SCIENTIFIC REVOLUTION 67
which a ball is fired, the farther
away it will land. If it is launched
sufficiently fast, it will not land at
all, but continue around Earth until
it arrives back at the top of the
mountain. In the same way, a
satellite launched into orbit at
the correct speed will continue
to circle Earth. The satellite is
continually being accelerated
by Earth’s gravity. It moves at a
constant speed, but its direction
is continually changing, making
it circle the planet rather than
whizzing off into space in a straight
line. In this case, Earth’s gravity
only changes the direction of the
satellite’s velocity, not its speed.Publishing the ideas
In 1684, Robert Hooke boasted
to his friends Edmond Halley
and Christopher Wren that he
had discovered the laws of
planetary motion. Halley was a
friend of Newton, and asked him
about this. Newton said that he
had already solved the problem,but had lost his notes. Halley
encouraged Newton to redo the
work, and as a result, Newton
produced On the Motion of Bodies
in an Orbit, a short manuscript sent
to the Royal Society in 1684. In this
paper, Newton showed that the
elliptical motion of the planets that
Kepler described would result from
a force pulling everything toward
the Sun, where that force was
inversely proportional to the
distance between the bodies.
Newton expanded on this work,
and included other work on forces
and motion, in the Principia
Mathematica, which was published
in three volumes and contained,
among other things, the Law of
Universal Gravitation and Newton’s
three Laws of Motion. The volumes
were written in Latin, and it was
not until 1729 that the first English
translation was published, based
on Newton’s third edition of the
Principia Mathematica.
Hooke and Newton had already
fallen out over Hooke’s criticisms
of Newton’s theory of light.
Following Newton’s publication,
however, much of Hooke’s work
on planetary motion was obscured.
However, Hooke had not been the
only one to suggest such a law, and
he had not demonstrated that it ❯❯To myself I am only a child
playing on the beach, while
vast oceans of truth lie
undiscovered before me.
Isaac NewtonNewton’s thought experiment described a cannon
fired horizontally from a high mountain. The greater the
force firing the cannon ball, the farther it travels before
falling to the ground. If it is fired hard enough, it will
travel right around the planet back to the mountain.
If a cannon ball is fired with
insufficient speed, gravity will pull
it to Earth (A and B). If fired with
sufficient speed, it will orbit Earth (C).ABC