The Astronomy Book

76

interested in discussing what force
might account for the motions of
planets and other celestial bodies
such as comets. Newton told his
astonished visitor that he had been
studying the matter himself and
had already solved the problem (the
answer was gravity), but that he
had not yet published his findings.
This meeting eventually led to
Halley editing and financing the
publication in 1687 of Newton’s

great book on gravity and the laws

of motion, Philosophiae Naturalis

Principia Mathematica.

Historical records

Halley suggested to Newton that

he might apply his new theory to

studying the orbits of more comets.

However, Newton’s mind had

turned to other matters so, from the

early 1690s, Halley conducted his

own detailed study. In all, over a

period of more than 10 years, he

studied the orbits of 24 comets—

some that he had observed himself,

and others for which he had

obtained data from historical

records. He suspected that, while

some comets followed paths that

are parabolas (open-ended curves)

as Newton had proposed, others

followed elliptical orbits, meaning

that they might pass through the

inner solar system, and thus

become visible from Earth, more

than once in a person’s lifetime.

During his studies, Halley

had noticed something strange.

In general, the orbit of each comet

HALLEY’S COMET

had a few characteristics that

clearly distinguished it from the

orbits of other comets, such as its

orientation in relation to the stars.

However, three of the comets he

had studied—one he had seen

himself in 1682, and others

observed by Kepler in 1607, and

Petrus Apianus in 1531—seemed

to have remarkably similar orbits.

He suspected that these were

Some comets follow a parabolic (A),

or hyperbolic (B) path, meaning that they

will never return. Others follow elliptical

curves of varying extent (C). Halley

suggested that if a comet followed a

moderately stretched elliptical curve (D)

it could return every 50 to 100 years.

The comet

will reappear

around 1758.

Three comets of 1531,

1607, and 1682 had very

similar orbits.

The small differences

in their orbits can be

accounted for in terms of

the gravitational pull

of Jupiter and Saturn.

The three comets

are therefore the same

comet, which reappears

every 75–76 years.

Even in an age renowned for

unusual savants, Halley stands

out as a man of extraordinary

breadth and depth.

J. Donald Fernie

Professor Emeritus of Astronomy

at the University of Toronto

Hyperbola

Parabola (ellipse stretched to infinity)

Ellipse (moderately stretched)

Ellipse

Earth

Sun

A

B

C

D