77
successive reappearances, which
occurred about once every 75 to
76 years, of the same comet, which
was traveling on a closed, elliptical
orbit. In 1705, Halley outlined his
ideas in a paper called Astronomiae
cometicae synopsis (A synopsis of
the astronomy of comets). He wrote:
“Many considerations incline me to
believe the Comet of 1531 observed
by Apianus to have been the same
as that described by Kepler and
Longomontanus in 1607 and which
I again observed when it returned
in 1682. All the elements agree.
Whence I would venture confidently
to predict its return, namely in
the year 1758.”
One uncertainty still worried
Halley. The time intervals between
the three appearances were not
precisely the same—they differed
by about a year. Remembering
research he had done some years
earlier on Jupiter and Saturn,
Halley suspected that the
gravitational pull from these two
giant planets might slightly throw
the comet off its course and delay
its timing. Halley asked Newton
to reflect on this problem, and
Newton came up with gravitational
calculations by which Halley was
able to refine his forecast. His
revised prediction was that the
comet would reappear either in
late 1758 or in early 1759.
Halley is proved right
Interest in Halley’s prediction
spread throughout Europe. As
the year of the comet’s predicted
return approached, three French
mathematicians—Alexis Clairaut,
Joseph Lalande, and Nicole-Reine
THE TELESCOPE REVOLUTION
Lepaute—spent several arduous
months recalculating when it
might reappear, and where
it might first be seen in the night
sky. Amateur and professional
astronomers alike began watching
for the comet’s return as early as- On December 25, 1758, it
was finally spotted by Johann
Palitzsch, a farmer and amateur
astronomer from Germany.
The comet passed closest to the
sun in March 1759, only a couple
of months later than Halley had
predicted. By then, Halley had
been dead for 17 years, but the
comet’s reappearance brought
him posthumous fame. The comet
was named Halley’s comet in his
honor by the French astronomer
Nicolas-Louis de Lacaille.
Halley’s comet was the first
object other than a planet that
had been proven to orbit the sun.
It also provided one of the earliest
proofs of Newton’s theory of gravity,
demonstrating that the theory
could be applied to all celestial
bodies. Comets themselves, once
feared as unpredictable omens of
ill fortune, were now understood.
Subsequent research found
that the comet had made regular
appearances going back to at
least 240 bce, including some
particularly bright apparitions in
87 bce, 12 bce, 837 ce, 1066, 1301,
and 1456. In 1986, the comet was
closely approached by spacecraft,
which provided data on the
structure of its nucleus (solid part)
and its tail. It is the only known
short-period comet (comet with
an orbit of less than 200 years) that
may be seen with a naked eye and
appear twice in a human lifetime. ■On its last appearance in 1986,
Halley’s comet passed to within
0.42 astronomical units (AU) of Earth.
It has passed much closer. In 1066,
for instance, it came within 0.1 AU.
Aristotle’s opinion
that comets were nothing
else than sublunary vapors
prevailed so far that this
sublimest part of astronomy
lay altogether neglected.
Edmond Halley