The Science Book

64 ISAAC NEWTON

A

t the time Isaac Newton
was born, the heliocentric
model of the universe, in
which Earth and the other planets
orbit the Sun, was the accepted
explanation for the observed
movements of the Sun, Moon, and
planets. This model was not new,
but had returned to prominence
when Nicolaus Copernicus
published his ideas at the end of
his life in 1543. In Copernicus’s
model, the Moon and each of
the planets revolved in its own
crystalline sphere around the Sun,
with an outer sphere holding the

“fixed” stars. This model was

superseded when Johannes Kepler

published his laws of planetary

motion in 1609. Kepler dispensed

with Copernicus’s crystalline

spheres, and showed that the orbits

of the planets were ellipses, with

the Sun at one focus of each ellipse.

He also described how the speed of

a planet changes as it moves.

What all these models of the

universe lacked was an explanation

of why the planets moved in the

way they did. This is where

Newton came in. He realized

that the force that pulled an

IN CONTEXT

BRANCH

Physics

BEFORE

1543 Nicolaus Copernicus

argues that the planets orbit

the Sun, not Earth.

1609 Johannes Kepler argues

that the planets move freely in

elliptical orbits around the Sun.

1610 Galileo’s astronomical

observations support

Copernicus’s views.

AFTER

1846 Johann Galle discovers

Neptune after French

mathematician Urbain Le

Verrier uses Newton’s laws to

calculate where it should be.

1859 Le Verrier reports that

Mercury’s orbit is not explained

by Newtonian mechanics.

1915 With his general theory

of relativity, Albert Einstein

explains gravity in terms of

the curvature of space-time.

apple toward the center of

Earth was the same force that

kept the planets in their orbits

around the Sun, and demonstrated

mathematically how this force

changed with distance. The

mathematics he used involved

Newton’s three Laws of Motion and

his Law of Universal Gravitation.

Changing ideas

For centuries, scientific thinking

had been dominated by the ideas

of Aristotle, who reached his

conclusions without carrying out

experiments to test them. Aristotle

Why does the apple always fall downward,

never sideways or upward?

Could it actually cause the orbit of the

Moon? In that case...

Gravity affects everything in the universe.

Could this attraction extend beyond the

apple, and reach as far as the Moon? If so,

it would affect the orbit of the Moon.

There must be an attraction toward the

center of Earth.