54 The Poetry of Physics and The Physics of Poetry
Law 1: The orbit of every planet is an ellipse with the Sun at a focus.
Law 2: A line joining a planet and the Sun sweeps out equal areas
during equal intervals of time.
Law 3: The square of the orbital period of a planet is directly
proportional to the cube of the semi-major axis of its orbit.
Origin of the Concept of Gravity
Kepler’s three laws of planetary motion so crucial to Newton’s
formulation of classical mechanics describe only the motion of the
planets and give no hint of the gravitational attraction, which keeps the
planets bound to the Sun. Unless the Sun were exerting an attractive
force on the planets they would fly off into space along a tangent to their
orbit since according to the law of inertia a body will continue in
constant speed straight-line motion unless a force is acting on it. The
reason the planets do not fall into the Sun as a result of this attraction,
which lies along the line joining the Sun and the planet, is due to the fact
that a balance is struck between the Sun’s attraction and the centrifugal
force outwards due to the planet’s motion. The net effect of the force is
essentially to change the direction of the planet’s motion without
changing its speed so that a nearly circular orbit results. The tremendous
contribution of Newton (and Hooke as well who made the discovery
independently) was to realize that the force by which the Sun attracts the
planets is the same as the one, which causes objects to fall to Earth. This
presumably was the idea that dawned on Newton when according to
legend the apple fell on his head.
The story of the modern development of the concept of gravity is an
interesting one. It begins with Copernicus who believed that each of the
heavenly bodies had its own local gravitational system. So in analogy to
Empedocles idea that Earth attracted earth, water water, air air and fire
fire, Copernicus believed that Moon attracts pieces of moon, the Sun
pieces of sun and the Earth pieces of earth. In Copernicus’ thinking there
was no mutual attraction of the heavenly bodies and presumably a piece
of moon would not fall to Earth but rather would fly up to the Moon and
vice versa with a piece of earth on the Moon.
William Gilbert who published a book on magnets in 1600 played an
important role in the development of the concept of gravity. He believed
that the bulk of the interior of the Earth was lodestone and that magnetic
attraction was the true cause of gravity. Gilbert believed the other