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.

December

28, 2010