HALLEY AND HIS COMET
Halley is one of the most picturesque characters in all astronomical
history. Next to Newton himself he was most intimately concerned in
giving the Newtonian law to the world.
Edmund Halley was born (1656) in stirring times. Charles I. had just
been executed, and it was the era of Cromwell's Lord Protectorate and
the wars with Spain and Holland. Then followed (1660) the promising but
profligate Charles II. (who nevertheless founded at Greenwich the
greatest of all observatories when Halley was nineteen), the frightful
ravages of the Black Plague, the tyrannies of James II., and the
Revolution of 1688--all in the early manhood of Halley, whose scientific
life and works marched with much of the vigor of the contending
personalities of state.
The telescope had been invented a half century earlier, and Galileo's
discoveries of Jupiter's moons and the phases of Venus had firmly
established the sun-centered theory of Copernicus.
The sun's distance, though, was known but crudely; and why the stars
seemed to have no yearly orbits of their own corresponding to that of
the earth was a puzzle. Newton was well advanced toward his supreme
discovery of the law of universal gravitation; and the authority of
Kepler taught that comets travel helter-skelter through space in
straight lines past the earth, a perpetual menace to humanity.
"Ugly monsters," that comets always were to the ancient world, the
medieval church perpetuated this misconception so vigorously that even
now these harmless, gauzy visitors from interstellar space possess a
certain "wizard hold upon our imagination." This entertaining phase of
the subject is excellently treated in President Andrew D. White's
"History of the Doctrine of Comets," in the Papers of the American
Historical Association. Halley's brilliant comet at its earlier
apparitions had been no exception.
Halley's father was a wealthy London soap maker, who took great pride in
the growing intellectuality of his son. Graduating at Queen's College,
Oxford, the latter began his astronomical labors at twenty by publishing
a work on planetary orbits; and the next year he voyaged to St. Helena
to catalogue the stars of the southern firmament, to measure the force
of terrestrial gravity, and observe a transit of Mercury over the disk
of the sun.
While clouds seriously interfered with his observations on that lonely
isle, what he saw of the transit led to his invention of "Halley's
method," which, as applied to the transit of Venus, though not till long
after his death, helped greatly in the accurate determination of the
sun's distance from the earth. Halley's researches on the proper motions
of the stars of both hemispheres soon made him famous, and it was said
of him, "If any star gets displaced on the globe, Halley will presently
find it out."
His return to London and election to the Royal Society (of which he was
many years secretary) added much to his fame, and he was commissioned
by the society to visit Danzig and arbitrate an astronomical controversy
between Hooke and Hevelius, both his seniors by a generation.
On the continent he associated with other great astronomers, especially
Cassini, who had already found three Saturnian moons; and it was then he
observed the great comet of 1680, which led up to the most famous event
of Halley's life.
The seerlike Seneca may almost be said to have predicted the advent of
Halley, when he wrote ("Quaestiones Naturales," vii): "Some day there
will arise a man who will demonstrate in what region of the heavens
comets pursue their way; why they travel apart from the planets; and
what their sizes and constitution are. Then posterity will be amazed
that simple things of this sort were not explained before."
To Newton it appeared probable that cometary voyagers through space
might have orbits of their own; and he proved that the comet of 1680
never swerved from such a path. As it could nowhere approach within the
moon's orbit, clearly threats of its wrecking the earth and punishing
its inhabitants ought to frighten no more.
Halley then became intensely interested in comets, and gathered whatever
data concerning the paths of all these bodies he could find. His first
great discovery was that the comets seen in 1531 by Apian, and in 1607
by Kepler, traveled round the sun in identical paths with one he had
himself observed in 1682. A still earlier appearance of Halley's comet
(1456) seems to have given rise to a popular and long-reiterated myth of
a papal bull excommunicating "the Devil, the Turk, and the Comet."
No longer room for doubt: so certain was Halley that all three were one
and the same comet, completing the round of its orbit in about
seventy-six years, that he fearlessly predicted that it would be seen
again in 1758 or 1759. And with equal confidence he might have foretold
its return in 1835 and 1910; for all three predictions have come true to
the letter.
Halley's span of existence did not permit his living to see even the
first of these now historic verifications. But we in our day may
emphatically term the epoch of the third verified return _Annus
Halleianus_.
Says Turner, Halley's successor in the Savilian chair at Oxford to-day:
"There can be no more complete or more sensational proof of a scientific
law, than to predict events by means of it. Halley was deservedly the
first to perform this great service for Newton's Law of Gravitation, and
he would have rejoiced to think how conspicuous a part England was to
play in the subsequent prediction of the existence of Neptune."
Halley rose rapidly among the chief astronomical figures of his day. But
he had little veneration for mere authority, and the significant veering
of his religious views toward heterodoxy was for years an obstacle to
his advance.
Still Halley the astronomer was great enough to question any
contemporary dicta that seemed to rest on authority alone. Everyone
called the stars "fixed" stars; but Halley doubting this, made the first
discovery of a star's individual motion--proper motion, as astronomers
say. To-day, two hundred years after, every star is considered to be in
motion, and astronomers are ascertaining their real motions in the
celestial spaces to a nicety undreamed of by even the exacting Halley.
The moon, of priceless service to the early navigator, was regarded by
all astronomers as endowed with an average rate of motion round the
earth that did not vary from age to age. But Halley questioned this too;
and on comparing with the ancient value from Chaldean eclipses, he made
another discovery--the secular acceleration of the moon's mean motion,
as it is technically termed. This was a colossal discovery in celestial
dynamics; and the reason underlying it lay hidden in Newton's law for
yet another century, till the keener mathematics of Laplace detected its
true origin.
* * * * *
With Newton, Halley laid down the firm foundations of celestial
mechanics, and they pushed the science as far as the mathematics of
their day would permit. Halley, however, was not content with
elucidating the motion of bodies nearest the earth, and pressed to the
utmost confines of the solar system known to him. Here, too, he made a
signal discovery of that mutual disturbance of the planets in their
motion round the sun, called the great inequality of Jupiter and Saturn.
Halley's versatile genius attacked all the great problems of the day.
His observation of the sun's total eclipse in 1715 is the earliest
reliable account of such a phenomenon by a trained astronomer. He
described the corona minutely and was the first to see that other
interesting phenomenon which only an alert observer can detect, which a
great astronomer of a later day compared to the "ignition of a fine
train of gunpowder," and which has ever since borne the name of "Baily's
beads."
Besides being a great astronomer, Halley was a man of affairs as well,
which Newton, although the greater mathematician, was not. Without
Halley, Newton's superb discovery might easily have been lost to the age
and nation, for the latter was bent merely on making discoveries, and on
speculative contemplation of them, with never a thought of publishing to
the world.
Halley, more practical and businesslike, insisted on careful writing out
and publication. Newton was then only forty-two, and Halley fully
fourteen years his junior. But the philosophers of that day were keenly
alive to the mystery of Kepler's laws, and Halley was fully conscious of
the grandeur and far-reaching significance of Newton's great
generalization which embodied all three of Kepler's laws in one.
Newton at last yielded, though reluctantly, and the "Principia" was
given to the world, though wholly at Halley's private charges.
But Halley was far from being completely engrossed with the absorbing
problems of the sky; things terrestrial held for years his undivided
attention. Imagine present-day Lords Commissioners of the Admiralty
intrusting a ship of the British navy to civilian command. Yet such was
their confidence in Halley that he was commissioned as captain of H.
M.'s pink _Paramour_ in 1698, with instructions to proceed to southern
seas for geographical discoveries, and for improving knowledge of the
longitude problem, and of the variations of the compass. Trade winds and
monsoons, charts of magnetic variation, tides and surveys of the Channel
coast, and experiments with diving bells were practical activities that
occupied his attention.
Halley in 1720 became Astronomer Royal. He was the second incumbent of
this great office, but the first to supply the Royal Observatory with
instruments of its own, some of which adorn its walls even to-day. His
long series of lunar observations and his magnetic researches were of
immense practical value in navigation.
Halley lived to a ripe old age and left the world vastly better than he
found it. His rise from humblest obscurity was most remarkable, and he
lived to gratify all the ambitions of his early manhood. "Of attractive
appearance, pleasing manners, and ready wit," says one of his
biographers, "loyal, generous, and free from self-seeking, he was one of
the most personally engaging men who ever held the office of Astronomer
Royal."
He died in office at Greenwich in 1742.
"Halley was buried," says Chambers, "in the churchyard of St.
Margaret's, Lee, not far from Greenwich, and it has lately been
announced that the Admiralty have decided to repair his tomb at the
public expense, no descendants of his being known." There is no suitable
monument in England to the memory of one of her greatest scientific men.
In any event the collection and republication of his epoch-making papers
would be welcomed by astronomers of every nation.