Astronomy - February 2014

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6 ASTRONOMY t FEBRUARY 2014

BY DAVID J. EICHER

FROM THE EDITOR

Editor David J. Eicher

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C

omet ISON (C/

S1) is now a memory,

and we’ve begun pub-

lishing some galleries

of the best images read-

ers shot of it (see p. 50).

Humans didn’t know what

comets were or how they

moved until the time of Isaac

Newton in the 17th century.

When it came to progress

in understanding comets, few

eras could match the mean-

ing of Newton’s renewed

interest in the subject that

accompanied the comet that

would variously come to be

known as C/1680 V1, the

Great Comet of 1680, or

Kirch’s Comet. This, the first

comet to be found telescopi-

cally, made a huge impact on

the interpretation of comets.

Newton observed the

comet and calculated details

of its physical nature. At first,

he believed comets seen in

November and December

1680 were two different

objects — on different sides

of the Sun — moving on rec-

tilinear paths. But after seeing

correspondence about the

comet between his colleagues

John Flamsteed and Edmond

Halley, Newton began to pon-

der this comet seriously.

On February 28, 1681,

Newton still believed that the

November and December

observations represented two

objects. He offered some

advice on what he thought

was Flamsteed’s incorrect

conclusion that the observa-

tions were of the same comet,

but with the object having

moved to the other side of the

Sun. He pointed out that

magnetism could not be

responsible for pulling the

comet around the Sun

because a red-hot lodestone

loses its magnetism, and the

Sun is certainly quite warm.

Newton argued that if the

observations represented the

same comet, then it would

have had to undergo rapid

acceleration and deceleration,

which wouldn’t make sense.

But Newton had been

using the observations that

were available, and some were

flawed. When he replied on

March 7, 1681, Flamsteed

corrected the erroneous

observations and suggested

that the Sun’s magnetism

might not be like that of a

lodestone. Newton held on

but began to convert to the

one-comet hypothesis.

Three years later, Newton

had come to accept the idea

that comets travel in closed

elliptical orbits. It was partly

for the purpose of explaining

cometary orbits that he set

about to produce his master-

work on gravity and allied

subjects, Mathematical Prin-

ciples of Natural Philosophy,

in 1687. None other than

Edmond Halley underwrote

the publication.

In the last of the work’s

three substantial books, New-

ton outlined his method for

determining the parabolic

orbits of comets. He

employed three identical

observations that were nearly

evenly spaced in time. As an

example, he provided the

details for his analysis of the

orbit of the Great Comet of

1680. The method was still
      somewhat crude relative to
      modern precision, but now
      completely correct in spirit
      and analytical approach.
      Fred Whipple, the man
      who defined what comets are,
      was not born yet. But in 1,
      years, philosophers, astrolo-
      gers, and astronomers had
      gone from pure superstition
      and speculation on the nature
      of comets to a mathematical
      understanding of them and
      their place among other
      celestial bodies.

Yours truly,

David J. Eicher

Editor

The

realization

of comets

Comet ISON (C/2012 S1) imaged in

late October 2013. DAMIAN PEACH