69
orbits, he did not know why the
planets moved as they did. In his
1609 book Astronomia Nova, he
suggested that Mars was being
carried around its orbit by an
angel in a chariot. A year later,
he had changed his mind,
suggesting that the planets
were magnets and were being
driven around by magnetic “arms”
extending from the spinning sun.
Newton’s insight
Before Newton, several scientists,
including Englishman Robert Hooke
and Italian Giovanni Alfonso Borelli,
suggested that there was a force
of attraction between the sun and
the individual planets. They also
stated that the force decreased
with distance.
On December 9, 1679, Hooke
wrote to Newton saying that he
thought the force might decrease
as the inverse square of distance.
However, Hooke did not publish
the idea and did not possess
the mathematical skills to fully
demonstrate his proposition. By
contrast, Newton was able to prove
rigorously that an inverse square
law of attractive force would result
in an elliptical planetary orbit.
Newton used mathematics to
demonstrate that, if the force of
attraction (F) between the sun
and the planets varied precisely as
an inverse square of the distance
(r) between them, this fully explained
the planetary orbits and why they
follow Kepler’s three laws. This
is written mathematically as
F 1/r^2. It means that doubling
the distance between the objects
reduces the strength of the attractive
force to a quarter of the original force.The Great Comet
Newton was a shy, reclusive
man, and reluctant to publish his
breakthrough. Two things forced
his hand. The first was the Great
Comet of 1680, and the second was
the astronomer Edmond Halley.
The Great Comet of 1680 was
the brightest comet of the 17th
century—so bright that for a short
time it was visible in the daytime.
Two comets were seen: one that
was approaching the sun in
November and December 1680; and
another that was moving away from
the sun between late December
1680 and March 1681. As with allcomets at the time, its orbit was
a mystery, and the two sightings
were at first not widely recognized
as the same object. Astronomer
John Flamsteed suggested that the
two sightings might be of the same
comet, which had come from the
outer edge of the solar system,
swung around the sun (where it
was too close to the sun to be
seen), and moved out again.
Halley was fascinated by the
mysterious form of cometary orbits,
and traveled to Cambridge to
discuss the problem with his friend
Newton. Using his law that related
force to acceleration and his
insistence that the strength of the
force varied as the inverse square
of distance, Newton calculated the
parameters of the comet’s orbit as
it passed through the inner solar ❯❯See also: Elliptical orbits 50–55 ■ Halley’s comet 74–77 ■ The discovery of Neptune 106–07 ■
The theory of relativity 146–53 ■ Lagrange (Directory) 336
THE TELESCOPE REVOLUTION
The Great Comet appeared in 1680,
then again in 1681. John Flamsteed
proposed that it was the same comet.
Newton disagreed, but changed his
mind after examining Flamsteed’s data.