Johannes Kepler: Father of Celestial Mechanics

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December 27
marks the birthday of the Father of Celestial Mechanics, Johannes
Kepler. Born in 1571, he went on to become one of the most important
scientists in the field of astronomy as the first person to explain
the laws of planetary motion. He also made important advances in
the fields of optics, geometry and calculus. Kepler is credited
with explaining how the moon influences the tides and with determining
the exact year of Christ’s birth.

Johannes was
the eldest of six children, three of whom died in infancy. His father
abandoned the family when Johannes was only 5 years old. Plagued
by ill health in his childhood, he was drawn toward his studies
rather than more physically demanding work. In 1594 he became a
professor of astronomy and mathematics in Austria, and two years
later published his first work entitled Mysterium
Cosmographicum,
which received critical acclaim. He was named Imperial Mathematician
in 1601 by the Holy Roman Emperor, Rudolph II, and in that capacity
published his first two laws of planetary motion under the title
Astronomia
Nova
. Ten years after that work, in 1619, the culmination
of Kepler’s research appeared in five books entitled De
Harmonices Mundi

(The Harmonies of the World)
, which included his third law.
The three principles are summed up as follows:

  • The planets
    circle the sun in elliptical orbits with the sun’s center of mass
    as one of two foci;

  • A straight
    line joining each planet with the sun (i.e., the radius vector)
    sweeps out equal areas in equal intervals of time;
  • The square
    of a planet’s orbital period is proportional to the cube of the
    semi-major axis of its orbit. This law determines the relationship
    of the distance of planets from the sun to their orbital periods.

The distinguishing
aspect of Kepler’s studies from that of scientists such as Copernicus
and Galileo was that he did not confine himself to the examination
of our solar system but searched for universal principles governing
celestial movement. He was the first to determine the elliptical
orbits of heavenly bodies, obliterating the time-honored yet erroneous
principle of circular celestial motion.

When Kepler
learned of the Copernican theory of heliocentrism, he dedicated
his studies to the discovery of these laws. Initially, he attributed
Earth’s rotation on its axis to an animating principle of universal
attraction; later, Sir Isaac Newton would expand and perfect this
hypothesis in his law of gravitation which states that bodies are
attracted to each other in direct proportion to their mass and inversely
as to the square of the distance between them.

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December
28, 2010

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