The great French astronomer Lalande considered Halley the greatest
astronomer of his time. This opinion is still held. Halley’s “time”
means the age of Kepler, Sir Isaac Newton, Flamsteed, Hevelius, and
Leibnitz, all of whom achieved first rank in Astronomy.
Halley’s greatest achievement in Astronomy was the discovery that our
solar system was but an atom in immeasurable space whence wandering
stars could be caught within the influence of our Sun, our Earth and
the other Planets swinging around our Sun.
Halley was the first to discover and to prove that the Comets that come
within the vision of man have fixed periods of return. He made this
discovery during the appearance of the great Comet of 1682, which has
since been known by his name.
In his studies of the motions of Comets, of which Halley computed the
orbits of twenty-four, he observed that a Comet of similar phenomena,
recorded by Appian in 1531 and by Kepler in 1607, had swung through the
same orbit as the Comet under his observation in 1682. Halley surmised
from this that these Comets might be one and the same, whose intervals
of return appeared to cover a period of seventy-five or seventy-six
years. Halley’s surmise seemed to be confirmed by the recorded
appearance of similar bright Comets in the years 1456, 1378, and 1301,
the intervals again being seventy-five or seventy-six years.
Halley was deeply imbued with Newton’s new discovery of gravitation,
for the publication of which Halley paid the expenses, so he brought
the principles of Newton’s theory of gravitation to bear on his own new
theory of the motions of Comets. He rightly conjectured that Comets
were drawn to our Sun across the disturbing orbits of our planetary
system, and that the comparatively small differences of one or two
years in the recorded intervals of this one Comet (Halley’s Comet) were
due to the attraction of the larger planets.
During the previous year, 1681, Halley computed that the Comet had
passed near the planet Jupiter, the attraction of which must have had a
considerable influence on the Comet’s motion. Making due allowance for
this disturbing influence of Jupiter, he computed that the Comet would
return to the vicinity of our Sun about the end of 1758 or beginning of
1759.
Halley did not live to see his prediction fulfilled (he died in 1742),
but he wrote shortly before he died: “If this Comet should return
according to our predictions about the year 1758, impartial posterity
will not refuse to acknowledge that this was first discovered by an
Englishman.”
All through the year 1758 the most noted astronomers of Europe were
on the lookout for the return of the predicted Comet. One of these
astronomers, Messier, looked for it through his telescope at the Paris
Observatory every night from sunset to sunrise throughout that whole
year. On Christmas night, 1758, the Comet was first seen by a German
peasant near Dresden, who had heard about the Comet and was looking
for it. He was a man of unusually good eyesight, yet his discovery was
doubted until Messier, nearly a month afterward, at Paris, “picked up”
the Comet with his telescope.
From that time forth this Comet, which returned in 1835, and is
reappearing in this year (1910), has been known as Halley’s Comet.
[Illustration: EDMUND HALLEY.]
Besides this achievement, Halley accomplished many other noteworthy
feats in astronomy, such as his discovery of the proper motions of the
fixed stars; his detection of the “long inequality” of Jupiter and
Saturn, and of the acceleration of the moon’s mean motion; his theory
of variation, including the hypothesis of various magnetic poles, with
his suggestion of the magnetic origin of the aurora borealis; and his
indication of a method still used for determining the solar parallax by
means of the transits of Venus.
On the strength of these achievements, Halley for many years was
elected to serve as secretary to the Royal Society. Commissioned as a
Captain in the Royal Navy, he also commanded a vessel on a long cruise
of exploration, and late in life he was made Astronomer Royal.
Although in his sixty-fourth year, he then undertook to observe the
moon through an entire revolution of her nodes (eighteen years), and
actually carried out his purpose. To appreciate the full significance
of so painstaking an achievement it should be borne in mind that
astronomical observations must be made in a temperature equal to that
of the open air. Observatories cannot be heated because the heat would
impair the accuracy of the instruments.
Great astronomers, like poets, are born, not made. Edmund Halley was
one of these. At the age of seventeen he had already observed the
change in the variations of the compass. At nineteen he was recognized
as an astronomer of reputation, having supplied a new and improved
method of determining the elements of the planetary orbits. His
detection of considerable errors in the tables then in use led him to
the conclusion that a more accurate determination of the places of the
fixed Stars was indispensable to the progress of astronomy. With this
end in view he set out on a voyage to the other side of the globe, St.
Helena, where he undertook the task of making complete new observations
of the entire Southern Hemisphere. Though the Heavens proved clouded
he succeeded within two years in registering three hundred and sixty
stars, a colossal achievement which won for him the title of the
“Southern Tycho.” This was when Halley was barely of age.
(The famous astronomer Tycho Brahe, long before this had won his fame
by mapping the stars of the Northern Heavens.)
No one could well have begun with prospects more remote from so high a
career, for Edmund Halley was born in 1656, the son of a soap boiler
in a shabby London suburb. From the refuse of rancid fat and lye the
boy was rescued by friends, who procured for him a scholarship at Saint
Paul’s school. By his brilliant attainments in mathematics he won
another scholarship to Oxford University.
While at Oxford the youth published a treatise on the planetary orbits
and argued the Sun’s axial rotation.
On his graduation from Oxford, the young would-be astronomer conceived
the project of turning his attention to the southern Stars, of which
no good observations had been made. Shortly before this time a Dutch
astronomer, named Houtman, had observed these Stars in the island of
Sumatra; and Blaeu, the best globe maker of the age, had used these
new observations in the correction of his celestial globes. Halley, on
examining these corrections, came to the conclusion that he himself
could do better. He also concluded that the Island of St. Helena might
be a better point for southern observations. His father, unable to pay
the expenses of so long a trip, broached the project to some friends.
The young astronomer was recommended to King Charles II. by Williamson
and Jones Moore, and the King in turn recommended the youth to the
Indian Company, which then had control over the island of St. Helena.
After this all was plain sailing. The India Company placed a ship
at his disposition and promised him all the assistance he required.
Young Halley provided himself with telescopes, and micrometers, and
other instruments of the latest approved pattern. In November, 1666,
at the age of twenty, he sailed for St. Helena. Among his luggage was
a sextant of five and a half feet and a telescope twenty-four feet in
length constructed under the supervision of Flamsteed, the Astronomer
Royal.
Halley was disappointed in the climate of St. Helena. Frequent rains
and a constantly hazy sky scarcely permitted any observations in the
months of August and September. Notwithstanding these difficulties, he
succeeded in observing and cataloguing some 360 Stars.
In addition to his work on the Stars, Halley made some investigations
on the Moon’s parallax, combining his observations at St. Helena
with those made in northern skies. He also evolved a new theory of
the Moon’s motion, which proved of great aid in the determination of
longitudes.
On November 7, 1677, Halley observed a transit of Mercury which
suggested to him the important idea of employing similar phenomena for
the calculation of the Sun’s distance.
Halley returned to England in November, 1678, and was hailed by his
fellow astronomers as the “Southern Tycho.” He was elected a fellow of
the Royal Society, and by the King’s command the degree of Master of
Arts was conferred upon him by the University of Oxford.
Six months later Halley set out for Dantsic for a personal conference
with Hevelius, the Polish astronomer. Halley wanted to satisfy himself
as to the accuracy of observations claimed by Hevelius without the
aid of a telescope. Halley convinced himself that the errors of the
observations made by Hevelius were less than had been supposed, and did
not exceed a minute of an arc. The two became life-long friends. Halley
proceeded to other cities of Europe where there were observatories. In
Paris he observed with Cassini the great Comet of 1680. This was the
beginning of Halley’s special study of Comets.
Returning to England, the young astronomer married the daughter of
Mr. Tooke, auditor of the Exchequer, with whom he lived harmoniously
until her death, fifty-five years later. The young couple settled at
Islington, where Halley erected an observatory of his own and engaged
in constant lunar observations with a view toward finding a method for
computing longitudes at sea.
Halley’s mind at the same time was busy with the momentous problem of
gravity, upon which Isaac Newton was working then. Independently of
Newton, Halley reached the conclusion that the central force of the
Solar System must decrease inversely as the square of the distance.
Having applied vainly to his fellow astronomers, Hooke and Wren, Halley
in August, 1684, made a special journey to Cambridge to consult Isaac
Newton, who confirmed his conjectures.
Halley and Newton became life-long friends. Halley had Newton elected
to the Royal Society, and when Newton became too poor to pay his
quarterly dues, Halley, through his influence with the leading members
of the Society, had them remitted. It was Halley who encouraged Newton
to put his momentous discovery and elucidation of the forces of gravity
into permanent form in his “Principia,” the first volume of which, “De
Motu,” was presented to the Royal Society at Halley’s suggestion.
In the proceedings of the Royal Society for December, 1684, there is an
entry that “Mr. Halley had lately seen Mr. Newton at Cambridge, who had
told him of a curious treatise ‘De Motu,’ which at Mr. Halley’s desire
he promised to send to the Society to be entered upon their register.
Mr. Halley was desired to put Mr. Newton in mind of his promise for the
securing this invention to himself, till such time as he could be at
leisure to publish it.”
Early in the following year Newton sent his treatise to the Society, to
whom it was read aloud by Halley. This treatise “De Motu” was the germ
of the “Principia” and was intended to be a short account of what the
greater work was to embrace.
During the next two years Newton was hard at work on his “Principia,”
while Halley was equally hard at work on his computations of the Comet
of 1682, and on his theory of the orbits and the periodical returns of
Comets which grew out of his observations.
On April 21, 1686, Halley read to the Royal Society his own “Discourse
Concerning Gravity and its Properties,” in which he stated that his
“worthy countryman, Mr. Issac Newton, has an incomparable treatise on
Motion almost ready for the press,” and that the law of the inverse
square “is the principle on which Mr. Newton has made out all the
phenomena of the celestial motions so easily and naturally that its
truth is past dispute.”
Shortly afterward Newton sent in the manuscript of his great work. The
Society voted “that a letter of thanks be written to Mr. Newton and
that the printing of his book be referred to the consideration of the
council and that in the meantime the book be put into the hands of Mr.
Halley.”
The truth was that the Royal Society, at that time, did not have money
enough to print the book. The Society went through the empty form of
“ordering” that the book be printed “forthwith,” but no printer was
forthcoming until Halley himself undertook the publication of the great
work at his own expense.
The delicacy of Halley’s feeling is revealed by his correspondence with
Newton, in which he informed Newton that the book had “been ordered
to be printed at the Society’s charge.” The preliminary delay about
printing he explained to Newton “arose from the President’s attendance
on the King, and the absence of the vice-presidents, whom the good
weather had drawn out of town.”
Later Newton came to realize how much he owed to Halley in this matter.
In his letters to Halley henceforth he always referred to his book
as if it had been Halley’s book. When the great work was finished at
last Newton wrote to Halley under the date of July 5, 1687: “I have at
length brought your book to an end, and hope it will please you.”
The finished work contained a note to this effect: “The inverse law
of gravity holds in all the celestial motions, as was discovered also
independently by my countrymen Wren, Hooke, and Halley.”
The book was dedicated to the Royal Society, and to it was prefixed a
set of Latin hexameters addressed by Halley to the author, ending with
the well known line:
“Nec fac est propius mortali attingere divos.”
(“It is not given to a mortal to get in closer touch
with the gods.”)
Halley was fifty years old when he made his famous prediction of
the return of the Comet of 1682. This was in his “Synopsis of Comet
Astronomy,” which ended with these words: “Hence I may venture to
foretell that this Comet will return again in the year 1758.”
Besides being an astronomer of the first class, Halley was also a good
navigator. In 1698 he was commissioned a captain in the Royal Navy
and was put in command of the King’s ship, “The Paramour Pink.” With
this vessel he set out on a long cruise to the Pacific for the purpose
of making observations on the laws which govern magnetic variations.
This task he accomplished in a voyage which lasted two years and
extended to the fifty-second degree of southern latitude, when the ice
compelled him to turn back. On the return voyage his crew mutinied and
his lieutenant sided with the mutineers. Halley quelled the mutiny by
sheer force of personality, and returning to England got rid of his
lieutenant. The results of his voyage were published in his “General
Chart of the Variation of the Compass” in 1701. Immediately afterwards
Halley set out on another King’s ship and executed by royal command
a careful survey of the tides and coasts of the British Channel, an
elaborate chart of which he published in 1702.
Next Halley was sent by the King to Dalmatia, for the purpose of
selecting and fortifying the port of Trieste.
On Halley’s return to England, he was made Savilian professor of
geometry at Oxford, and received an honorary doctor’s degree. He
filled two terms of eight years each as secretary to the Royal
Society, and early in 1720 he succeeded Flamsteed as Astronomer Royal.
He died on January 14, 1742, at the age of eighty-five in the full
possession of his faculties, the foremost astronomer of the day and a
man universally beloved and respected. His gravestone stands at the
Greenwich Observatory.
Halley’s works fill several shelves in the library of the Royal
Society. His fame is kept green by the periodical return of the
wandering star known by his name.