From photograph taken at Union Observatory, Johannesburg, May 5, 1910.
Exposure 60 minutes.
]
Halley, by whose name the comet is known, was the first definitely to
establish the fact, suspected before, that certain comets are regular
visitors to the domain of the sun, returning at stated intervals. For
this reason they are termed periodic comets. After Halley had calculated
the paths of twenty-four comets, he found that three were moving in
orbits almost identical. From this he assumed that the three comets must
be one and the same, just as, when a train passes through a station at
regular stated intervals, one is led to infer that it must be the same
train. Naturally allowances must be made for delays due to fog or stormy
weather, but these factors are taken into account should the train
arrive after scheduled time. In the case of a comet it may be delayed by
means of the disturbing effects of the giant planets Jupiter, Saturn,
Uranus, and Neptune, but in Halley’s day the presence of the last two
planets in the solar system was as yet unknown (Uranus was discovered in
1781, and Neptune in 1846). Therefore the following prediction made by
Halley, when he was convinced that the paths of the comets which
appeared in 1531, 1607, and 1682 were identical, is all the more
wonderful, since only an approximate allowance had been made for these
disturbing factors. Referring to the comet of 1682, he said: “If it
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.”
This was certainly the most extraordinary prediction ever made, for
cometary investigations were then in their infancy, and Halley was the
only man living who could have computed the orbit of this comet. Newton
had his doubts regarding the suggestion that a comet seen on one side of
the sun might be identical with another seen on the other side some
weeks later, but Robert Hooke, in a letter addressed to Newton in 1679,
suspected that a comet could reappear after a definite period. He
declared that, if gravity decreased according to the reciprocal of the
square of the distance, the path of a projectile would be an ellipse.
As the year 1758 approached, one can imagine the interest aroused among
astronomers, and the calculations which were made for determining as
accurately as possible the disturbing effects of the larger planets
within the sphere of whose influence the comet might pass. It is
impossible to convey an idea of the labor involved in making the
required computations of the perturbations of this comet throughout a
period of two revolutions, or one hundred and fifty years. It is with a
feeling of pride that the author notes the important part taken in this
work by Madame Lepaute,[9] wife of one of the assistants of the great
mathematician Lalande. Her work proved of inestimable value according to
the following remarks made by her husband on the subject:
“During six months we calculated from morning till night, sometimes
even at meals; the consequence of which was that I contracted an
illness which changed my constitution during the remainder of my
life. The assistance rendered by Madame Lepaute was such that
without her we never should have dared to undertake the enormous
labour with which it was necessary to calculate the distance of each
of the two planets, Jupiter and Saturn, from the comet, separately
for every degree, for one hundred and fifty years.”
Amid all these difficulties, the computers toiled on, and finally, as
the time was drawing near for the return of the comet, Clairaut, who was
working in conjunction with Lalande, announced that the expected comet
would be delayed one hundred days by the influence of Saturn, and five
hundred and eighteen days by the action of Jupiter, and therefore fixed
its nearest approach to the sun for April 13th, 1759. These results were
presented to the Academy of Sciences on November 14, 1758, and as we
have already seen in an earlier chapter of this book, on December 25th
of that year the first glimpse of the long-expected wanderer was
obtained by George Palitzsch, a farmer of Saxony. His telescope was
small, his vision keen, but the enthusiasm of a devoted amateur made up
for his lack of suitable equipment. Observations were made of the comet,
and astronomers were soon able to prove that the perihelion passage
would take place on March 13, 1759, thirty-two days before the epoch
calculated by Clairaut. Such a triumphant success of the theory produced
a deep impression in the scientific world, and, as Lalande
enthusiastically remarked:
“The universe beholds this year the most satisfactory phenomenon
ever presented to us by astronomy; an event which, unique until this
day, changes our doubts to certainty, and our hypotheses to
demonstration.... M. Clairaut asked one month’s grace for the
theory; the month’s grace was just sufficient, and the comet has
appeared after a period of 586 days longer than the previous time of
revolution, and thirty-two days before the time fixed; but what are
thirty-two days to an interval of more than 150 years, during only
one two-hundredth part of which observations were made, the comet
being out of sight all the rest of the time! What are thirty-two
days for all the other attractions of the solar system which have
not been included; for all the comets, the situations and masses of
which are unknown to us; for the resistance of the ethereal medium
which we are unable even to estimate, and for all those quantities
which of necessity have been neglected in the approximations of the
calculation?”
Twenty-five years before the comet was again due, its expected return in
1835 began to arouse the interest of astronomers, and prizes were
offered by two academies for the most accurate forecast of its nearest
approach to the sun. The successful competitors were Baron Damoiseau and
M. Pontecoulant, and several astronomers undertook and completed the
task of computing the planetary perturbations. Although the computers,
as might be expected, differed slightly as to the time when the comet
would make its nearest approach to the sun, yet the difference was not
due to any defects in the methods of computation, but to the
imperfections of the data employed, especially with regard to the
unknown disturbing factor, the planet Neptune.
Not only was the time for the nearest approach of the comet computed,
but its exact path among the stars was worked out with such accuracy
that directions could be given as to the precise point toward which the
telescope must be directed when the comet came within range of
observation. On August 5, 1835, when M. Dumouchel, director of the
observatory of the Roman College, turned his telescope in the direction
indicated and looked through the tube, to his great delight he saw the
comet as a faint and almost invisible stain of light on the deep blue of
the heavens. Thus did science triumph in a most remarkable manner, the
comet making its nearest approach within nine days of the predicted
time. It appeared as a nearly circular misty object near to the
predicted place, and began to develop a tail about the middle of
September, which attained a length of about twenty-four degrees, or
nearly five times the distance between the pointers, Alpha and Beta, in
the constellation of Ursa Major. To the naked eye the head of the comet
resembled a reddish star rather brighter than Antares in the
constellation of the Scorpion. Bessel compared it to a blazing coal, and
called attention to the peculiar fan-like haze of luminous matter
forming the train, which seemed to sway to and fro like a pendulum
across the radius vector, an imaginary line joining the sun and the
nucleus of the comet. This oscillation took place during a period of
four and three-fifths days. He came to the conclusion that a repulsive
force about twice as powerful as the attractive force of gravity was
responsible for the production of these remarkable effects, thus
anticipating the theory according to which the very fine particles
forming the train of a comet may be driven away from the direction of
the sun by radiation pressure.
Meanwhile Halley’s comet was passing through a remarkable series of
transformations, first appearing as a nebula, then as a well-regulated
comet with nucleus and train, next shining as a star, and finally
dilating till it resembled a ball, then assuming paraboloidal form about
May 5, 1836, after which it vanished as if melting into adjacent space
through the excessive diffusion of its light. Moreover, it lost its tail
previous to its arrival at perihelion on November 16, nor did it begin
to recover its elongated shape until more than two months later.
At the return of Halley’s comet in 1910 it was conjectured that it would
probably be greatly disturbed by the influence of the planet Jupiter,
and that of Uranus and the newly discovered planet Neptune. It was
therefore possible for Dr. P. H. Cowell and Dr. A. C. D. Crommelin (both
of the Royal Observatory, Greenwich) to make a prediction so exact that
the comet was found within six minutes of arc in R. A., and four minutes
in declination of its predicted place, as shown on the first photograph
obtained. This was equivalent to an angular distance in the sky less
than one quarter of the diameter of the moon. (Incidentally, a prize of
1,000 marks, which had been offered by Mr. Lindemann for the most
accurate prediction of the comet’s arrival at perihelion, was won and
divided between the two mathematicians.) De Pontecoulant had made
calculations regarding the return of the comet many years earlier which
were fairly near the truth, but one month too late. It was action of
Jupiter about the 1835 perihelion that had such an effect on the 1910
return. The action of Jupiter at any return does not produce a notable
effect till the following return.
One of the first photographs obtained of Halley’s comet at this return
was due to the foresight of Herr Max Wolf of the Heidelberg Observatory,
in exposing a photographic plate for several weeks beforehand, so as to
entrap the wanderer at the first opportunity. It was caught at 2 A.M. in
the morning of September 12, 1909, engraving its image on the
photographic plate, a welcome message announcing its advent to the
astronomical world. (The first photograph obtained of Halley’s comet was
taken at Helwan on August 24, but Herr Wolf was the first to identify
the comet’s image on the plate. There were also many early photographs
taken at Greenwich.) For thirty-two years it had remained beyond the
orbit of the outermost planet Neptune, then, obedient to the attractive
power of its lord and master the sun, it had started on the return trip.
Despite its enormous distance from our planet, and the fact that it was
beyond reach of telescope or camera, it was possible for mathematicians
to trace its path with unerring accuracy. It had approached the orbit of
Neptune after the year 1888, the orbit of Uranus about ten years later,
crossing that of Saturn in 1908. The following year it arrived at the
orbit of Jupiter, thus bringing it within the range of both the
photographic plate and giant telescopes. Its actual return to perihelion
in 1910 differed by two and seven-tenths days from the prediction which
can be explained only by the existence of forces which are not pure
gravitation, or the possibility of another planet beyond Neptune, as yet
undiscovered, acting as a disturbing factor.
When the news of Herr Wolf’s success in obtaining a photograph of the
comet had been announced on September 12, it was followed on September
15 by a message from the Lick Observatory to the effect that a
photograph of the comet had been obtained by Dr. Heber D. Curtis with
the aid of the Crossley reflector. On Wednesday morning, September 15,
Professor S. W. Burnham of the Yerkes Observatory at Williams Bay,
Wisconsin, sighted Halley’s comet by means of the great refractor with
its forty-inch lens, while at the same time it was photographed with the
two-foot reflector in an adjacent dome, by Dr. Oliver J. Lee. The comet
was again detected by Professor Burnham the following morning, September
16, and it was also registered on the photographic plate by Dr. Lee.
Then came the morning of September 17, one of the most eventful in the
life of the writer, who had arrived the previous day as the guest of the
Barnards. That night the great refractor with its forty-inch lens was in
the care of Professor Barnard, who courteously invited the writer to
come to the observatory the next morning at 3 A.M., escorted by his
niece, Miss Calvert, for the purpose of looking through the telescope
and obtaining a view of Halley’s comet. Making a first visit to the
observatory in the darkness preceding dawn was an experience in itself,
but the glimpse of the comet after its absence of seventy-five years is
one never to be forgotten, nor is it easy to describe. For the first
second or so, all seemed darkness as I gazed down the length of that
great tube (63½ feet) into the opening beyond. I saw nothing, and an
intense feeling of disappointment overwhelmed me as I realized and
stated this fact, but Professor Barnard remarked in his whimsical way:
“Surely you did not expect to see the comet with a tail?” Then he
advised me to keep on looking, and even while he spoke I saw a faint,
very misty outline. “Is it exactly in the center of the field of view?”
queried Professor Barnard when I told him that I had seen a
nebulous-looking object, and when I replied in the negative, he informed
me that that faint object I was looking at _was the comet_, which eight
months later I saw in all its splendor from the tower at the top of the
_Times_ Building in New York City.
Meanwhile, the comet had been slowly increasing in size, and by March 4,
1910, it presented the appearance shown in a remarkable photograph
obtained at the Helwan Observatory. It was then suggestive of the
nebulous-looking objects which had been catalogued as such by Herschel
and Messier, but the latter, being more interested in comets, would soon
have recognized, by means of the method already referred to, the
difference as the comet slowly moved against the background of the
stars. This is no reflection on the marvelous sight of Herschel, but
when one reflects on the enthusiasm with which Messier hunted for
comets, we may be sure any suspicious-looking object he came across was
subjected to keen scrutiny before it was catalogued finally as one of
those “embarrassing objects” he named “nebulæ.” During the autumn of
1909 and the early part of the year 1910 the comet was photographed and
observed visually at all the great observatories. At the Royal
Observatory, Greenwich, a fine series of photographs were obtained
despite the trying climate of our country. Up on the heights, at the
Government Observatory at Kodaikanal in southern India, the progress of
the comet was recorded by telescope and camera, so that our planet might
be said on this occasion to have kept its Argus eye constantly directed
toward the celestial visitant.
According to Professor Barnard, who made a special study of the comet,
its first appearance resembled that of a small and rather faint speck of
light, very much like a faint stellar nebula. The increase in brightness
was not very rapid, and as late as the final observations in February,
1910, before the comet passed behind the sun, it gave very little
promise of the splendid display it was destined to make later on in the
month of May. However, its reappearance from behind the sun in the
morning skies of April and May could not have been under more
unfortunate circumstances for observation at the Yerkes Observatory.
According to Professor Barnard:
“that part of the year is always unpropitious here, and it seemed as
if everything combined, on this particular occasion, to hide from us
the growth of the comet and its approach to the earth. Forest fires
in the northern part of the State (Wisconsin) produced a densely
smoky sky, which, even when the clouds were merciful to us and would
have let us see the comet, cut off with a thick yellow veil all but
a glimpse of the bright head.”
The comet was seen for the first time with the naked eye at the Yerkes
Observatory on April 29, the nucleus being bright and of the second
magnitude. The tail was visible for a couple of degrees, but with
field-glasses it could be traced for four or five degrees. On May 3, at
3 h. 40 m. (civil time), the comet was seen for about one minute in a
thin streak of clearer sky, but the next morning at about the same hour
it was a beautiful object with a long tail streaming upward toward the
right, as shown on the magnificent photograph obtained by Professor
Barnard. The photograph facing Chapter VI, taken at the Union
Observatory, Johannesburg, on May 5, 1910, may give some idea of what
was expected but not realized by watchers of the sky in England.
When it was announced on April 29 that the comet had come within range
of naked-eye observations, it occurred to the writer, who was in New
York City at the time, that a desperate attempt must be made to see the
comet, despite the smoke, and electric lights turning night into day.
“When there’s a will there’s a way,” and while walking along Broadway on
the afternoon of April 30, wondering how these difficulties might be
overcome, a glance in the direction of the _Times_ Building solved the
problem. On explaining to Mr. Van Anda, the assistant editor of the
_Times_, what a very desirable spot the summit of the _Times_ Building
would be for observing the comet, a permit was obtained to be handed to
the janitor the next morning at 3 A.M. on May day. It was indeed a case
of “Call me early, mother dear,” but an alarm clock served the purpose
equally well on this momentous occasion.
Promptly at three o’clock the permit was presented to the janitor, and
the writer, ascending in the lift, was transported to the twenty-third
story, and escorted up a spiral staircase leading to the tower. The door
was unlocked by the janitor, and the writer, stepping out on to the
parapet surrounding the tower, gazed eastward for the comet, which
failed to materialize, owing to a dense haze. Awaiting until dawn, the
idea of seeing the comet was given up, but, nothing daunted, the same
program was carried out at the same hour on May 2, and May 3, but
without avail.
[Illustration: