75
See also: The Tychonic model 44–47 ■ Elliptical orbits 50–55 ■
Gravitational theory 66–73
observational data from different
parts of Europe, the Danish
astronomer Tycho Brahe calculated
that the comet must be at least four
times farther away than the moon,
and this allowed him to fit comets
into his model of the universe.
He saw them as objects that
could move freely through the
same regions of space as planets.
But what was not agreed on in
Brahe’s time, nor for many decades
afterward, was the shape of the
paths that comets carved through
space. Brahe’s one-time student
Johannes Kepler believed that they
traveled in straight lines. Polish
astronomer Johannes Hevelius,
however, suggested that a comet
of 1664 had traveled in a curved
orbit around the sun.
Newton tackles comets
From about 1680, stimulated by
the appearance of a particularly
bright comet that year, the great
English scientist Isaac Newton
began studying cometary orbits
while developing his universal
theory of gravitation. Using his
new theory, Newton analyzed and
predicted the future path that the
1680 comet would take. He came
to the conclusion that comets
(like planets) had orbits in the
shapes of ellipses, with the sun
at one focus of the ellipse. These
ellipses were so stretched out,
however, that they could be
approximated to an open-ended
curve called a parabola. If Newton
was right, then once a comet had
visited the inner solar system and
curved around the sun, it would
either never return (if its orbit was
parabolic) or would not return for
thousands of years (if its orbit was
an extremely stretched-out ellipse,
but not a parabola).
In 1684, Newton received a
visit from a young acquaintance
named Edmond Halley, who was ❯❯THE TELESCOPE REVOLUTION
Halley’s comet appeared in 1066
and is shown in the Bayeaux Tapestry,
with Anglo-Saxons pointing fearfully
at the sky. Its appearance was taken
by some to foretell the fall of England.Edmond Halley
Edmond Halley was born
in 1656 in London. In 1676,
he sailed to the island of
St. Helena in the South
Atlantic where he charted
the stars of the southern
hemisphere, publishing a
catalog and star charts after
his return. In 1687, he helped
persuade Isaac Newton to
publish Principia, which
included details on how to
calculate cometary orbits.
Halley was appointed
Astronomer Royal in 1720,
and he resided at the Royal
Observatory, Greenwich,
until his death in 1742.
Although remembered mainly
as an astronomer, Halley
did important work in many
fields. He published studies
on variations in Earth’s
magnetic field; invented and
tested a diving bell; devised
methods for calculating life
insurance premiums; and
produced oceanic charts of
unprecedented accuracy.Key works1679 Catalogus Stellarum
Australium
1705 Astronomiae cometicae
synopsis
1716 An Account of Several
Nebulae