• 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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