The heavens have long been a subject of human fascination and study.
The regular motion of the stars and planets were at once a symbol of the
divine order of the universe and a profound challenge for human
understanding. For the ancient Greeks the separation between the
terrestrial and celestial realms was absolute--the downward motion of
falling objects was thought of as a "natural tendency" towards the center
of the earth. The Greeks believed explaining motion on earth was a completely
different problem from explaining why the earth went around the sun.
It was Nicholas Copernicus who first proposed, in 1543, that the
earth moved around the sun. His book, De Revolutionibus Orbium
Caelestium (On the Revolution of the Celestial Orbs) created a
revolution in science that brought scientists such as Galileo into
conflict with the Catholic Church. Even Kepler
believed the heavens a mystical realm that obeyed a mathematical order unlike
anything on earth. His three laws of planetary motion were based on the
synthesis of a huge amount of observational data, compiled over many centuries.
However, although fundamentally correct, Kepler's Laws were purely
empirical; they facilitated prediction of planetary motion but did not explain
why the planets should move in the way they did.
It was Sir Isaac Newton who not only provided
this explanation in his famous inverse square law of gravitation, but managed to
"synthesize" the explanation of motion on earth and motion in the heavens. This
had profound philosophical and scientific consequences. The unification into
what became the laws of gravitation became a
symbol of the predictive and quantitative power of science. The fact that a
single law could explain the motion of a cannonball and the motion of Mars
revolutionized our understanding of our place in the universe.
In the centuries after Newton, scientists and mathematicians created powerful
techniques and concepts for understanding
complex phenomena such as tides and perturbations. In 1915 Albert Einstein
published a new theory of gravitation (The General Theory of Relativity) that
conceived of gravitational effects as caused by curvature in a four-dimensional
space-time surface. Although Einstein's theory explains some observations that
Newton's cannot, the inverse square law has remained an important tool in modern
astrophysics and cosmology. Moreover, Newton's and Kepler's laws of gravitation
can explain almost all the phenomena of orbits
and terrestrial motion with which we will be concerned here.
