed that the Sun emits a force that
diminishes inversely with distance and pushes the
planets around in their orbits. He published his
account in a treatise entitled Mysterium
Cosmographicum (Cosmographic Mystery)
in 1596.
Kepler left Graz in 1597 rather
than undergo compulsory conversion to Roman
Catholicism. While seeking another post, he
became assistant to the Danish astronomer Tycho Brahe in the latter's observatory near
Prague. On the death of Brahe in 1601, Rudolph
II, the Holy Roman Emperor, appointed Kepler to
be Brahe's successor as imperial mathematician.
Kepler made his most
significant discoveries when he tried to find an
orbit to fit Brahe's observations of Mars. Earlier astronomers believed a
planet's orbit was a circle or a combination of
circles, but Kepler could not find a circular
arrangement to agree with Brahe's observations.
He realized that the orbit could not be circular,
and resorted to an ellipse in his calculations.
The ellipse worked, and Kepler destroyed a
2,000yearold belief. In 1609, he published Astronomia
Nova (New Astronomy), which
contains two of his laws of planetary motion: (1)
every planet follows an ovalshaped path around
the Sun, called an ellipse, with the Sun
at one focus; and (2) an imaginary line from the
center of the Sun to the center of a planet
sweeps out the same area in a given time, meaning
that planets move faster when they are closer to
the Sun.
Right: the apparent
movements of the thenknown planets as observed
from the Earth.
In 1612, Kepler became
mathematician to the states of Oberösterreich
(Upper Austria). In 1619, while living in Linz,
he published his Harmonica Mundi (Harmony
of the World), the final section of which
contains his third law of planetary motion: (3)
the ratio of the cube of a planet's distance from
the Sun and the square of the planet's orbital
period is a constant and is the same for all
planets.
Epitome Astronomiae
Copernicanae (Epitome of Copernican
Astronomy), published between 1618 and 1621,
brought all of Kepler's discoveries together in a
single volume, and became the first textbook of
astronomy to be based on Copernican principles. Tabulae
Rudolfinae (Rudolfine Tables),
published in 1625, consists of new tables of
planetary motion based on Brahe's data.
Kepler also founded modern
optics by giving the earliest acurate account of
the way in which the human eye works. He
correctly identified the pupil as a diaphragm
through which light rays enter the eye and
described the way in which they are refracted. He
suggested that people with blurred vision had an
optical defect which meant that rays were focused
either in front of or just behind their retina.
He then went on explain how telescopes work by
refracting light and brining an enlarged image to
focus in the eyepiece. His work provided the
theoretical basis for subsequent improvements in
telescope design and for the development of
opthalmic optics as a recognized branch of
medicine.
Johannes Kepler died in
Regensburg, on November 15, 1630.
The Galileo Project http://galileo.rice.edu/sci/kepler.html
Nicolaus
Copernicus
Tycho
Brahe
Mars
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