James Clerk-Maxwell's conception of ether--Thomas Young and
"Luminiferous ether,"--Young's and Fresnel's conception of transverse
luminiferous undulations--Faraday's experiments pointing to the
existence of ether--Professor Lodge's suggestion of two ethers--Lord
Kelvin's calculation of the probable density of ether--The vortex theory
of atoms--Helmholtz's calculations in vortex motions--Professor
Tait's apparatus for creating vortex rings in the air---The ultimate
constitution of matter as conceived by Boscovich--Davy's speculations
as to the changes that occur in the substance of matter at different
temperatures--Clausius's and Maxwell's investigations of the
kinetic theory of gases--Lord Kelvin's estimate of the size of the
molecule--Studies of the potential energy of molecules--Action of gases
at low temperatures.
APPENDIX
A HISTORY OF SCIENCE
BOOK III. MODERN DEVELOPMENT OF THE PHYSICAL SCIENCES
With the present book we enter the field of the distinctively modern.
There is no precise date at which we take up each of the successive
stories, but the main sweep of development has to do in each case with
the nineteenth century. We shall see at once that this is a time both
of rapid progress and of great differentiation. We have heard almost
nothing hitherto of such sciences as paleontology, geology, and
meteorology, each of which now demands full attention. Meantime,
astronomy and what the workers of the elder day called natural
philosophy become wonderfully diversified and present numerous
phases that would have been startling enough to the star-gazers and
philosophers of the earlier epoch.
Thus, for example, in the field of astronomy, Herschel is able, thanks
to his perfected telescope, to discover a new planet and then to reach
out into the depths of space and gain such knowledge of stars and
nebulae as hitherto no one had more than dreamed of. Then, in rapid
sequence, a whole coterie of hitherto unsuspected minor planets is
discovered, stellar distances are measured, some members of the starry
galaxy are timed in their flight, the direction of movement of the solar
system itself is investigated, the spectroscope reveals the chemical
composition even of suns that are unthinkably distant, and a tangible
theory is grasped of the universal cycle which includes the birth and
death of worlds.
Similarly the new studies of the earth's surface reveal secrets of
planetary formation hitherto quite inscrutable. It becomes known that
the strata of the earth's surface have been forming throughout untold
ages, and that successive populations differing utterly from one another
have peopled the earth in different geological epochs. The entire point
of view of thoughtful men becomes changed in contemplating the history
of the world in which we live--albeit the newest thought harks back to
some extent to those days when the inspired thinkers of early Greece
dreamed out the wonderful theories with which our earlier chapters have
made our readers familiar.
In the region of natural philosophy progress is no less pronounced and
no less striking. It suffices here, however, by way of anticipation,
simply to name the greatest generalization of the century in physical
science--the doctrine of the conservation of energy.
I. THE SUCCESSORS OF NEWTON IN ASTRONOMY
HEVELIUS AND HALLEY
STRANGELY enough, the decade immediately following Newton was one of
comparative barrenness in scientific progress, the early years of the
eighteenth century not being as productive of great astronomers as the
later years of the seventeenth, or, for that matter, as the later years
of the eighteenth century itself. Several of the prominent astronomers
of the later seventeenth century lived on into the opening years of the
following century, however, and the younger generation soon developed
a coterie of astronomers, among whom Euler, Lagrange, Laplace, and
Herschel, as we shall see, were to accomplish great things in this field
before the century closed.
One of the great seventeenth-century astronomers, who died just before
the close of the century, was Johannes Hevelius (1611-1687), of Dantzig,
who advanced astronomy by his accurate description of the face and
the spots of the moon. But he is remembered also for having retarded
progress by his influence in refusing to use telescopic sights in his
observations, preferring until his death the plain sights long before
discarded by most other astronomers. The advantages of these telescope
sights have been discussed under the article treating of Robert Hooke,
but no such advantages were ever recognized by Hevelius. So great was
Hevelius's reputation as an astronomer that his refusal to recognize the
advantage of the telescope sights caused many astronomers to hesitate
before accepting them as superior to the plain; and even the famous
Halley, of whom we shall speak further in a moment, was sufficiently
in doubt over the matter to pay the aged astronomer a visit to test his
skill in using the old-style sights. Side by side, Hevelius and Halley
made their observations, Hevelius with his old instrument and Halley
with the new. The results showed slightly in the younger man's favor,
but not enough to make it an entirely convincing demonstration. The
explanation of this, however, did not lie in the lack of superiority
of the telescopic instrument, but rather in the marvellous skill of the
aged Hevelius, whose dexterity almost compensated for the defect of his
instrument. What he might have accomplished could he have been induced
to adopt the telescope can only be surmised.
Halley himself was by no means a tyro in matters astronomical at that
time. As the only son of a wealthy soap-boiler living near London, he
had been given a liberal education, and even before leaving college
made such novel scientific observations as that of the change in the
variation of the compass. At nineteen years of age he discovered a new
method of determining the elements of the planetary orbits which was a
distinct improvement over the old. The year following he sailed for the
Island of St, Helena to make observations of the heavens in the southern
hemisphere.
It was while in St. Helena that Halley made his famous observation
of the transit of Mercury over the sun's disk, this observation being
connected, indirectly at least, with his discovery of a method of
determining the parallax of the planets. By parallax is meant the
apparent change in the position of an object, due really to a change in
the position of the observer. Thus, if we imagine two astronomers making
observations of the sun from opposite sides of the earth at the same
time, it is obvious that to these observers the sun will appear to be
at two different points in the sky. Half the angle measuring this
difference would be known as the sun's parallax. This would depend,
then, upon the distance of the earth from the sun and the length of
the earth's radius. Since the actual length of this radius has been
determined, the parallax of any heavenly body enables the astronomer to
determine its exact distance.
The parallaxes can be determined equally well, however, if two observers
are separated by exactly known distances, several hundreds or thousands
of miles apart. In the case of a transit of Venus across the sun's
disk, for example, an observer at New York notes the image of the planet
moving across the sun's disk, and notes also the exact time of this
observation. In the same manner an observer at London makes similar
observations. Knowing the distance between New York and London, and
the different time of the passage, it is thus possible to calculate the
difference of the parallaxes of the sun and a planet crossing its disk.
The idea of thus determining the parallax of the planets originated, or
at least was developed, by Halley, and from this phenomenon he thought
it possible to conclude the dimensions of all the planetary orbits. As
we shall see further on, his views were found to be correct by later
astronomers.
In 1721 Halley succeeded Flamsteed as astronomer royal at the Greenwich
Observatory. Although sixty-four years of age at that time his activity
in astronomy continued unabated for another score of years. At Greenwich
he undertook some tedious observations of the moon, and during those
observations was first to detect the acceleration of mean motion. He
was unable to explain this, however, and it remained for Laplace in the
closing years of the century to do so, as we shall see later.
Halley's book, the Synopsis Astronomiae Cometicae, is one of the most
valuable additions to astronomical literature since the time of Kepler.
He was first to attempt the calculation of the orbit of a comet, having
revived the ancient opinion that comets belong to the solar system,
moving in eccentric orbits round the sun, and his calculation of the
orbit of the comet of 1682 led him to predict correctly the return of
that comet in 1758. Halley's Study of Meteors.
Like other astronomers of his time he was greatly puzzled over the
well-known phenomena of shooting-stars, or meteors, making many
observations himself, and examining carefully the observations of other
astronomers. In 1714 he gave his views as to the origin and composition
of these mysterious visitors in the earth's atmosphere. As this
subject will be again referred to in a later chapter, Halley's views,
representing the most advanced views of his age, are of interest.
"The theory of the air seemeth at present," he says, "to be perfectly
well understood, and the differing densities thereof at all altitudes;
for supposing the same air to occupy spaces reciprocally proportional to
the quantity of the superior or incumbent air, I have elsewhere proved
that at forty miles high the air is rarer than at the surface of
the earth at three thousand times; and that the utmost height of the
atmosphere, which reflects light in the Crepusculum, is not fully
forty-five miles, notwithstanding which 'tis still manifest that some
sort of vapors, and those in no small quantity, arise nearly to that
height. An instance of this may be given in the great light the society
had an account of (vide Transact. Sep., 1676) from Dr. Wallis, which was
seen in very distant counties almost over all the south part of England.
Of which though the doctor could not get so particular a relation as was
requisite to determine the height thereof, yet from the distant places
it was seen in, it could not but be very many miles high.
"So likewise that meteor which was seen in 1708, on the 31st of July,
between nine and ten o'clock at night, was evidently between forty and
fifty miles perpendicularly high, and as near as I can gather, over
Shereness and the buoy on the Nore. For it was seen at London moving
horizontally from east by north to east by south at least fifty degrees
high, and at Redgrove, in Suffolk, on the Yarmouth road, about twenty
miles from the east coast of England, and at least forty miles to the
eastward of London, it appeared a little to the westward of the south,
suppose south by west, and was seen about thirty degrees high, sliding
obliquely downward. I was shown in both places the situation thereof,
which was as described, but could wish some person skilled in
astronomical matters bad seen it, that we might pronounce concerning its
height with more certainty. Yet, as it is, we may securely conclude
that it was not many more miles westerly than Redgrove, which, as I
said before, is about forty miles more easterly than London. Suppose it,
therefore, where perpendicular, to have been thirty-five miles east
from London, and by the altitude it appeared at in London--viz., fifty
degrees, its tangent will be forty-two miles, for the height of the
meteor above the surface of the earth; which also is rather of the
least, because the altitude of the place shown me is rather more than
less than fifty degrees; and the like may be concluded from the altitude
it appeared in at Redgrove, near seventy miles distant. Though at this
very great distance, it appeared to move with an incredible velocity,
darting, in a very few seconds of time, for about twelve degrees of
a great circle from north to south, being very bright at its first
appearance; and it died away at the east of its course, leaving for some
time a pale whiteness in the place, with some remains of it in the track
where it had gone; but no hissing sound as it passed, or bounce of an
explosion were heard.
"It may deserve the honorable society's thoughts, how so great a
quantity of vapor should be raised to the top of the atmosphere, and
there collected, so as upon its ascension or otherwise illumination, to
give a light to a circle of above one hundred miles diameter, not much
inferior to the light of the moon; so as one might see to take a pin
from the ground in the otherwise dark night. 'Tis hard to conceive what
sort of exhalations should rise from the earth, either by the action
of the sun or subterranean heat, so as to surmount the extreme cold
and rareness of the air in those upper regions: but the fact is
indisputable, and therefore requires a solution."
From this much of the paper it appears that there was a general belief
that this burning mass was heated vapor thrown off from the earth in
some mysterious manner, yet this is unsatisfactory to Halley, for after
citing various other meteors that have appeared within his knowledge, he
goes on to say:
"What sort of substance it must be, that could be so impelled and
ignited at the same time; there being no Vulcano or other Spiraculum of
subterraneous fire in the northeast parts of the world, that we ever yet
heard of, from whence it might be projected.
"I have much considered this appearance, and think it one of the hardest
things to account for that I have yet met with in the phenomena of
meteors, and I am induced to think that it must be some collection of
matter formed in the aether, as it were, by some fortuitous concourse
of atoms, and that the earth met with it as it passed along in its orb,
then but newly formed, and before it had conceived any great impetus of
descent towards the sun. For the direction of it was exactly opposite to
that of the earth, which made an angle with the meridian at that time
of sixty-seven gr., that is, its course was from west southwest to east
northeast, wherefore the meteor seemed to move the contrary way. And
besides falling into the power of the earth's gravity, and losing its
motion from the opposition of the medium, it seems that it descended
towards the earth, and was extinguished in the Tyrrhene Sea, to the
west southwest of Leghorn. The great blow being heard upon its first
immersion into the water, and the rattling like the driving of a cart
over stones being what succeeded upon its quenching; something like this
is always heard upon quenching a very hot iron in water. These facts
being past dispute, I would be glad to have the opinion of the learned
thereon, and what objection can be reasonably made against the above
hypothesis, which I humbly submit to their censure."(1)
These few paragraphs, coming as they do from a leading
eighteenth-century astronomer, convey more clearly than any comment the
actual state of the meteorological learning at that time. That this ball
of fire, rushing "at a greater velocity than the swiftest cannon-ball,"
was simply a mass of heated rock passing through our atmosphere, did not
occur to him, or at least was not credited. Nor is this surprising when
we reflect that at that time universal gravitation had been but recently
discovered; heat had not as yet been recognized as simply a form of
motion; and thunder and lightning were unexplained mysteries, not to
be explained for another three-quarters of a century. In the chapter on
meteorology we shall see how the solution of this mystery that puzzled
Halley and his associates all their lives was finally attained.
BRADLEY AND THE ABERRATION OF LIGHT
Halley was succeeded as astronomer royal by a man whose useful additions
to the science were not to be recognized or appreciated fully until
brought to light by the Prussian astronomer Bessel early in the
nineteenth century. This was Dr. James Bradley, an ecclesiastic, who
ranks as one of the most eminent astronomers of the eighteenth century.
His most remarkable discovery was the explanation of a peculiar motion
of the pole-star, first observed, but not explained, by Picard a
century before. For many years a satisfactory explanation was sought
unsuccessfully by Bradley and his fellow-astronomers, but at last he was
able to demonstrate that the stary Draconis, on which he was making his
observations, described, or appeared to describe, a small ellipse.
If this observation was correct, it afforded a means of computing the
aberration of any star at all times. The explanation of the physical
cause of this aberration, as Bradley thought, and afterwards
demonstrated, was the result of the combination of the motion of light
with the annual motion of the earth. Bradley first formulated this
theory in 1728, but it was not until 1748--twenty years of continuous
struggle and observation by him--that he was prepared to communicate the
results of his efforts to the Royal Society. This remarkable paper is
thought by the Frenchman, Delambre, to entitle its author to a place in
science beside such astronomers as Hipparcbus and Kepler.
Bradley's studies led him to discover also the libratory motion of the
earth's axis. "As this appearance of Draconis indicated a diminution
of the inclination of the earth's axis to the plane of the ecliptic,"
he says; "and as several astronomers have supposed THAT inclination to
diminish regularly; if this phenomenon depended upon such a cause, and
amounted to 18" in nine years, the obliquity of the ecliptic would, at
that rate, alter a whole minute in thirty years; which is much
faster than any observations, before made, would allow. I had reason,
therefore, to think that some part of this motion at the least, if not
the whole, was owing to the moon's action upon the equatorial parts of
the earth; which, I conceived, might cause a libratory motion of
the earth's axis. But as I was unable to judge, from only nine years
observations, whether the axis would entirely recover the same position
that it had in the year 1727, I found it necessary to continue my
observations through a whole period of the moon's nodes; at the end of
which I had the satisfaction to see, that the stars, returned into the
same position again; as if there had been no alteration at all in the
inclination of the earth's axis; which fully convinced me that I had
guessed rightly as to the cause of the phenomena. This circumstance
proves likewise, that if there be a gradual diminution of the obliquity
of the ecliptic, it does not arise only from an alteration in the
position of the earth's axis, but rather from some change in the plane
of the ecliptic itself; because the stars, at the end of the period
of the moon's nodes, appeared in the same places, with respect to the
equator, as they ought to have done, if the earth's axis had retained
the same inclination to an invariable plane."(2)
FRENCH ASTRONOMERS
Meanwhile, astronomers across the channel were by no means idle. In
France several successful observers were making many additions to the
already long list of observations of the first astronomer of the Royal
Observatory of Paris, Dominic Cassini (1625-1712), whose reputation
among his contemporaries was much greater than among succeeding
generations of astronomers. Perhaps the most deserving of these
successors was Nicolas Louis de Lacaille (1713-1762), a theologian who
had been educated at the expense of the Duke of Bourbon, and who, soon
after completing his clerical studies, came under the patronage of
Cassini, whose attention had been called to the young man's interest in
the sciences. One of Lacaille's first under-takings was the remeasuring
of the French are of the meridian, which had been incorrectly measured
by his patron in 1684. This was begun in 1739, and occupied him for
two years before successfully completed. As a reward, however, he was
admitted to the academy and appointed mathematical professor in Mazarin
College.
In 1751 he went to the Cape of Good Hope for the purpose of determining
the sun's parallax by observations of the parallaxes of Mars and Venus,
and incidentally to make observations on the other southern hemisphere
stars. The results of this undertaking were most successful, and were
given in his Coelum australe stelligerum, etc., published in 1763. In
this he shows that in the course of a single year he had observed some
ten thousand stars, and computed the places of one thousand nine hundred
and forty-two of them, measured a degree of the meridian, and made
many observations of the moon--productive industry seldom equalled in
a single year in any field. These observations were of great service to
the astronomers, as they afforded the opportunity of comparing the stars
of the southern hemisphere with those of the northern, which were being
observed simultaneously by Lelande at Berlin.
Lacaille's observations followed closely upon the determination of an
absorbing question which occupied the attention of the astronomers in
the early part of the century. This question was as to the shape of the
earth--whether it was actually flattened at the poles. To settle this
question once for all the Academy of Sciences decided to make the
actual measurement of the length of two degrees, one as near the pole
as possible, the other at the equator. Accordingly, three astronomers,
Godin, Bouguer, and La Condamine, made the journey to a spot on the
equator in Peru, while four astronomers, Camus, Clairaut, Maupertuis,
and Lemonnier, made a voyage to a place selected in Lapland. The result
of these expeditions was the determination that the globe is oblately
spheroidal.
A great contemporary and fellow-countryman of Lacaille was Jean Le Rond
d'Alembert (1717-1783), who, although not primarily an astronomer, did
so much with his mathematical calculations to aid that science that
his name is closely connected with its progress during the eighteenth
century. D'Alembert, who became one of the best-known men of science of
his day, and whose services were eagerly sought by the rulers of Europe,
began life as a foundling, having been exposed in one of the markets of
Paris. The sickly infant was adopted and cared for in the family of a
poor glazier, and treated as a member of the family. In later years,
however, after the foundling had become famous throughout Europe, his
mother, Madame Tencin, sent for him, and acknowledged her relationship.
It is more than likely that the great philosopher believed her story,
but if so he did not allow her the satisfaction of knowing his belief,
declaring always that Madame Tencin could "not be nearer than a
step-mother to him, since his mother was the wife of the glazier."
D'Alembert did much for the cause of science by his example as well
as by his discoveries. By living a plain but honest life, declining
magnificent offers of positions from royal patrons, at the same time
refusing to grovel before nobility, he set a worthy example to other
philosophers whose cringing and pusillanimous attitude towards persons
of wealth or position had hitherto earned them the contempt of the upper
classes.
His direct additions to astronomy are several, among others the
determination of the mutation of the axis of the earth. He also
determined the ratio of the attractive forces of the sun and moon,
which he found to be about as seven to three. From this he reached the
conclusion that the earth must be seventy times greater than the moon.
The first two volumes of his Researches on the Systems of the World,
published in 1754, are largely devoted to mathematical and astronomical
problems, many of them of little importance now, but of great interest
to astronomers at that time.
Another great contemporary of D'Alembert, whose name is closely
associated and frequently confounded with his, was Jean Baptiste Joseph
Delambre (1749-1822). More fortunate in birth as also in his educational
advantages, Delambre as a youth began his studies under the celebrated
poet Delille. Later he was obliged to struggle against poverty,
supporting himself for a time by making translations from Latin, Greek,
Italian, and English, and acting as tutor in private families. The
turning-point of his fortune came when the attention of Lalande was
called to the young man by his remarkable memory, and Lalande soon
showed his admiration by giving Delambre certain difficult astronomical
problems to solve. By performing these tasks successfully his future as
an astronomer became assured. At that time the planet Uranus had just
been discovered by Herschel, and the Academy of Sciences offered as the
subject for one of its prizes the determination of the planet's orbit.
Delambre made this determination and won the prize--a feat that brought
him at once into prominence.
By his writings he probably did as much towards perfecting modern
astronomy as any one man. His History of Astronomy is not merely a
narrative of progress of astronomy but a complete abstract of all the
celebrated works written on the subject. Thus he became famous as an
historian as well as an astronomer.
LEONARD EULER
Still another contemporary of D'Alembert and Delambre, and somewhat
older than either of them, was Leonard Euler (1707-1783), of Basel,
whose fame as a philosopher equals that of either of the great
Frenchmen. He is of particular interest here in his capacity of
astronomer, but astronomy was only one of the many fields of science in
which he shone. Surely something out of the ordinary was to be expected
of the man who could "repeat the AEneid of Virgil from the beginning
to the end without hesitation, and indicate the first and last line of
every page of the edition which he used." Something was expected, and he
fulfilled these expectations.
In early life he devoted himself to the study of theology and the
Oriental languages, at the request of his father, but his love of
mathematics proved too strong, and, with his father's consent, he
finally gave up his classical studies and turned to his favorite study,
geometry. In 1727 he was invited by Catharine I. to reside in St.
Petersburg, and on accepting this invitation he was made an associate
of the Academy of Sciences. A little later he was made professor of
physics, and in 1733 professor of mathematics. In 1735 he solved a
problem in three days which some of the eminent mathematicians would not
undertake under several months. In 1741 Frederick the Great invited him
to Berlin, where he soon became a member of the Academy of Sciences and
professor of mathematics; but in 1766 he returned to St. Petersburg.
Towards the close of his life he became virtually blind, being obliged
to dictate his thoughts, sometimes to persons entirely ignorant of the
subject in hand. Nevertheless, his remarkable memory, still further
heightened by his blindness, enabled him to carry out the elaborate
computations frequently involved.
Euler's first memoir, transmitted to the Academy of Sciences of Paris
in 1747, was on the planetary perturbations. This memoir carried off the
prize that had been offered for the analytical theory of the motions of
Jupiter and Saturn. Other memoirs followed, one in 1749 and another in
1750, with further expansions of the same subject. As some slight
errors were found in these, such as a mistake in some of the formulae
expressing the secular and periodic inequalities, the academy proposed
the same subject for the prize of 1752. Euler again competed, and won
this prize also. The contents of this memoir laid the foundation for
the subsequent demonstration of the permanent stability of the planetary
system by Laplace and Lagrange.
It was Euler also who demonstrated that within certain fixed limits
the eccentricities and places of the aphelia of Saturn and Jupiter are
subject to constant variation, and he calculated that after a lapse
of about thirty thousand years the elements of the orbits of these two
planets recover their original values.
II. THE PROGRESS OF MODERN ASTRONOMY
A NEW epoch in astronomy begins with the work of William Herschel, the
Hanoverian, whom England made hers by adoption. He was a man with a
positive genius for sidereal discovery. At first a mere amateur in
astronomy, he snatched time from his duties as music-teacher to grind
him a telescopic mirror, and began gazing at the stars. Not content with
his first telescope, he made another and another, and he had such genius
for the work that he soon possessed a better instrument than was ever
made before. His patience in grinding the curved reflective surface was
monumental. Sometimes for sixteen hours together he must walk steadily
about the mirror, polishing it, without once removing his hands.
Meantime his sister, always his chief lieutenant, cheered him with her
presence, and from time to time put food into his mouth. The telescope
completed, the astronomer turned night into day, and from sunset to
sunrise, year in and year out, swept the heavens unceasingly, unless
prevented by clouds or the brightness of the moon. His sister sat always
at his side, recording his observations. They were in the open air,
perched high at the mouth of the reflector, and sometimes it was so cold
that the ink froze in the bottle in Caroline Herschel's hand; but the
two enthusiasts hardly noticed a thing so common-place as terrestrial
weather. They were living in distant worlds.
The results? What could they be? Such enthusiasm would move mountains.
But, after all, the moving of mountains seems a liliputian task compared
with what Herschel really did with those wonderful telescopes. He moved
worlds, stars, a universe--even, if you please, a galaxy of universes;
at least he proved that they move, which seems scarcely less wonderful;
and he expanded the cosmos, as man conceives it, to thousands of times
the dimensions it had before. As a mere beginning, he doubled the
diameter of the solar system by observing the great outlying planet
which we now call Uranus, but which he christened Georgium Sidus,
in honor of his sovereign, and which his French contemporaries, not
relishing that name, preferred to call Herschel.
This discovery was but a trifle compared with what Herschel did later
on, but it gave him world-wide reputation none the less. Comets and
moons aside, this was the first addition to the solar system that had
been made within historic times, and it created a veritable furor of
popular interest and enthusiasm. Incidentally King George was flattered
at having a world named after him, and he smiled on the astronomer, and
came with his court to have a look at his namesake. The inspection
was highly satisfactory; and presently the royal favor enabled the
astronomer to escape the thraldom of teaching music and to devote his
entire time to the more congenial task of star-gazing.
Thus relieved from the burden of mundane embarrassments, he turned with
fresh enthusiasm to the skies, and his discoveries followed one another
in bewildering profusion. He found various hitherto unseen moons of our
sister planets; he made special studies of Saturn, and proved that this
planet, with its rings, revolves on its axis; he scanned the spots on
the sun, and suggested that they influence the weather of our earth; in
short, he extended the entire field of solar astronomy. But very soon
this field became too small for him, and his most important researches
carried him out into the regions of space compared with which the span
of our solar system is a mere point. With his perfected telescopes he
entered abysmal vistas which no human eve ever penetrated before, which
no human mind had hitherto more than vaguely imagined. He tells us that
his forty-foot reflector will bring him light from a distance of "at
least eleven and three-fourths millions of millions of millions of
miles"--light which left its source two million years ago. The smallest
stars visible to the unaided eye are those of the sixth magnitude; this
telescope, he thinks, has power to reveal stars of the 1342d magnitude.
But what did Herschel learn regarding these awful depths of space and
the stars that people them? That was what the world wished to know.
Copernicus, Galileo, Kepler, had given us a solar system, but the stars
had been a mystery. What says the great reflector--are the stars points
of light, as the ancients taught, and as more than one philosopher of
the eighteenth century has still contended, or are they suns, as others
hold? Herschel answers, they are suns, each and every one of all the
millions--suns, many of them, larger than the one that is the centre of
our tiny system. Not only so, but they are moving suns. Instead of
being fixed in space, as has been thought, they are whirling in gigantic
orbits about some common centre. Is our sun that centre? Far from it.
Our sun is only a star like all the rest, circling on with its attendant
satellites--our giant sun a star, no different from myriad other stars,
not even so large as some; a mere insignificant spark of matter in an
infinite shower of sparks.
Nor is this all. Looking beyond the few thousand stars that are visible
to the naked eye, Herschel sees series after series of more distant
stars, marshalled in galaxies of millions; but at last he reaches a
distance beyond which the galaxies no longer increase. And yet--so he
thinks--he has not reached the limits of his vision. What then? He has
come to the bounds of the sidereal system--seen to the confines of the
universe. He believes that he can outline this system, this universe,
and prove that it has the shape of an irregular globe, oblately
flattened to almost disklike proportions, and divided at one edge--a
bifurcation that is revealed even to the naked eye in the forking of the
Milky Way.
This, then, is our universe as Herschel conceives it--a vast galaxy
of suns, held to one centre, revolving, poised in space. But even
here those marvellous telescopes do not pause. Far, far out beyond the
confines of our universe, so far that the awful span of our own system
might serve as a unit of measure, are revealed other systems, other
universes, like our own, each composed, as he thinks, of myriads of
suns, clustered like our galaxy into an isolated system--mere islands of
matter in an infinite ocean of space. So distant from our universe are
these now universes of Herschel's discovery that their light reaches
us only as a dim, nebulous glow, in most cases invisible to the unaided
eye. About a hundred of these nebulae were known when Herschel began
his studies. Before the close of the century he had discovered about
two thousand more of them, and many of these had been resolved by his
largest telescopes into clusters of stars. He believed that the farthest
of these nebulae that he could see was at least three hundred thousand
times as distant from us as the nearest fixed star. Yet that nearest
star--so more recent studies prove--is so remote that its light,
travelling one hundred and eighty thousand miles a second, requires
three and one-half years to reach our planet.
As if to give the finishing touches to this novel scheme of cosmology,
Herschel, though in the main very little given to unsustained
theorizing, allows himself the privilege of one belief that he cannot
call upon his telescope to substantiate. He thinks that all the myriad
suns of his numberless systems are instinct with life in the human
sense. Giordano Bruno and a long line of his followers had held that
some of our sister planets may be inhabited, but Herschel extends
the thought to include the moon, the sun, the stars--all the heavenly
bodies. He believes that he can demonstrate the habitability of our own
sun, and, reasoning from analogy, he is firmly convinced that all the
suns of all the systems are "well supplied with inhabitants." In this,
as in some other inferences, Herschel is misled by the faulty physics
of his time. Future generations, working with perfected instruments, may
not sustain him all along the line of his observations, even, let alone
his inferences. But how one's egotism shrivels and shrinks as one grasps
the import of his sweeping thoughts!
Continuing his observations of the innumerable nebulae, Herschel is led
presently to another curious speculative inference. He notes that some
star groups are much more thickly clustered than others, and he is
led to infer that such varied clustering tells of varying ages of the
different nebulae. He thinks that at first all space may have been
evenly sprinkled with the stars and that the grouping has resulted from
the action of gravitation.
"That the Milky Way is a most extensive stratum of stars of various
sizes admits no longer of lasting doubt," he declares, "and that our sun
is actually one of the heavenly bodies belonging to it is as evident. I
have now viewed and gauged this shining zone in almost every direction
and find it composed of stars whose number... constantly increases and
decreases in proportion to its apparent brightness to the naked eye.
"Let us suppose numberless stars of various sizes, scattered over an
indefinite portion of space in such a manner as to be almost equally
distributed throughout the whole. The laws of attraction which no doubt
extend to the remotest regions of the fixed stars will operate in such a
manner as most probably to produce the following effects:
"In the first case, since we have supposed the stars to be of various
sizes, it will happen that a star, being considerably larger than its
neighboring ones, will attract them more than they will be attracted by
others that are immediately around them; by which means they will be,
in time, as it were, condensed about a centre, or, in other words, form
themselves into a cluster of stars of almost a globular figure, more
or less regular according to the size and distance of the surrounding
stars....
"The next case, which will also happen almost as frequently as the
former, is where a few stars, though not superior in size to the rest,
may chance to be rather nearer one another than the surrounding ones,...
and this construction admits of the utmost variety of shapes....
"From the composition and repeated conjunction of both the foregoing
formations, a third may be derived when many large stars, or combined
small ones, are spread in long, extended, regular, or crooked rows,
streaks, or branches; for they will also draw the surrounding stars, so
as to produce figures of condensed stars curiously similar to the former
which gave rise to these condensations.
"We may likewise admit still more extensive combinations; when, at the
same time that a cluster of stars is forming at the one part of
space, there may be another collection in a different but perhaps not
far-distant quarter, which may occasion a mutual approach towards their
own centre of gravity.
"In the last place, as a natural conclusion of the former cases, there
will be formed great cavities or vacancies by the retreating of the
stars towards the various centres which attract them."(1)
Looking forward, it appears that the time must come when all the suns
of a system will be drawn together and destroyed by impact at a common
centre. Already, it seems to Herschel, the thickest clusters have
"outlived their usefulness" and are verging towards their doom.
But again, other nebulae present an appearance suggestive of an opposite
condition. They are not resolvable into stars, but present an almost
uniform appearance throughout, and are hence believed to be composed of
a shining fluid, which in some instances is seen to be condensed at the
centre into a glowing mass. In such a nebula Herschel thinks he sees a
sun in process of formation.
THE NEBULAR HYPOTHESIS OF KANT
Taken together, these two conceptions outline a majestic cycle of world
formation and world destruction--a broad scheme of cosmogony, such as
had been vaguely adumbrated two centuries before by Kepler and in
more recent times by Wright and Swedenborg. This so-called "nebular
hypothesis" assumes that in the beginning all space was uniformly filled
with cosmic matter in a state of nebular or "fire-mist" diffusion,
"formless and void." It pictures the condensation--coagulation, if
you will--of portions of this mass to form segregated masses, and the
ultimate development out of these masses of the sidereal bodies that we
see.
Perhaps the first elaborate exposition of this idea was that given by
the great German philosopher Immanuel Kant (born at Konigsberg in 1724,
died in 1804), known to every one as the author of the Critique of Pure
Reason. Let us learn from his own words how the imaginative philosopher
conceived the world to have come into existence.
"I assume," says Kant, "that all the material of which the globes
belonging to our solar system--all the planets and comets--consist, at
the beginning of all things was decomposed into its primary elements,
and filled the whole space of the universe in which the bodies formed
out of it now revolve. This state of nature, when viewed in and by
itself without any reference to a system, seems to be the very simplest
that can follow upon nothing. At that time nothing has yet been formed.
The construction of heavenly bodies at a distance from one another,
their distances regulated by their attraction, their form arising out of
the equilibrium of their collected matter, exhibit a later state.... In
a region of space filled in this manner, a universal repose could last
only a moment. The elements have essential forces with which to put
each other in motion, and thus are themselves a source of life. Matter
immediately begins to strive to fashion itself. The scattered elements
of a denser kind, by means of their attraction, gather from a sphere
around them all the matter of less specific gravity; again, these
elements themselves, together with the material which they have united
with them, collect in those points where the particles of a still denser
kind are found; these in like manner join still denser particles, and
so on. If we follow in imagination this process by which nature fashions
itself into form through the whole extent of chaos, we easily perceive
that all the results of the process would consist in the formation of
divers masses which, when their formation was complete, would by the
equality of their attraction be at rest and be forever unmoved.
"But nature has other forces in store which are specially exerted when
matter is decomposed into fine particles. They are those forces by which
these particles repel one another, and which, by their conflict with
attractions, bring forth that movement which is, as it were, the lasting
life of nature. This force of repulsion is manifested in the elasticity
of vapors, the effluences of strong-smelling bodies, and the diffusion
of all spirituous matters. This force is an uncontestable phenomenon of
matter. It is by it that the elements, which may be falling to the point
attracting them, are turned sideways promiscuously from their movement
in a straight line; and their perpendicular fall thereby issues in
circular movements, which encompass the centre towards which they were
falling. In order to make the formation of the world more distinctly
conceivable, we will limit our view by withdrawing it from the infinite
universe of nature and directing it to a particular system, as the
one which belongs to our sun. Having considered the generation of this
system, we shall be able to advance to a similar consideration of the
origin of the great world-systems, and thus to embrace the infinitude of
the whole creation in one conception.
"From what has been said, it will appear that if a point is situated in
a very large space where the attraction of the elements there situated
acts more strongly than elsewhere, then the matter of the elementary
particles scattered throughout the whole region will fall to that point.
The first effect of this general fall is the formation of a body at this
centre of attraction, which, so to speak, grows from an infinitely
small nucleus by rapid strides; and in the proportion in which this mass
increases, it also draws with greater force the surrounding particles
to unite with it. When the mass of this central body has grown so great
that the velocity with which it draws the particles to itself with great
distances is bent sideways by the feeble degree of repulsion with which
they impede one another, and when it issues in lateral movements which
are capable by means of the centrifugal force of encompassing the
central body in an orbit, then there are produced whirls or vortices
of particles, each of which by itself describes a curved line by the
composition of the attracting force and the force of revolution that had
been bent sideways. These kinds of orbits all intersect one another,
for which their great dispersion in this space gives place. Yet these
movements are in many ways in conflict with one another, and they
naturally tend to bring one another to a uniformity--that is, into a
state in which one movement is as little obstructive to the other as
possible. This happens in two ways: first by the particles limiting
one another's movement till they all advance in one direction; and,
secondly, in this way, that the particles limit their vertical movements
in virtue of which they are approaching the centre of attraction, till
they all move horizontally--i. e., in parallel circles round the sun as
their centre, no longer intercept one another, and by the centrifugal
force becoming equal with the falling force they keep themselves
constantly in free circular orbits at the distance at which they move.
The result, finally, is that only those particles continue to move in
this region of space which have acquired by their fall a velocity, and
through the resistance of the other particles a direction, by which they
can continue to maintain a FREE CIRCULAR MOVEMENT....
"The view of the formation of the planets in this system has the
advantage over every other possible theory in holding that the origin
of the movements, and the position of the orbits in arising at that same
point of time--nay, more, in showing that even the deviations from the
greatest possible exactness in their determinations, as well as the
accordances themselves, become clear at a glance. The planets are formed
out of particles which, at the distance at which they move, have exact
movements in circular orbits; and therefore the masses composed out of
them will continue the same movements and at the same rate and in the
same direction."(2)
It must be admitted that this explanation leaves a good deal to be
desired. It is the explanation of a metaphysician rather than that of
an experimental scientist. Such phrases as "matter immediately begins to
strive to fashion itself," for example, have no place in the reasoning
of inductive science. Nevertheless, the hypothesis of Kant is a
remarkable conception; it attempts to explain along rational lines
something which hitherto had for the most part been considered
altogether inexplicable.
But there are various questions that at once suggest themselves which
the Kantian theory leaves unanswered. How happens it, for example, that
the cosmic mass which gave birth to our solar system was divided into
several planetary bodies instead of remaining a single mass? Were the
planets struck from the sun by the chance impact of comets, as Buffon
has suggested? or thrown out by explosive volcanic action, in accordance
with the theory of Dr. Darwin? or do they owe their origin to some
unknown law? In any event, how chanced it that all were projected in
nearly the same plane as we now find them?
LAPLACE AND THE NEBULAR HYPOTHESIS
It remained for a mathematical astronomer to solve these puzzles. The
man of all others competent to take the subject in hand was the French
astronomer Laplace. For a quarter of a century he had devoted his
transcendent mathematical abilities to the solution of problems of
motion of the heavenly bodies. Working in friendly rivalry with his
countryman Lagrange, his only peer among the mathematicians of the age,
he had taken up and solved one by one the problems that Newton left
obscure. Largely through the efforts of these two men the last lingering
doubts as to the solidarity of the Newtonian hypothesis of universal
gravitation had been removed. The share of Lagrange was hardly less than
that of his co-worker; but Laplace will longer be remembered, because
he ultimately brought his completed labors into a system, and,
incorporating with them the labors of his contemporaries, produced
in the Mecanique Celeste the undisputed mathematical monument of the
century, a fitting complement to the Principia of Newton, which it
supplements and in a sense completes.
In the closing years of the eighteenth century Laplace took up the
nebular hypothesis of cosmogony, to which we have just referred, and
gave it definite proportions; in fact, made it so thoroughly his own
that posterity will always link it with his name. Discarding the crude
notions of cometary impact and volcanic eruption, Laplace filled up the
gaps in the hypothesis with the aid of well-known laws of gravitation
and motion. He assumed that the primitive mass of cosmic matter which
was destined to form our solar system was revolving on its axis even at
a time when it was still nebular in character, and filled all space to
a distance far beyond the present limits of the system. As this vaporous
mass contracted through loss of heat, it revolved more and more swiftly,
and from time to time, through balance of forces at its periphery, rings
of its substance were whirled off and left revolving there, subsequently
to become condensed into planets, and in their turn whirl off minor
rings that became moons. The main body of the original mass remains in
the present as the still contracting and rotating body which we call the
sun.
Let us allow Laplace to explain all this in detail:
"In order to explain the prime movements of the planetary system,"
he says, "there are the five following phenomena: The movement of the
planets in the same direction and very nearly in the same plane; the
movement of the satellites in the same direction as that of the planets;
the rotation of these different bodies and the sun in the same
direction as their revolution, and in nearly the same plane; the slight
eccentricity of the orbits of the planets and of the satellites; and,
finally, the great eccentricity of the orbits of the comets, as if their
inclinations had been left to chance.
"Buffon is the only man I know who, since the discovery of the true
system of the world, has endeavored to show the origin of the planets
and their satellites. He supposes that a comet, in falling into the sun,
drove from it a mass of matter which was reassembled at a distance in
the form of various globes more or less large, and more or less removed
from the sun, and that these globes, becoming opaque and solid, are now
the planets and their satellites.
"This hypothesis satisfies the first of the five preceding phenomena;
for it is clear that all the bodies thus formed would move very nearly
in the plane which passed through the centre of the sun, and in the
direction of the torrent of matter which was produced; but the four
other phenomena appear to be inexplicable to me by this means. Indeed,
the absolute movement of the molecules of a planet ought then to be in
the direction of the movement of its centre of gravity; but it does not
at all follow that the motion of the rotation of the planets should be
in the same direction. Thus the earth should rotate from east to west,
but nevertheless the absolute movement of its molecules should be
from east to west; and this ought also to apply to the movement of the
revolution of the satellites, in which the direction, according to the
hypothesis which he offers, is not necessarily the same as that of the
progressive movement of the planets.
"A phenomenon not only very difficult to explain under this hypothesis,
but one which is even contrary to it, is the slight eccentricity of the
planetary orbits. We know, by the theory of central forces, that if
a body moves in a closed orbit around the sun and touches it, it also
always comes back to that point at every revolution; whence it follows
that if the planets were originally detached from the sun, they would
touch it at each return towards it, and their orbits, far from being
circular, would be very eccentric. It is true that a mass of matter
driven from the sun cannot be exactly compared to a globe which touches
its surface, for the impulse which the particles of this mass receive
from one another and the reciprocal attractions which they exert among
themselves, could, in changing the direction of their movements, remove
their perihelions from the sun; but their orbits would be always most
eccentric, or at least they would not have slight eccentricities except
by the most extraordinary chance. Thus we cannot see, according to
the hypothesis of Buffon, why the orbits of more than a hundred comets
already observed are so elliptical. This hypothesis is therefore
very far from satisfying the preceding phenomena. Let us see if it is
possible to trace them back to their true cause.
"Whatever may be its ultimate nature, seeing that it has caused or
modified the movements of the planets, it is necessary that this cause
should embrace every body, and, in view of the enormous distances which
separate them, it could only have been a fluid of immense extent.
In order to have given them an almost circular movement in the same
direction around the sun, it is necessary that this fluid should
have enveloped the sun as in an atmosphere. The consideration of the
planetary movements leads us then to think that, on account of excessive
heat, the atmosphere of the sun originally extended beyond the orbits of
all the planets, and that it was successively contracted to its present
limits.
"In the primitive condition in which we suppose the sun to have been, it
resembled a nebula such as the telescope shows is composed of a nucleus
more or less brilliant, surrounded by a nebulosity which, on condensing
itself towards the centre, forms a star. If it is conceived by analogy
that all the stars were formed in this manner, it is possible to imagine
their previous condition of nebulosity, itself preceded by other states
in which the nebulous matter was still more diffused, the nucleus being
less and less luminous. By going back as far as possible, we thus
arrive at a nebulosity so diffused that its existence could hardly be
suspected.
"For a long time the peculiar disposition of certain stars, visible
to the unaided eye, has struck philosophical observers. Mitchell
has already remarked how little probable it is that the stars in the
Pleiades, for example, could have been contracted into the small
space which encloses them by the fortuity of chance alone, and he has
concluded that this group of stars, and similar groups which the skies
present to us, are the necessary result of the condensation of a nebula,
with several nuclei, and it is evident that a nebula, by continually
contracting, towards these various nuclei, at length would form a group
of stars similar to the Pleiades. The condensation of a nebula with two
nuclei would form a system of stars close together, turning one upon
the other, such as those double stars of which we already know the
respective movements.
"But how did the solar atmosphere determine the movements of the
rotation and revolution of the planets and satellites? If these bodies
had penetrated very deeply into this atmosphere, its resistance would
have caused them to fall into the sun. We can therefore conjecture that
the planets were formed at their successive limits by the condensation
of a zone of vapors which the sun, on cooling, left behind, in the plane
of his equator.
"Let us recall the results which we have given in a preceding chapter.
The atmosphere of the sun could not have extended indefinitely. Its
limit was the point where the centrifugal force due to its movement
of rotation balanced its weight. But in proportion as the cooling
contracted the atmosphere, and those molecules which were near to them
condensed upon the surface of the body, the movement of the rotation
increased; for, on account of the Law of Areas, the sum of the areas
described by the vector of each molecule of the sun and its atmosphere
and projected in the plane of the equator being always the same, the
rotation should increase when these molecules approach the centre of the
sun. The centrifugal force due to this movement becoming thus larger,
the point where the weight is equal to it is nearer the sun. Supposing,
then, as it is natural to admit, that the atmosphere extended at some
period to its very limits, it should, on cooling, leave molecules behind
at this limit and at limits successively occasioned by the increased
rotation of the sun. The abandoned molecules would continue to revolve
around this body, since their centrifugal force was balanced by their
weight. But this equilibrium not arising in regard to the atmospheric
molecules parallel to the solar equator, the latter, on account of their
weight, approached the atmosphere as they condensed, and did not cease
to belong to it until by this motion they came upon the equator.
"Let us consider now the zones of vapor successively left behind. These
zones ought, according to appearance, by the condensation and mutual
attraction of their molecules, to form various concentric rings of vapor
revolving around the sun. The mutual gravitational friction of each ring
would accelerate some and retard others, until they had all acquired the
same angular velocity. Thus the actual velocity of the molecules most
removed from the sun would be the greatest. The following cause would
also operate to bring about this difference of speed. The molecules
farthest from the sun, and which by the effects of cooling and
condensation approached one another to form the outer part of the ring,
would have always described areas proportional to the time since the
central force by which they were controlled has been constantly directed
towards this body. But this constancy of areas necessitates an increase
of velocity proportional to the distance. It is thus seen that the same
cause would diminish the velocity of the molecules which form the inner
part of the ring.
"If all the molecules of the ring of vapor continued to condense without
disuniting, they would at length form a ring either solid or fluid. But
this formation would necessitate such a regularity in every part of the
ring, and in its cooling, that this phenomenon is extremely rare; and
the solar system affords us, indeed, but one example--namely, in the
ring of Saturn. In nearly every case the ring of vapor was broken into
several masses, each moving at similar velocities, and continuing to
rotate at the same distance around the sun. These masses would take
a spheroid form with a rotatory movement in the direction of the
revolution, because their inner molecules had less velocity than the
outer. Thus were formed so many planets in a condition of vapor. But
if one of them were powerful enough to reunite successively by its
attraction all the others around its centre of gravity, the ring of
vapor would be thus transformed into a single spheroidical mass of
vapor revolving around the sun with a rotation in the direction of its
revolution. The latter case has been that which is the most common, but
nevertheless the solar system affords us an instance of the first case
in the four small planets which move between Jupiter and Mars; at least,
if we do not suppose, as does M. Olbers, that they originally formed a
single planet which a mighty explosion broke up into several portions
each moving at different velocities.
"According to our hypothesis, the comets are strangers to our planetary
system. In considering them, as we have done, as minute nebulosities,
wandering from solar system to solar system, and formed by the
condensation of the nebulous matter everywhere existent in profusion in
the universe, we see that when they come into that part of the heavens
where the sun is all-powerful, he forces them to describe orbits either
elliptical or hyperbolic, their paths being equally possible in all
directions, and at all inclinations of the ecliptic, conformably to what
has been observed. Thus the condensation of nebulous matter, by which
we have at first explained the motions of the rotation and revolution
of the planets and their satellites in the same direction, and in nearly
approximate planes, explains also why the movements of the comets escape
this general law."(3)
The nebular hypothesis thus given detailed completion by Laplace is a
worthy complement of the grand cosmologic scheme of Herschel. Whether
true or false, the two conceptions stand as the final contributions
of the eighteenth century to the history of man's ceaseless efforts to
solve the mysteries of cosmic origin and cosmic structure. The world
listened eagerly and without prejudice to the new doctrines; and that
attitude tells of a marvellous intellectual growth of our race. Mark the
transition. In the year 1600, Bruno was burned at the stake for teaching
that our earth is not the centre of the universe. In 1700, Newton was
pronounced "impious and heretical" by a large school of philosophers
for declaring that the force which holds the planets in their orbits
is universal gravitation. In 1800, Laplace and Herschel are honored for
teaching that gravitation built up the system which it still controls;
that our universe is but a minor nebula, our sun but a minor star, our
earth a mere atom of matter, our race only one of myriad races peopling
an infinity of worlds. Doctrines which but the span of two human lives
before would have brought their enunciators to the stake were now
pronounced not impious, but sublime.
ASTEROIDS AND SATELLITES
The first day of the nineteenth century was fittingly signalized by the
discovery of a new world. On the evening of January 1, 1801, an Italian
astronomer, Piazzi, observed an apparent star of about the eighth
magnitude (hence, of course, quite invisible to the unaided eye), which
later on was seen to have moved, and was thus shown to be vastly nearer
the earth than any true star. He at first supposed, as Herschel had
done when he first saw Uranus, that the unfamiliar body was a comet; but
later observation proved it a tiny planet, occupying a position in space
between Mars and Jupiter. It was christened Ceres, after the tutelary
goddess of Sicily.
Though unpremeditated, this discovery was not unexpected, for
astronomers had long surmised the existence of a planet in the wide
gap between Mars and Jupiter. Indeed, they were even preparing to make
concerted search for it, despite the protests of philosophers, who
argued that the planets could not possibly exceed the magic number
seven, when Piazzi forestalled their efforts. But a surprise came
with the sequel; for the very next year Dr. Olbers, the wonderful
physician-astronomer of Bremen, while following up the course of Ceres,
happened on another tiny moving star, similarly located, which soon
revealed itself as planetary. Thus two planets were found where only one
was expected.
The existence of the supernumerary was a puzzle, but Olbers solved it
for the moment by suggesting that Ceres and Pallas, as he called his
captive, might be fragments of a quondam planet, shattered by internal
explosion or by the impact of a comet. Other similar fragments, he
ventured to predict, would be found when searched for. William Herschel
sanctioned this theory, and suggested the name asteroids for the tiny
planets. The explosion theory was supported by the discovery of another
asteroid, by Harding, of Lilienthal, in 1804, and it seemed clinched
when Olbers himself found a fourth in 1807. The new-comers were named
Juno and Vesta respectively.
There the case rested till 1845, when a Prussian amateur astronomer
named Hencke found another asteroid, after long searching, and opened a
new epoch of discovery. From then on the finding of asteroids became a
commonplace. Latterly, with the aid of photography, the list has been
extended to above four hundred, and as yet there seems no dearth in the
supply, though doubtless all the larger members have been revealed. Even
these are but a few hundreds of miles in diameter, while the smaller
ones are too tiny for measurement. The combined bulk of these minor
planets is believed to be but a fraction of that of the earth.
Olbers's explosion theory, long accepted by astronomers, has been
proven open to fatal objections. The minor planets are now believed to
represent a ring of cosmical matter, cast off from the solar nebula
like the rings that went to form the major planets, but prevented
from becoming aggregated into a single body by the perturbing mass of
Jupiter.
The Discovery of Neptune
As we have seen, the discovery of the first asteroid confirmed a
conjecture; the other important planetary discovery of the nineteenth
century fulfilled a prediction. Neptune was found through scientific
prophecy. No one suspected the existence of a trans-Uranian planet till
Uranus itself, by hair-breadth departures from its predicted orbit, gave
out the secret. No one saw the disturbing planet till the pencil of the
mathematician, with almost occult divination, had pointed out its place
in the heavens. The general predication of a trans-Uranian planet was
made by Bessel, the great Konigsberg astronomer, in 1840; the analysis
that revealed its exact location was undertaken, half a decade later,
by two independent workers--John Couch Adams, just graduated senior
wrangler at Cambridge, England, and U. J. J. Leverrier, the leading
French mathematician of his generation.
Adams's calculation was first begun and first completed. But it had one
radical defect--it was the work of a young and untried man. So it found
lodgment in a pigeon-hole of the desk of England's Astronomer Royal, and
an opportunity was lost which English astronomers have never ceased to
mourn. Had the search been made, an actual planet would have been seen
shining there, close to the spot where the pencil of the mathematician
had placed its hypothetical counterpart. But the search was not made,
and while the prophecy of Adams gathered dust in that regrettable
pigeon-hole, Leverrier's calculation was coming on, his tentative
results meeting full encouragement from Arago and other French savants.
At last the laborious calculations proved satisfactory, and, confident
of the result, Leverrier sent to the Berlin observatory, requesting that
search be made for the disturber of Uranus in a particular spot of the
heavens. Dr. Galle received the request September 23, 1846. That very
night he turned his telescope to the indicated region, and there, within
a single degree of the suggested spot, he saw a seeming star, invisible
to the unaided eye, which proved to be the long-sought planet,
henceforth to be known as Neptune. To the average mind, which finds
something altogether mystifying about abstract mathematics, this was a
feat savoring of the miraculous.
Stimulated by this success, Leverrier calculated an orbit for an
interior planet from perturbations of Mercury, but though prematurely
christened Vulcan, this hypothetical nursling of the sun still haunts
the realm of the undiscovered, along with certain equally hypothetical
trans-Neptunian planets whose existence has been suggested by "residual
perturbations" of Uranus, and by the movements of comets. No other
veritable additions of the sun's planetary family have been made in our
century, beyond the finding of seven small moons, which chiefly attest
the advance in telescopic powers. Of these, the tiny attendants of our
Martian neighbor, discovered by Professor Hall with the great Washington
refractor, are of greatest interest, because of their small size and
extremely rapid flight. One of them is poised only six thousand
miles from Mars, and whirls about him almost four times as fast as he
revolves, seeming thus, as viewed by the Martian, to rise in the west
and set in the east, and making the month only one-fourth as long as the
day.
The Rings of Saturn
The discovery of the inner or crape ring of Saturn, made simultaneously
in 1850 by William C. Bond, at the Harvard observatory, in America,
and the Rev. W. R. Dawes in England, was another interesting optical
achievement; but our most important advances in knowledge of Saturn's
unique system are due to the mathematician. Laplace, like his
predecessors, supposed these rings to be solid, and explained their
stability as due to certain irregularities of contour which Herschel
bad pointed out. But about 1851 Professor Peirce, of Harvard, showed
the untenability of this conclusion, proving that were the rings such as
Laplace thought them they must fall of their own weight. Then Professor
J. Clerk-Maxwell, of Cambridge, took the matter in hand, and his
analysis reduced the puzzling rings to a cloud of meteoric particles--a
"shower of brickbats"--each fragment of which circulates exactly as if
it were an independent planet, though of course perturbed and jostled
more or less by its fellows. Mutual perturbations, and the disturbing
pulls of Saturn's orthodox satellites, as investigated by Maxwell,
explain nearly all the phenomena of the rings in a manner highly
satisfactory.
After elaborate mathematical calculations covering many pages of his
paper entitled "On the Stability of Saturn's Rings," he summarizes his
deductions as follows:
"Let us now gather together the conclusions we have been able to draw
from the mathematical theory of various kinds of conceivable rings.
"We found that the stability of the motion of a solid ring depended
on so delicate an adjustment, and at the same time so unsymmetrical a
distribution of mass, that even if the exact conditions were fulfilled,
it could scarcely last long, and, if it did, the immense preponderance
of one side of the ring would be easily observed, contrary to
experience. These considerations, with others derived from the
mechanical structure of so vast a body, compel us to abandon any theory
of solid rings.
"We next examined the motion of a ring of equal satellites, and found
that if the mass of the planet is sufficient, any disturbances produced
in the arrangement of the ring will be propagated around it in the form
of waves, and will not introduce dangerous confusion. If the satellites
are unequal, the propagations of the waves will no longer be regular,
but disturbances of the ring will in this, as in the former case,
produce only waves, and not growing confusion. Supposing the ring to
consist, not of a single row of large satellites, but a cloud of evenly
distributed unconnected particles, we found that such a cloud must
have a very small density in order to be permanent, and that this is
inconsistent with its outer and inner parts moving with the same angular
velocity. Supposing the ring to be fluid and continuous, we found that
it will be necessarily broken up into small portions.
"We conclude, therefore, that the rings must consist of disconnected
particles; these must be either solid or liquid, but they must be
independent. The entire system of rings must, therefore, consist either
of a series of many concentric rings each moving with its own velocity
and having its own system of waves, or else of a confused multitude of
revolving particles not arranged in rings and continually coming into
collision with one another.
"Taking the first case, we found that in an indefinite number of
possible cases the mutual perturbations of two rings, stable in
themselves, might mount up in time to a destructive magnitude, and that
such cases must continually occur in an extensive system like that of
Saturn, the only retarding cause being the irregularity of the rings.
"The result of long-continued disturbance was found to be the
spreading-out of the rings in breadth, the outer rings pressing outward,
while the inner rings press inward.
"The final result, therefore, of the mechanical theory is that the only
system of rings which can exist is one composed of an indefinite number
of unconnected particles, revolving around the planet with different
velocities, according to their respective distances. These particles
may be arranged in series of narrow rings, or they may move through one
another irregularly. In the first case the destruction of the system
will be very slow, in the second case it will be more rapid, but there
may be a tendency towards arrangement in narrow rings which may retard
the process.
"We are not able to ascertain by observation the constitution of the two
outer divisions of the system of rings, but the inner ring is certainly
transparent, for the limb of Saturn has been observed through it. It is
also certain that though the space occupied by the ring is transparent,
it is not through the material parts of it that the limb of Saturn is
seen, for his limb was observed without distortion; which shows that
there was no refraction, and, therefore, that the rays did not pass
through a medium at all, but between the solar or liquid particles of
which the ring is composed. Here, then, we have an optical argument
in favor of the theory of independent particles as the material of
the rings. The two outer rings may be of the same nature, but not
so exceedingly rare that a ray of light can pass through their whole
thickness without encountering one of the particles.
"Finally, the two outer rings have been observed for two hundred years,
and it appears, from the careful analysis of all the observations of M.
Struve, that the second ring is broader than when first observed, and
that its inner edge is nearer the planet than formerly. The inner ring
also is suspected to be approaching the planet ever since its discovery
in 1850. These appearances seem to indicate the same slow progress of
the rings towards separation which we found to be the result of theory,
and the remark that the inner edge of the inner ring is more distinct
seems to indicate that the approach towards the planet is less rapid
near the edge, as we had reason to conjecture. As to the apparent
unchangeableness of the exterior diameter of the outer ring, we must
remember that the outer rings are certainly far more dense than the
inner one, and that a small change in the outer rings must balance a
great change in the inner one. It is possible, however, that some of the
observed changes may be due to the existence of a resisting medium.
If the changes already suspected should be confirmed by repeated
observations with the same instruments, it will be worth while to
investigate more carefully whether Saturn's rings are permanent or
transitory elements of the solar system, and whether in that part of
the heavens we see celestial immutability or terrestrial corruption
and generation, and the old order giving place to the new before our
eyes."(4)
Studies of the Moon
But perhaps the most interesting accomplishments of mathematical
astronomy--from a mundane standpoint, at any rate--are those that refer
to the earth's own satellite. That seemingly staid body was long ago
discovered to have a propensity to gain a little on the earth, appearing
at eclipses an infinitesimal moment ahead of time. Astronomers were
sorely puzzled by this act of insubordination; but at last Laplace and
Lagrange explained it as due to an oscillatory change in the earth's
orbit, thus fully exonerating the moon, and seeming to demonstrate the
absolute stability of our planetary system, which the moon's misbehavior
had appeared to threaten.
This highly satisfactory conclusion was an orthodox belief of celestial
mechanics until 1853, when Professor Adams of Neptunian fame, with whom
complex analyses were a pastime, reviewed Laplace's calculation, and
discovered an error which, when corrected, left about half the moon's
acceleration unaccounted for. This was a momentous discrepancy, which at
first no one could explain. But presently Professor Helmholtz, the great
German physicist, suggested that a key might be found in tidal friction,
which, acting as a perpetual brake on the earth's rotation, and
affecting not merely the waters but the entire substance of our planet,
must in the long sweep of time have changed its rate of rotation. Thus
the seeming acceleration of the moon might be accounted for as actual
retardation of the earth's rotation--a lengthening of the day instead of
a shortening of the month.
Again the earth was shown to be at fault, but this time the moon could
not be exonerated, while the estimated stability of our system, instead
of being re-established, was quite upset. For the tidal retardation is
not an oscillatory change which will presently correct itself, like the
orbital wobble, but a perpetual change, acting always in one direction.
Unless fully counteracted by some opposing reaction, therefore (as
it seems not to be), the effect must be cumulative, the ultimate
consequences disastrous. The exact character of these consequences was
first estimated by Professor G. H. Darwin in 1879. He showed that tidal
friction, in retarding the earth, must also push the moon out from the
parent planet on a spiral orbit. Plainly, then, the moon must formerly
have been nearer the earth than at present. At some very remote period
it must have actually touched the earth; must, in other words, have been
thrown off from the then plastic mass of the earth, as a polyp buds out
from its parent polyp. At that time the earth was spinning about in a
day of from two to four hours.
Now the day has been lengthened to twenty-four hours, and the moon has
been thrust out to a distance of a quarter-million miles; but the end is
not yet. The same progress of events must continue, till, at some remote
period in the future, the day has come to equal the month, lunar tidal
action has ceased, and one face of the earth looks out always at the
moon with that same fixed stare which even now the moon has been brought
to assume towards her parent orb. Should we choose to take even greater
liberties with the future, it may be made to appear (though some
astronomers dissent from this prediction) that, as solar tidal action
still continues, the day must finally exceed the month, and lengthen out
little by little towards coincidence with the year; and that the moon
meantime must pause in its outward flight, and come swinging back on a
descending spiral, until finally, after the lapse of untold aeons, it
ploughs and ricochets along the surface of the earth, and plunges to
catastrophic destruction.
But even though imagination pause far short of this direful culmination,
it still is clear that modern calculations, based on inexorable tidal
friction, suffice to revolutionize the views formerly current as to the
stability of the planetary system. The eighteenth-century mathematician
looked upon this system as a vast celestial machine which had been in
existence about six thousand years, and which was destined to run on
forever. The analyst of to-day computes both the past and the future of
this system in millions instead of thousands of years, yet feels well
assured that the solar system offers no contradiction to those laws of
growth and decay which seem everywhere to represent the immutable order
of nature.
COMETS AND METEORS
Until the mathematician ferreted out the secret, it surely never could
have been suspected by any one that the earth's serene attendant,
"That orbed maiden, with white fire laden,
Whom mortals call the moon,"
could be plotting injury to her parent orb. But there is another
inhabitant of the skies whose purposes have not been similarly free from
popular suspicion. Needless to say I refer to the black sheep of the
sidereal family, that "celestial vagabond" the comet.
Time out of mind these wanderers have been supposed to presage war,
famine, pestilence, perhaps the destruction of the world. And little
wonder. Here is a body which comes flashing out of boundless space into
our system, shooting out a pyrotechnic tail some hundreds of millions of
miles in length; whirling, perhaps, through the very atmosphere of the
sun at a speed of three or four hundred miles a second; then darting off
on a hyperbolic orbit that forbids it ever to return, or an elliptical
one that cannot be closed for hundreds or thousands of years; the tail
meantime pointing always away from the sun, and fading to nothingness as
the weird voyager recedes into the spatial void whence it came. Not many
times need the advent of such an apparition coincide with the outbreak
of a pestilence or the death of a Caesar to stamp the race of comets as
an ominous clan in the minds of all superstitious generations.
It is true, a hard blow was struck at the prestige of these alleged
supernatural agents when Newton proved that the great comet of 1680
obeyed Kepler's laws in its flight about the sun; and an even harder
one when the same visitant came back in 1758, obedient to Halley's
prediction, after its three-quarters of a century of voyaging but in
the abyss of space. Proved thus to bow to natural law, the celestial
messenger could no longer fully, sustain its role. But long-standing
notoriety cannot be lived down in a day, and the comet, though proved a
"natural" object, was still regarded as a very menacing one for
another hundred years or so. It remained for the nineteenth century to
completely unmask the pretender and show how egregiously our forebears
had been deceived.
The unmasking began early in the century, when Dr. Olbers, then the
highest authority on the subject, expressed the opinion that
the spectacular tail, which had all along been the comet's chief
stock-in-trade as an earth-threatener, is in reality composed of
the most filmy vapors, repelled from the cometary body by the sun,
presumably through electrical action, with a velocity comparable to that
of light. This luminous suggestion was held more or less in abeyance for
half a century. Then it was elaborated by Zollner, and particularly by
Bredichin, of the Moscow observatory, into what has since been regarded
as the most plausible of cometary theories. It is held that comets
and the sun are similarly electrified, and hence mutually repulsive.
Gravitation vastly outmatches this repulsion in the body of the comet,
but yields to it in the case of gases, because electrical force varies
with the surface, while gravitation varies only with the mass. From
study of atomic weights and estimates of the velocity of thrust of
cometary tails, Bredichin concluded that the chief components of the
various kinds of tails are hydrogen, hydrocarbons, and the vapor of
iron; and spectroscopic analysis goes far towards sustaining these
assumptions.
But, theories aside, the unsubstantialness of the comet's tail has been
put to a conclusive test. Twice during the nineteenth century the
earth has actually plunged directly through one of these threatening
appendages--in 1819, and again in 1861, once being immersed to a depth
of some three hundred thousand miles in its substance. Yet nothing
dreadful happened to us. There was a peculiar glow in the atmosphere,
so the more imaginative observers thought, and that was all. After such
fiascos the cometary train could never again pose as a world-destroyer.
But the full measure of the comet's humiliation is not yet told. The
pyrotechnic tail, composed as it is of portions of the comet's actual
substance, is tribute paid the sun, and can never be recovered. Should
the obeisance to the sun be many times repeated, the train-forming
material will be exhausted, and the comet's chiefest glory will have
departed. Such a fate has actually befallen a multitude of comets which
Jupiter and the other outlying planets have dragged into our system and
helped the sun to hold captive here. Many of these tailless comets were
known to the eighteenth-century astronomers, but no one at that time
suspected the true meaning of their condition. It was not even known how
closely some of them are enchained until the German astronomer Encke,
in 1822, showed that one which he had rediscovered, and which has
since borne his name, was moving in an orbit so contracted that it must
complete its circuit in about three and a half years. Shortly afterwards
another comet, revolving in a period of about six years, was discovered
by Biela, and given his name. Only two more of these short-period comets
were discovered during the first half of last century, but latterly they
have been shown to be a numerous family. Nearly twenty are known
which the giant Jupiter holds so close that the utmost reach of their
elliptical tether does not let them go beyond the orbit of Saturn. These
aforetime wanderers have adapted themselves wonderfully to planetary
customs, for all of them revolve in the same direction with the planets,
and in planes not wide of the ecliptic.
Checked in their proud hyperbolic sweep, made captive in a planetary
net, deprived of their trains, these quondam free-lances of the heavens
are now mere shadows of their former selves. Considered as to mere
bulk, they are very substantial shadows, their extent being measured in
hundreds of thousands of miles; but their actual mass is so slight that
they are quite at the mercy of the gravitation pulls of their captors.
And worse is in store for them. So persistently do sun and planets tug
at them that they are doomed presently to be torn into shreds.
Such a fate has already overtaken one of them, under the very eyes of
the astronomers, within the relatively short period during which these
ill-fated comets have been observed. In 1832 Biela's comet passed quite
near the earth, as astronomers measure distance, and in doing so created
a panic on our planet. It did no greater harm than that, of course, and
passed on its way as usual. The very next time it came within telescopic
hail it was seen to have broken into two fragments. Six years later
these fragments were separated by many millions of miles; and in 1852,
when the comet was due again, astronomers looked for it in vain. It had
been completely shattered.
What had become of the fragments? At that time no one positively knew.
But the question was to be answered presently. It chanced that just at
this period astronomers were paying much attention to a class of bodies
which they had hitherto somewhat neglected, the familiar shooting-stars,
or meteors. The studies of Professor Newton, of Yale, and Professor
Adams, of Cambridge, with particular reference to the great
meteor-shower of November, 1866, which Professor Newton had predicted
and shown to be recurrent at intervals of thirty-three years, showed
that meteors are not mere sporadic swarms of matter flying at random,
but exist in isolated swarms, and sweep about the sun in regular
elliptical orbits.
Presently it was shown by the Italian astronomer Schiaparelli that
one of these meteor swarms moves in the orbit of a previously observed
comet, and other coincidences of the kind were soon forthcoming. The
conviction grew that meteor swarms are really the debris of comets; and
this conviction became a practical certainty when, in November, 1872,
the earth crossed the orbit of the ill-starred Biela, and a shower of
meteors came whizzing into our atmosphere in lieu of the lost comet.
And so at last the full secret was out. The awe-inspiring comet, instead
of being the planetary body it had all along been regarded, is really
nothing more nor less than a great aggregation of meteoric particles,
which have become clustered together out in space somewhere, and which
by jostling one another or through electrical action become luminous. So
widely are the individual particles separated that the cometary body as
a whole has been estimated to be thousands of times less dense than the
earth's atmosphere at sea-level. Hence the ease with which the comet may
be dismembered and its particles strung out into streaming swarms.
So thickly is the space we traverse strewn with this cometary dust
that the earth sweeps up, according to Professor Newcomb's estimate, a
million tons of it each day. Each individual particle, perhaps no larger
than a millet seed, becomes a shooting-star, or meteor, as it burns to
vapor in the earth's upper atmosphere. And if one tiny planet sweeps
up such masses of this cosmic matter, the amount of it in the entire
stretch of our system must be beyond all estimate. What a story it tells
of the myriads of cometary victims that have fallen prey to the sun
since first he stretched his planetary net across the heavens!
THE FIXED STARS
When Biela's comet gave the inhabitants of the earth such a fright in
1832, it really did not come within fifty millions of miles of us. Even
the great comet through whose filmy tail the earth passed in 1861 was
itself fourteen millions of miles away. The ordinary mind, schooled to
measure space by the tiny stretches of a pygmy planet, cannot grasp the
import of such distances; yet these are mere units of measure compared
with the vast stretches of sidereal space. Were the comet which hurtles
past us at a speed of, say, a hundred miles a second to continue its
mad flight unchecked straight into the void of space, it must fly on its
frigid way eight thousand years before it could reach the very nearest
of our neighbor stars; and even then it would have penetrated but a
mere arm's-length into the vistas where lie the dozen or so of sidereal
residents that are next beyond. Even to the trained mind such distances
are only vaguely imaginable. Yet the astronomer of our century has
reached out across this unthinkable void and brought back many a secret
which our predecessors thought forever beyond human grasp.
A tentative assault upon this stronghold of the stars was being made
by Herschel at the beginning of the century. In 1802 that greatest of
observing astronomers announced to the Royal Society his discovery that
certain double stars had changed their relative positions towards one
another since he first carefully charted them twenty years before.
Hitherto it had been supposed that double stars were mere optical
effects. Now it became clear that some of them, at any rate, are
true "binary systems," linked together presumably by gravitation and
revolving about one another. Halley had shown, three-quarters of a
century before, that the stars have an actual or "proper" motion in
space; Herschel himself had proved that the sun shares this motion
with the other stars. Here was another shift of place, hitherto quite
unsuspected, to be reckoned with by the astronomer in fathoming sidereal
secrets.
Double Stars
When John Herschel, the only son and the worthy successor of the great
astronomer, began star-gazing in earnest, after graduating senior
wrangler at Cambridge, and making two or three tentative professional
starts in other directions to which his versatile genius impelled him,
his first extended work was the observation of his father's double
stars. His studies, in which at first he had the collaboration of Mr.
James South, brought to light scores of hitherto unrecognized pairs, and
gave fresh data for the calculation of the orbits of those longer
known. So also did the independent researches of F. G. W. Struve,
the enthusiastic observer of the famous Russian observatory at the
university of Dorpat, and subsequently at Pulkowa. Utilizing data
gathered by these observers, M. Savary, of Paris, showed, in 1827, that
the observed elliptical orbits of the double stars are explicable by
the ordinary laws of gravitation, thus confirming the assumption that
Newton's laws apply to these sidereal bodies. Henceforth there could be
no reason to doubt that the same force which holds terrestrial objects
on our globe pulls at each and every particle of matter throughout the
visible universe.
The pioneer explorers of the double stars early found that the systems
into which the stars are linked are by no means confined to single
pairs. Often three or four stars are found thus closely connected into
gravitation systems; indeed, there are all gradations between binary
systems and great clusters containing hundreds or even thousands of
members. It is known, for example, that the familiar cluster of the
Pleiades is not merely an optical grouping, as was formerly supposed,
but an actual federation of associated stars, some two thousand five
hundred in number, only a few of which are visible to the unaided eve.
And the more carefully the motions of the stars are studied, the more
evident it becomes that widely separated stars are linked together into
infinitely complex systems, as yet but little understood. At the same
time, all instrumental advances tend to resolve more and more seemingly
single stars into close pairs and minor clusters. The two Herschels
between them discovered some thousands of these close multiple systems;
Struve and others increased the list to above ten thousand; and Mr.
S. W. Burnham, of late years the most enthusiastic and successful of
double-star pursuers, added a thousand new discoveries while he was
still an amateur in astronomy, and by profession the stenographer of a
Chicago court. Clearly the actual number of multiple stars is beyond all
present estimate.
The elder Herschel's early studies of double stars were undertaken in
the hope that these objects might aid him in ascertaining the actual
distance of a star, through measurement of its annual parallax--that
is to say, of the angle which the diameter of the earth's orbit would
subtend as seen from the star. The expectation was not fulfilled. The
apparent shift of position of a star as viewed from opposite sides of
the earth's orbit, from which the parallax might be estimated, is so
extremely minute that it proved utterly inappreciable, even to the
almost preternaturally acute vision of Herschel, with the aid of any
instrumental means then at command. So the problem of star distance
allured and eluded him to the end, and he died in 1822 without seeing
it even in prospect of solution. His estimate of the minimum distance of
the nearest star, based though it was on the fallacious test of apparent
brilliancy, was a singularly sagacious one, but it was at best a
scientific guess, not a scientific measurement.
The Distance of the Stars
Just about this time, however, a great optician came to the aid of the
astronomers. Joseph Fraunhofer perfected the refracting telescope,
as Herschel had perfected the reflector, and invented a wonderfully
accurate "heliometer," or sun-measurer. With the aid of these
instruments the old and almost infinitely difficult problem of star
distance was solved. In 1838 Bessel announced from the Konigsberg
observatory that he had succeeded, after months of effort, in detecting
and measuring the parallax of a star. Similar claims had been made often
enough before, always to prove fallacious when put to further test; but
this time the announcement carried the authority of one of the greatest
astronomers of the age, and scepticism was silenced.
Nor did Bessel's achievement long await corroboration. Indeed, as so
often happens in fields of discovery, two other workers had almost
simultaneously solved the same problem--Struve at Pulkowa, where the
great Russian observatory, which so long held the palm over all others,
had now been established; and Thomas Henderson, then working at the
Cape of Good Hope, but afterwards the Astronomer Royal of Scotland.
Henderson's observations had actual precedence in point of time, but
Bessel's measurements were so much more numerous and authoritative that
he has been uniformly considered as deserving the chief credit of the
discovery, which priority of publication secured him.
By an odd chance, the star on which Henderson's observations were made,
and consequently the first star the parallax of which was ever measured,
is our nearest neighbor in sidereal space, being, indeed, some ten
billions of miles nearer than the one next beyond. Yet even this nearest
star is more than two hundred thousand times as remote from us as the
sun. The sun's light flashes to the earth in eight minutes, and to
Neptune in about three and a half hours, but it requires three and a
half years to signal Alpha Centauri. And as for the great majority of
the stars, had they been blotted out of existence before the Christian
era, we of to-day should still receive their light and seem to see them
just as we do. When we look up to the sky, we study ancient history;
we do not see the stars as they ARE, but as they WERE years, centuries,
even millennia ago.