THE SOLAR SYSTEM
In the intervals of personal observation Percival was often giving
lectures or writing on astronomical subjects for the publications of the
Observatory, and for scientific societies and periodicals. The substance
of most of these found their way into his books, which are summations or
expositions of his conclusions. In December 1902, for example, he gave
six lectures on “The Solar System” at the Massachusetts Institute of
Technology, of which he was a non-resident professor, and they were
published by Houghton, Mifflin & Company. Then in the autumn of 1906 he
gave a course of eight lectures at the Lowell Institute in Boston on
“Mars as the Abode of Life.” These were so crowded that they had to be
repeated, were then printed as six papers in the _Century Magazine_, and
finally re-published by The Macmillan Company under the same title. Two
years later, in the winter of 1909, he gave at the Massachusetts
Institute of Technology, another course of six lectures on “Cosmic
Physics: The Evolution of Worlds,” which were brought out in December by
the same publisher with the latter half of the title. Although their
names are so diverse, and far more is told of Mars in the book whose
title contains its name, they all deal essentially with the same
subject, the evolution of the planets and the development and end of
life upon them. In the Preface to “Mars as the Abode of Life,”—for a
preface, although printed at the beginning, is always written after the
book is finished, and is the author’s last word to the reader, giving
his latest thought as the work is being launched,—he tells us:[17]
“Though dealing specifically with Mars, the theme of the lectures was
that of planetary evolution in general, and this book is thus a
presentation of something which Professor Lowell has long had in mind
and of which his studies of Mars form but a part, the research into the
genesis and development of what we call a world; not the mere
aggregating of matter, but what that aggregation inevitably brings
forth. The subject which links the Nebular Hypothesis to the Darwinian
Theory, bridging the evolutionary gap between the two, he has called
planetology, thus designating the history of the planet’s individual
career. It is in this light that Mars is here regarded: how it came to
be what it is and how it came to differ from the Earth in the process.”
At each opposition, in fact at every opposition during Percival’s life
and long thereafter, Mars was observed at Flagstaff and more detail was
discovered confirming what had been found before. He tells of a slight
change in the estimated tilt in its axis; the fact that the temperature
is warmer than was earlier supposed;[18] and he had found how to
discover the gases by spectroscopic analysis applied according to an
ingenious device of his own known as “Velocity Shift” and much used
thereafter.[19] He tells also of an ingenious and elaborate experiment
with wires, and with lines on a wooden disk, which showed that such
lines can be perceived at a greater distance and therefore of smaller
size than had been supposed, so that the canals might have less width
than had been assumed. It is, however, needless, in describing his
planetary theory, to do more than allude to his evidence of Martian
habitation drawn from the canals, with which the reader is already
familiar. Curiously enough, however, it is interesting to note that on
September 9, 1909, about the time when “The Evolution of Worlds” was
going to press, a strange phenomenon appeared in Mars. Two striking
canals were seen where none had ever been seen before, and the most
conspicuous on that part of the disk. Moreover, they were photographed.
After examining all the maps of canals made at Flagstaff and elsewhere,
Percival discussed them in the Observatory Bulletin No. 45, and
concluded that they must not only be new to us, but new to Mars since
its previous corresponding season of two of our years before: “something
_extra ordinem naturae_.” We may here leave Mars for the time, and turn
to the more extensive study of the evolution of the planetary system.
The desire to rise from a particular case to a more general law was
characteristic of his attitude of mind, constructive and insatiable, and
appears throughout these volumes. It may have been influenced by his
great master Benjamin Peirce, who ever treated any mathematical formula
as a special instance of a more comprehensive one. In such a subject as
the evolution of the planets, especially of life on them, it involved
dipping into many sciences, beyond the physical laws of matter; and he
says in the same preface: “As in all theses, the cogency of the
conclusion hangs upon the validity of each step in the argument. It is
vital that each of these should be based on all that we know of natural
laws and the general principles underlying them.” This did not mean that
all his premises would be universally accepted, but that he found out
all he could about them, convincing himself of their accuracy and of the
validity of the conclusions he draws therefrom. That is all any man of
science can do in a subject larger than his own special, and therefore
limited, field.
But from the time of his resumption of research and the direction of the
observatory in 1901, he was constantly enlarging his own field by the
study of astrophysical subjects, and the methods for their
determination. With this object he was initiating and encouraging
planetary photography. He was constantly writing Dr. V. M. Slipher about
procuring and using spectrographic apparatus and about the results
obtained by him therefrom. By this process the rotations of planets were
determined; and the spectra of the major ones—often reproduced in
astronomical works—have been a puzzle to astrophysicists until their
interpretation in very recent years. He was interested also in nebulae,
especially in spiral ones, taking part in Dr. Slipher’s pioneering
spectrographic work at the observatory, which showed that they were vast
aggregations of stars of different spectral types, moving with great
speed, and far beyond the limits of our universe. For over fifteen years
the observatory was almost alone in this field of research, as well as
in that of globular clusters. It is in fact, the discovery of the rapid
motion of the spiral nebulae away from the solar system that has given
rise to the conception of an expanding universe.
But these discoveries were still largely in the future, and to return to
his books on the planetary system it may be noted that in the two larger
and more popular ones the general planetary theory is expounded in the
text, while the demonstrations of the more complex statements made, and
the mathematical calculations involved, are relegated to a mass of notes
at the end of the volume.
The first of his books on the solar system is the small volume bearing
that title; but since all three of the books here described are several
expositions of the same subject it may be well to treat his views on
each topic in connection with the work in which he deals with it most
fully. Indeed, “The Solar System” is not a general treatise, but rather
a discussion of some striking points, and it is these which one thinks
of in connection therewith.
In considering the origin of the planets he had become much interested
in the meteors, shooting stars, meteoric streams and comets, all or
almost all of which he regarded as parts of the solar system, revolving
about the Sun in elliptic orbits, often so eccentric as to appear
parabolas.[20] The old idea that comets came from outer space and
therefore travelled in hyperbolas can, he points out, be true of few, if
any, of them. “Very few, three or four perhaps, hint at hyperbolas. Not
one is such beyond question.” Many of them are associated with the
meteoric streams with which everyone is familiar at certain seasons of
the year. Indeed seventy-six of these associations were then known, and
comets sometimes break up into such streams.
Now if the comets are travelling in orbits around the Sun they must be
throughout their course within its control, and not within that of some
other star; and therefore he computes how far the Sun’s control extends.
Taking for this purpose our nearest star, α Centauri, a double with a
total mass twice that of the Sun, at a distance of 275,000 astronomical
units, in other words that number of times our distance from the Sun, he
finds that the point at which its attraction and that of the Sun become
equal is 114,000 of these units. This he calls the extent of the Sun’s
domain, certainly an area large enough for any, or almost any, comet
known.[21]
He then turns to some of the planets,—Mercury to show the effect of
tidal action in slowing the rotation of a planet or satellite, and
causing it to turn the same face always to its master.[22] This involved
a highly interesting comparison of Newton’s theory of the tides, long
generally accepted, but not taking enough account of the planet’s
rotation, and that of Sir George Darwin based upon the effect of such
rotation. The general conceptions are even more different than the
results, and the later theory is less concerned with the tides in
oceans, which probably affect only our Earth, than with those of a
planet in a fluid or viscous condition, which may still continue to some
extent after the surface has become partly solidified. He therefore
studies the tide raising force, and the tendency to retardation of
rotation, by the Sun on the planets, and by these on their satellites
while still in a fluid state, tabulating some very striking results.
What he says about Mars is more fully dealt with in his other writings;
and the same is true of Saturn’s rings, except for the reference to the
calculation by Edward Roche of the limit of possible approach by a fluid
satellite to its planet without being disrupted, and for the fact that
this limit in Saturn’s case falls just beyond the outer edge of the
rings. In discussing Saturn’s satellites he brings out a curious analogy
between the order of distribution of these attendants of the three best
known major planets and the order of the planets themselves about the
Sun. In each case the largest of the bodies so revolving is nearly in
the centre of the line, as in the case of Jupiter among the planets; the
second largest the next, or not far, beyond, as in the case of Saturn;
while there is another maximum farther in, for as the Earth is larger
than any planet on either side until Jupiter is reached, so a like order
is found in the satellites of Jupiter, Saturn and Uranus. In other
words, the size in each case rises with increasing distance, falls off,
then rises again to the largest and thence declines. This he believed
cannot be an accidental coincidence, but the result of a law of
development as yet unexplained.
To the ordinary reader the most novel thing he says about Jupiter
relates to its family of comets, for no less than thirty-two of these
bodies have their aphelia, or greatest distance from the Sun, near its
orbit. Moreover, their ascending nodes—that is the place where their
paths if inclined to the plane of the ecliptic pass through it—are close
to its orbit. At some time, therefore, in the vast ages of the past they
must have passed close to the planet, and if so have had their orbits
greatly changed by its attraction. He considers the various effects
Jupiter may have upon a comet, and shows—contrary to the opinion of
Professor H. A. Newton—that any such body moving by the attraction of
the Sun would be going too fast for Jupiter to capture completely. Then
he takes up other effects of deflection. The comet’s speed may be
accelerated and its direction changed even so much as to drive it out of
the solar system; it may be retarded so that its path is contracted and
the aphelion drawn nearer to the planet’s orbit. After calculating the
possible conditions and analyzing the actual orbits of Jupiter’s family,
he comes to the provisional conclusion that these comets have been drawn
from the neighborhood. “It is certain,” he says, “that Jupiter has swept
his neighborhood.... If we consider the comet aphelia of short-period
comets, we shall notice that they are clustered about the path of
Jupiter and the path of Saturn, thinning out to a neutral ground
between, where there are none. Two-thirds of the way from Jupiter’s
orbit to Saturn’s, space is clear of them, the centre of the gap falling
at 8.4 astronomical units from the sun....
“Jupiter is not the only planet that has a comet family. All the large
planets have the like. Saturn has a family of two, Uranus also of two,
Neptune of six; and the spaces between these planets are clear of comet
aphelia; the gaps prove the action.
“Nor does the action, apparently, stop there. Plotting the aphelia of
all the comets that have been observed, we find, as we go out from the
Sun, clusters of them at first, representing, respectively, Jupiter’s,
Saturn’s, Uranus’, and Neptune’s family;[23] but the clusters do not
stop with Neptune. Beyond that planet is a gap, and then at 49 and 50
astronomical units we find two more aphelia, and then nothing again till
we reach 75 units out.
“This can hardly be accident; and if not chance, it means a planet out
there as yet unseen by man, but certain sometime to be detected and
added to the others. Thus not only are comets a part of our system now
recognized, but they act as finger-posts to planets not yet known.”
We shall hear more of this last suggestion hereafter.
In both “Mars as the Abode of Life” and “The Evolution of Worlds,” he
accepts the proposition that our present solar system began with a
collision with some dark body from interstellar space, as had been
suggested by Chamberlin and Moulton a few years before. He points out
that stars which have finished contracting, grown cold and ceased to be
luminous, must exist, and although we cannot see them directly we know
about some of them,—such as the dark companion of Algol, revolving
around it and cutting off two-thirds of its light every three days. Many
dark wanderers there must be, and the _novae_, as he says, are
sometimes, at least, due to a collision with such a body,—not
necessarily an actual impact, but an approach so near that the star is
sprung asunder by the tidal effect. In such a case the opposite sides of
the victim would be driven away from it, and if it was rotating would
form spirals. Now we know that the apparently empty spaces in our solar
system still contain a vast number of little meteoric particles, which
as judged from their velocity do not fall from outer space, but are
members of our system travelling in their own orbits around the sun. As
he puts it, “Could we rise a hundred miles above the Earth’s surface we
should be highly sorry we came, for we should incontinently be killed by
flying brickbats. Instead of masses of a sunlike size we should have to
do with bits of matter on the average smaller than ourselves[24] but
hardly on that account innocuous, as they would strike us with fifteen
hundred times the speed of an express train.” That these meteorites are
moving in the same direction as the Earth he shows by an ingenious
calculation of the proportion that in such a case would be seen at
sunrise and sunset, which accords with the observed facts. Moreover,
their chemical composition shows that they were once parts of a great
hot body from which they have been expelled.
The meteorites that are seen because they become hot and luminous in
traversing our atmosphere, and occasionally fall upon the Earth, are the
remnants of vastly larger numbers formerly circling about the sun, but
which, by collision and attraction, were, as he describes, gathered into
great masses, thus forming the planets. The force of gravity gradually
compacted these fragments closer and closer together, thereby generating
heat which if the body were homogeneous would be in proportion to the
square of its mass. The larger the planet therefore the more heat it
would generate, and owing to the fact that mass is in proportion to the
cube and its radiating surface to the square of the diameter the slower
it would radiate, and thus lose, its heat, so that the larger ones would
be hotter and remain hot longer than the smaller ones.
Some of the planets may once have been white-hot, and luminous of
themselves, some were certainly red-hot, some only darkly warm; all
growing cooler after the amount radiated exceeded the amount generated.
Now by the difference in the heat generated and retained by the larger
and smaller bodies he explains the diverse appearance of those whose
surfaces we know, the Earth, Mars and the Moon. As the surface cools it
forms a crust, but if the interior still remains molten it will continue
to contract, the crust will be too large for it and crinkle, like the
skin of a dried apple; and this will be more true of a large than a
small body. “In like manner is volcanic action relatively increased, and
volcanoes arise, violent and widespread, in proportion; since these are
vents by which the molten matter under pressure within finds exit
abroad.” By a calculation, which agrees with the formula of Laplace, he
finds that the effective internal heat of the Earth might be 10,000
degrees Fahrenheit, enough to account for all the phenomena; and for
Mars only 2,000, which is below the melting point of iron, and would not
cause volcanic action. Now the observations of Mars at Flagstaff show
that there can be no mountains on it more than two or three thousand
feet high, and that the surface is singularly flat.
But here he met a difficulty; for the Moon ought to be flatter still if
it had evolved in the ordinary way, whereas it has enormous volcanic
cones, craters 17,000 feet high, some exceeding 100 miles in diameter,
and a range of mountains rising to nearly 30,000 feet. An explanation he
finds in the analysis of the action of the tides in the Earth-Moon
system by Sir George Darwin, who showed that when traced backward it
“lands us at a time when the Moon might have formed a part of the
Earth’s mass, the two rotating together as a single pear-shaped body in
about five hours.... For in that event the internal heat which the Moon
carried away with it must have been that of the parent body—the amount
the Earth-Moon had been able to amass. Thus the Moon was endowed from
the start of its separate existence with an amount of heat the falling
together of its own mass could never have generated. Thus its great
craters and huge volcanic cones stand explained. It did not originate as
a separate body, but had its birth in a rib of Earth.”[25]
The Flagstaff site having been selected for the purpose of planetary
observation yielded facts less easily detected elsewhere. Mercury, for
instance, is so near the Sun that it could be observed in the dark only
a short time after sunset and before sunrise, an obstacle that gave rise
to errors of fact. Schiaparelli led the way to better results by
observing this planet in broad daylight. Up to that time it had been
supposed to rotate on its axis in about twenty-four hours, and therefore
to have a day and night like those of the Earth, but daylight
observation showed him markings constant on its illuminated face, and
therefore that it turns nearly the same side to the Sun. Before knowing
his conclusions, and therefore independently, the study of Mercury was
taken up at Flagstaff in 1896, and the result was a complete
corroboration of his work. It showed that, as in the case of the Moon
with the Earth, tidal action on the still partially fluid mass had
slowed its rotation until it has little with regard to the central body
around which it revolves. He discovered also other facts about Mercury,
which Schiaparelli had not, that its size, mass and density had not been
accurately measured.
A similar discovery about the period of rotation had been made in the
case of Venus. For more than two centuries astronomers had felt sure
that this period was just under twenty-four hours, figured, indeed, to
the minute. But again it was Schiaparelli who doubted, and once more by
observing the planet at noon; when he noted that the markings on the
disk did not change from day to day, and concluded that the same side
was always pointed at the Sun. At Flagstaff in 1896 his observations
were verified and the inference later confirmed by the spectroscope,
which was, indeed, first brought to the Observatory for that purpose.
Thus Venus, which from its distance from the Sun, its size and density,
is most like the Earth, turns out to be in a totally different
condition, one face baked by unending glare, the other chilled in
interstellar night, and as he puts it: “To Venus the Sun stands
substantially stock-still in the sky,— ... No day, no seasons,
practically no year, diversifies existence or records the flight of
time. Monotony eternalized,—such is Venus’ lot.”[26]
On the movements and physical condition of the Earth it was needless to
dwell, and he passed to the asteroids. He describes how they began to be
discovered at the beginning of the last century by searching for a
planet that would fill a gap in Bode’s law. This, a formula of
arithmetical progression for the distances of the planets from the Sun,
has proved not to be a law at all, especially since the discovery of
Neptune which is much nearer than the formula required; but for nearly a
century it had a strong influence on astronomic thought, and the gap in
the series between Mars and Jupiter was searched for the missing link.
Two were found, then two more, about the middle of the last century
another, and then many, smaller and smaller, until by the time Percival
wrote six hundred were known, and their number seems limitless. Only the
four first found, he remarks, exceed a hundred miles in diameter, the
greater part being hardly over ten or twenty. But here he points out a
notable fact, that they are not evenly distributed throughout this
space; and although massed in a series growing thicker toward its centre
there are many gaps, even close to the centre, where few or no asteroids
are found. Now it is the large size and attraction of Jupiter by which
Percival explains the presence of asteroids with gaps in their ranks,
instead of a planet, in the space between it and Mars; but we shall hear
much more of this subject when we come to his work on Saturn’s rings and
the order in the distribution of the planets.
Jupiter, he tells us, having a mass 318 times that of the Earth, and a
volume 1400 times as large, is much less dense, not much more than
water, in short still fluid; and as it has a tremendous spin, rotating
in less than ten hours, it is more oblate than the Earth; that is, the
diameter at its equator is larger in proportion to that from pole to
pole. The observations at Flagstaff brought out some interesting facts:
first, that the dark belts of cloud that surround it are red, looking as
if the planet within were still molten;[27] second, that the bright
central belt lies exactly upon its equator, without regard to, and hence
independent of, its tilt toward the Sun, and that the belts of cloud on
each side appear at the planet’s morning just as they left it in the
evening. All which shows that Jupiter’s cloud formation is not due to
the Sun, but to its own internal heat, an interpretation of the
phenomena that has a direct bearing on his explanation of the Earth’s
carboniferous age.
Saturn is still less dense, even more oblate; but its most extraordinary
feature is of course the rings. Assumed by the early astronomers to be
solid and continuous, they were later shown to have concentric
intervals, and to be composed of discrete particles. They have usually
been supposed flat, but when the position of the planet was such that
they were seen on edge knots or beads appeared upon them; and in 1907
these were studied critically at Flagstaff, when it was found that the
shadows of the rings on the planet were not uniform, but had dark cores;
these thicker places lying on the outer margin of each ring where it
came to one of the intervals. These phenomena he explained in the same
way as the distribution of the intervals among the asteroids.[28]
About Uranus and Neptune he tells us in this book little that was not
known, and save for their orbits, masses and satellites not much was
known of their condition. But later, in 1911, the spectroscope at
Flagstaff determined the rotation period of Uranus, afterwards precisely
duplicated at the Lick; and later still the spectral bands in the vast
atmosphere of the giant planets were identified as due to methane, or
marsh, gas.[29]