68 ISAAC NEWTON
worked. Newton had shown that
his Law of Universal Gravitation
and laws of motion could be used
mathematically to describe the
orbits of planets and comets,
and that these descriptions
matched observations.
Sceptical reception
Newton’s ideas on gravity were
not welcomed everywhere. The
“action at a distance” of Newton’s
force of gravity, with no way
of explaining how or why it
happened, was seen as an
“occult” idea. Newton himself
refused to speculate on the nature
of gravity. For him, it was enough
that he had shown that the idea
of an inverse-square attraction
could explain planetary motions,
so the mathematics was correct.
However, Newton’s laws described
so many phenomena that they soon
came to be widely accepted, and
today the internationally used unit
of force is named after him.Using the equations
Edmond Halley used Newton’s
equations to calculate the orbit
of a comet seen in 1682, and
showed that it was the same comet
as that observed in 1531 and 1607.
The comet is now called Halley’s
comet. Halley successfully
predicted that it would return in
1758, which was 16 years after his
death. This was the first time that
comets had been shown to orbit the
Sun. Halley’s comet passes close to
Earth every 75–76 years, and was
the same comet as that seen in
1066 before the Battle of Hastings
in southern England.
The equations were also used
successfully to discover a new
planet. Uranus is the seventh planet
from the Sun, and was identified
as a planet by William Herschel
in 1781. Herschel found the planet
by chance while making careful
observations of the night sky.
Further observations of Uranus
allowed astronomers to calculate
its orbit and to produce tables
predicting where it could be
observed at future dates. These
predictions were not always
correct, however, leading to the
idea that there must be another
planet beyond Uranus whose
gravity was affecting the orbit of
Uranus. By 1845, astronomers had
calculated where this eighth planet
should be in the sky, and Neptune
was discovered in 1846.Problems with the theory
For a planet with an elliptical orbit,
the point of closest approach to the
Sun is called the perihelion. If there
were only one planet orbiting theNewton’s laws provided the tools
to calculate the orbits of heavenly
bodies such as Halley’s comet,
shown here on the Bayeux Tapestry
after its appearance in 1066.Why should that apple always
descend perpendicularly to the
ground, thought he to himself...
William Stukeley