The Astronomy Book

92

THE MECHANISM

OF THE HEAVENS

GRAVITATIONAL DISTURBANCES

IN CONTEXT

KEY ASTRONOMER

Pierre-Simon Laplace

(1749 –1827)

BEFORE

1609 Johannes Kepler

determines that the planets

move in elliptical orbits.

1687 Isaac Newton publishes

Principia Mathematica, which

includes his law of universal

gravitation and a mathematical

derivation of Kepler’s laws of

planetary motion.

1734 Swedish philosopher

Emanuel Swedenborg outlines

the nebular theory of the

formation of the solar system.

AFTER

1831 Mary Somerville translates

Laplace’s Méchanique céleste

to English.

1889 French mathematician

Henri Poincaré shows that

it is not possible to prove that

the solar system is stable,

laying the foundations for

chao s t he or y.

B

y the end of the 18th

century, the structure

of the solar system was

well-known. The planets moved

in elliptical orbits around the

sun, held in place by gravity.

Isaac Newton’s laws allowed

a mathematical basis for this

model of the solar system to be

developed, but there were still

problems. Newton himself tested

his ideas against observations,

but noted “perturbations” to the

There are

disturbances in

the mechanism

of the heavens.

Without divine

intervention these

disturbances look like they

should make the orbits of

the planets unstable.

planets’ orbits. By this he meant

a disturbance to the orbits caused

by an additional force, which

would make the orbits unstable if

not corrected. As a result, Newton

decided that the hand of God was

occasionally required to maintain

the solar system in a stable state.

Orbital resonance

French mathematician Pierre-

Simon Laplace rejected the notion

of divine intervention, however. In

But the disturbances continually self-correct over time.

The self-correction is made by the force of

gravity that caused the disturbance itself.