Edmund Halley and his Comet
By Patrick Mondout
Comets are almost always named after their discoverers, so what is so
special about Edmund Halley? For that, lets go back to the 16th century.
For a good portion of human history, comets were thought to be
bringers of bad fortune, ill health, and even death. This fear of comets
was still in the collective unconscious of the vast majority of people in
Halley's time (indeed, the 1910 return of Halley's
Comet was successfully exploited by snake-oil salesmen even here in
the United States). Their origins were a mystery and they appeared
unpredictably. All of that changed because of the mathematical work of Mr.
Great Comet of 1680
Our story takes us to 1680 -- the year a great comet was seen in the
Northern skies. Halley had taken to the southern seas to observe and
catalog the stars in the Southern Hemisphere. On his way back home, he saw
the comet and quickly headed for Paris where the great observational
astronomer Jean-Dominique Cassini, who had also discovered four of
Saturn's moons, had first discovered the comet at the Paris Observatory. The
24-year-old Halley was impressed with Cassini but particularly taken with
a theory of his: that the comet of 1680 was actually the same comet that
had appeared in 1577. No one at this point of history had
calculated the orbit of any comet so this theory, if confirmed, would
change our understanding of our universe forever. From this point forward,
Halley was hooked. He spent countless hours for the rest of his life
working on these theories.
In 1682, another great comet streaked across the skies. Halley compared
observations of the comets of 1531 and 1607 with the best mathematical
observations of the "new" comet. He found them to be similar. He
also found that that elliptical orbits using Newtonian theories of
planetary motion fit the observed path of the comet and the
periodic return of the comet.
Newton's Weighty Ideas
In 1705, Halley published a work entitled "A Synopsis of the
Astronomy of Comets." In it, using Isaac Newton's newly formulated
laws of motion, Halley claimed that the comets seen in 1531, 1607, and
1682 were the same comet and that it would return again in 1758.
Halley did not live to see his if his prediction would be
validated. He died in 1742 at the age of 86 and
contemporary obituaries failed to even note the prediction - it had been
all but forgotten.
Was He Right?
However, 16 years later, the astronomical world eagerly anticipated the
return of the comet. Additional calculations of the orbit of the comet
relative to the planets showed that the gravitational pull of Saturn was
going to delay the return of the comet by about 3 months. However, on
Christmas Day, 1758, a German farmer named Johann Palitzsch became the
first to know for sure that Halley had been right. Perhaps the greatest
prediction of all time had been proven accurate. From this point forward,
generations of skywatchers would anticipate the return of Halley's comet
every 76 years or so.
What About Cassini?
You might wonder why Cassini isn't given more credit for this
discovery. First, he made no predictive theory. That is, he was unable to
predict the return of a comet. Second, his theory that the comet of 1680
was the same as the comet of 1577 was proven to not be the case. (Many
comets may have elliptical orbits, but those were two separate comets.)