43
that the planets were carried on
invisible spheres, but Tycho had
demonstrated that comets travel
unhindered through interplanetary
space, seeming to contradict this
idea. Kepler thought that some
influence from the sun impelled
the planets, but he had no scientific
means to describe it.Universal gravitation
It fell to Isaac Newton to describe
the force responsible for the
movement of the planets, with a
theory that remained unchallenged
until Einstein. Newton concluded
that celestial bodies pull on each
other and he showed mathematically
that Kepler’s laws follow as a natural
consequence if the pulling force
between two bodies decreases
in proportion to the square of the
distance between them. Writingabout this force, Newton used the
word gravitas, Latin for weight, from
which we get the word gravity.Improving telescopes
Newton not only created a
new theoretical framework for
astronomers with his mathematical
way of describing how objects
move, but he also applied his
genius to practical matters.
Early telescope makers found it
impossible to obtain images free
from colored distortion with their
simple lenses, although it helped
to make the telescope enormously
long. Giovanni Domenico Cassini,
for example, used long “aerial”
telescopes without a tube to
observe Saturn in the 1670s.
In 1668, Newton designed and
made the first working version of
a reflecting telescope, which didnot suffer from the color problem.
Reflecting telescopes of Newton’s
design were widely used in the
18th century, after English inventor
John Hadley developed methods
for making large curved mirrors
of precisely the right shape from
shiny speculum metal. James
Bradley, Oxford professor and
later Astronomer Royal, was one
astronomer who was impressed
and acquired a reflector.
There were also developments
in lens-making. In the early-18th
century, English inventor Chester
Moore Hall designed a two-part
lens that greatly reduced color
distortion. The optician John
Dollond used this invention to build
much-improved refracting telescopes.
With high-quality telescopes
now widely available, practical
astronomy was transformed. ■THE TELESCOPE REVOLUTION
1659
1675
1676
1687
1705
1725
Dane Ole Rømer measures
the speed of light by
observing eclipses of
Jupiter’s moon Io.Isaac Newton publishes
Principia, in which he lays
out his universal law
of gravitation.English astronomer
Edmond Halley
predicts the return of
the comet that now
bears his name.Giovanni Domenico
Cassini spots a gap in
Saturn’s rings and
concludes correctly that
they are not solid.Dutch astronomer
Christiaan Huygens
correctly describes
the shape of
Saturn’s rings for
the first time.
James Bradley
proves that Earth
is moving by
demonstrating
an effect called
stellar aberration.