THE EFFECT OF OTHER BODIES ON THE SUN
If solar activity is really an important factor in causing climatic
changes, it behooves us to subject the sun to the same kind of inquiry
to which we have subjected the earth. We have inquired into the nature
of the changes through which the earth's crust, the oceans, and the
atmosphere have influenced the climate of geological times. It has not
been necessary, however, to study the origin of the earth, nor to trace
its earlier stages. Our study of the geological record begins only when
the earth had attained practically its present mass, essentially its
present shape, and a climate so similar to that of today that life as we
know it was possible. In other words, the earth had passed the stages of
infancy, childhood, youth, and early maturity, and had reached full
maturity. As it still seems to be indefinitely far from old age, we
infer that during geological times its relative changes have been no
greater than those which a man experiences between the ages of perhaps
twenty-five and forty.
Similar reasoning applies with equal or greater force to the sun.
Because of its vast size it presumably passes through its stages of
development much more slowly than the earth. In the first chapter of
this book we saw that the earth's relative uniformity of climate for
hundreds of millions of years seems to imply a similar uniformity in
solar activity. This accords with a recent tendency among astronomers
who are more and more recognizing that the stars and the solar system
possess an extraordinary degree of conservatism. Changes that once were
supposed to take place in thousands of years are now thought to have
required millions. Hence in this chapter we shall assume that throughout
geological times the condition of the sun has been almost as at present.
It may have been somewhat larger, or different in other ways, but it was
essentially a hot, gaseous body such as we see today and it gave out
essentially the same amount of energy. This assumption will affect the
general validity of what follows only if it departs widely from the
truth. With this assumption, then, let us inquire into the degree to
which the sun's atmosphere has probably been disturbed throughout
geological times.
In _Earth and Sun_, as already explained, a detailed study has led to
the conclusion that cyclonic storms are influenced by the electrical
action of the sun. Such action appears to be most intense in sunspots,
but apparently pertains also to other disturbed areas in the sun's
atmosphere. A study of sunspots suggests that their true periodicity is
almost if not exactly identical with that of the orbital revolution of
Jupiter, 11.8 years. Other investigations show numerous remarkable
coincidences between sunspots and the orbital revolution of the other
planets, including especially Saturn and Mercury. This seems to indicate
that there is some truth in the hypothesis that sunspots and other
related disturbances of the solar atmosphere owe their periodicity to
the varying effects of the planets as they approach and recede from the
sun in their eccentric orbits and as they combine or oppose their
effects according to their relative positions. This does not mean that
the energy of the solar disturbances is supposed to come from the
planets, but merely that their variations act like the turning of a
switch to determine when and how violently the internal forces of the
sun shall throw the solar atmosphere into commotion. This hypothesis is
by no means new, for in one form or another it has been advocated by
Wolfer, Birkeland, E. W. Brown, Schuster, Arctowski, and others.
The agency through which the planets influence the solar atmosphere is
not yet clear. The suggested agencies are the direct pull of
gravitation, the tidal effect of the planets, and an electro-magnetic
effect. In _Earth and Sun_ the conclusion is reached that the first two
are out of the question, a conclusion in which E. W. Brown acquiesces.
Unless some unknown cause is appealed to, this leaves an
electro-magnetic hypothesis as the only one which has a reasonable
foundation. Schuster inclines to this view. The conclusions set forth in
_Earth and Sun_ as to the electrical nature of the sun's influence on
the earth point somewhat in the same direction. Hence in this chapter we
shall inquire what would happen to the sun, and hence to the earth, on
their journey through space, if the solar atmosphere is actually subject
to disturbance by the electrical or other effects of other heavenly
bodies. It need hardly be pointed out that we are here venturing into
highly speculative ground, and that the verity or falsity of the
conclusions reached in this chapter has nothing to do with the validity
of the reasoning in previous chapters. Those chapters are based on the
assumption that terrestrial causes of climatic changes are supplemented
by solar disturbances which produce their effect partly through
variations in temperature but also through variations in the intensity
and paths of cyclonic storms. The present chapter seeks to shed some
light on the possible causes and sequence of solar disturbances.
Let us begin by scanning the available evidence as to solar disturbances
previous to the time when accurate sunspot records are available. Two
rather slender bits of evidence point to cycles of solar activity
lasting hundreds of years. One of these has already been discussed in
Chapter VI, where the climatic stress of the fourteenth century was
described. At that time sunspots are known to have been unusually
numerous, and there were great climatic extremes. Lakes overflowed in
Central Asia; storms, droughts, floods, and cold winters were unusually
severe in Europe; the Caspian Sea rose with great rapidity; the trees of
California grew with a vigor unknown for centuries; the most terrible of
recorded famines occurred in England and India; the Eskimos were
probably driven south by increasing snowiness in Greenland; and the
Mayas of Yucatan appear to have made their last weak attempt at a
revival of civilization under the stimulus of greater storminess and
less constant rainfall.
The second bit of evidence is found in recent exhaustive studies of
periodicities by Turner[112] and other astronomers. They have sought
every possible natural occurrence for which a numerical record is
available for a long period. The most valuable records appear to be
those of tree growth, Nile floods, Chinese earthquakes, and sunspots.
Turner reaches the conclusion that all four types of phenomena show the
same periodicity, namely, cycles with an average length of about 260 to
280 years. He suggests that if this is true, the cycles in tree growth
and in floods, both of which are climatic, are probably due to a
non-terrestrial cause. The fact that the sunspots show similar cycles
suggests that the sun's variations are the cause.
These two bits of evidence are far too slight to form the foundation of
any theory as to changes in solar activity in the geological past.
Nevertheless it may be helpful to set forth certain possibilities as a
stimulus to further research. For example, it has been suggested that
meteoric bodies may have fallen into the sun and caused it suddenly to
flare up, as it were. This is not impossible, although it does not
appear to have taken place since men became advanced enough to make
careful observations. Moreover, the meteorites which now fall on the
earth are extremely small, the average size being computed as no larger
than a grain of wheat. The largest ever found on the earth's surface, at
Bacubirito in Mexico, weighs only about fifty tons, while within the
rocks the evidences of meteorites are extremely scanty and
insignificant. If meteorites had fallen into the sun often enough and of
sufficient size to cause glacial fluctuations and historic pulsations of
climate, it seems highly probable that the earth would show much more
evidence of having been similarly disturbed. And even if the sun should
be bombarded by large meteors the result would probably not be sudden
cold periods, which are the most notable phenomena of the earth's
climatic history, but sudden warm periods followed by slow cooling.
Nevertheless, the disturbance of the sun by collision with meteoric
matter can by no means be excluded as a possible cause of climatic
variations.
Allied to the preceding hypothesis is Shapley's[113] nebular hypothesis.
At frequent intervals, averaging about once a year during the last
thirty years, astronomers have discovered what are known as novae. These
are stars which were previously faint or even invisible, but which flash
suddenly into brilliancy. Often their light-giving power rises seven or
eight magnitudes--a thousand-fold. In addition to the spectacular novae
there are numerous irregular variables whose brilliancy changes in every
ratio from a few per cent up to several magnitudes. Most of them are
located in the vicinity of nebulae, as is also the case with novae. This,
as well as other facts, makes it probable that all these stars are
"friction variables," as Shapley calls them. Apparently as they pass
through the nebulae they come in contact with its highly diffuse matter
and thereby become bright much as the earth would become bright if its
atmosphere were filled with millions of almost infinitesimally small
meteorites. A star may also lose brilliancy if nebulous matter
intervenes between it and the observer. If our sun has been subjected to
any of these changes some sort of climatic effect must have been
produced.
In a personal communication Shapley amplifies the nebular climatic
hypothesis as follows:
Within 700 light years of the sun in many directions (Taurus,
Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity,
some bright, most of them dark. The probability that stars moving in
the general region of such clouds will encounter this material is
very high, for the clouds fill enormous volumes of space,--e.g.,
probably more than a hundred thousand cubic light years in the Orion
region, and are presumably composed of rarefied gases or of dust
particles. Probably throughout all our part of space such nebulosity
exists (it is all around us, we are sure), but only in certain
regions is it dense enough to affect conspicuously the stars
involved in it. If a star moving at high velocity should collide
with a dense part of such a nebulous cloud, we should probably have
a typical nova. If the relative velocity of nebulous material and
star were low or moderate, or if the material were rare, we should
not expect a conspicuous effect on the star's light.
In the nebulous region of Orion, which is probably of unusually high
density, there are about 100 known stars, varying between 20% and
80% of their total light--all of them irregularly--some slowly, some
suddenly. Apparently they are "friction variables." Some of the
variables suddenly lose 40% of their light as if blanketed by
nebulous matter. In the Trifid Nebula there are variables like those
of Orion, in Messier 8 also, and probably many of the 100 or so
around the Rho Ophiuchi region belong to this kind.
I believe that our sun could not have been a typical nova, at least
not since the Archeozoic, that is for perhaps a billion years. I
believe we have in geological climates final proof of this, because
an increase in the amount of solar radiation by 1000 times as in the
typical nova, would certainly punctuate emphatically the life cycle
on the earth, even if the cause of the nova would not at the same
time eliminate the smaller planets. But the sun may have been one of
these miniature novae or friction variables; and I believe it very
probable that its wanderings through this part of space could not
long leave its mean temperature unaffected to the amount of a few
per cent.
One reason we have not had this proposal insisted upon before is
that the data back of it are mostly new--the Orion variables have
been only recently discovered and studied, the distribution and
content of the dark nebulae are hardly as yet generally known.
This interesting hypothesis cannot be hastily dismissed. If the sun
should pass through a nebula it seems inevitable that there would be at
least slight climatic effects and perhaps catastrophic effects through
the action of the gaseous matter not only on the sun but on the earth's
own atmosphere. As an explanation of the general climatic conditions of
the past, however, Shapley points out that the hypothesis has the
objection of being vague, and that nebulosity should not be regarded as
more than "a possible factor." One of the chief difficulties seems to be
the enormously wide distribution of as yet undiscovered nebulous matter
which must be assumed if any large share of the earth's repeated
climatic changes is to be ascribed to such matter. If such matter is
actually abundant in space, it is hard to see how any but the nearest
stars would be visible. Another objection is that there is no known
nebulosity near at hand with which to connect the climatic vicissitudes
of the last glacial period. Moreover, the known nebulae are so much less
numerous than stars that the chances that the sun will encounter one of
them are extremely slight. This, however, is not an objection, for
Shapley points out that during geological times the sun can never have
varied as much as do the novae, or even as most of the friction
variables. Thus the hypothesis stands as one that is worth
investigating, but that cannot be finally rejected or accepted until it
is made more definite and until more information is available.
Another suggested cause of solar variations is the relatively sudden
contraction of the sun such as that which sometimes occurs on the earth
when continents are uplifted and mountains upheaved. It seems improbable
that this could have occurred in a gaseous body like the sun. Lacking,
as it does, any solid crust which resists a change of form, the sun
probably shrinks steadily. Hence any climatic effects thus produced must
be extremely gradual and must tend steadily in one direction for
millions of years.
Still another suggestion is that the tidal action of the stars and other
bodies which may chance to approach the sun's path may cause
disturbances of the solar atmosphere. The vast kaleidoscope of space is
never quiet. The sun, the stars, and all the other heavenly bodies are
moving, often with enormous speed. Hence the effect of gravitation upon
the sun must vary constantly and irregularly, as befits the geological
requirements. In the case of the planets, however, the tidal effect does
not seem competent to produce the movements of the solar atmosphere
which appear to be concerned in the inception of sunspots. Moreover,
there is only the most remote probability that a star and the sun will
approach near enough to one another to produce a pronounced
gravitational disturbance in the solar atmosphere. For instance, if it
be assumed that changes in Jupiter's tidal effect on the sun are the
main factor in regulating the present difference between sunspot maxima
and sunspot minima, the chances that a star or some non-luminous body of
similar mass will approach near enough to stimulate solar activity and
thereby bring on glaciation are only one in twelve billion years, as
will be explained below. This seems to make a gravitational hypothesis
impossible.
Another possible cause of solar disturbances is that the stars in their
flight through space may exert an electrical influence which upsets the
equilibrium of the solar atmosphere. At first thought this seems even
more impossible than a gravitational effect. Electrostatic effects,
however, differ greatly from those of tides. They vary as the diameter
of a body instead of as its mass; their differentials also vary
inversely as the square of the distance instead of as the cube.
Electrostatic effects also increase as the fourth power of the
temperature or at least would do so if they followed the law of black
bodies; they are stimulated by the approach of one body to another; and
they are cumulative, for if ions arrive from space they must accumulate
until the body to which they have come begins to discharge them. Hence,
on the basis of assumptions such as those used in the preceding
paragraph, the chances of an electrical disturbance of the solar
atmosphere sufficient to cause glaciation on the earth may be as high as
one in twenty or thirty million years. This seems to put an electrical
hypothesis within the bounds of possibility. Further than that we cannot
now go. There may be other hypotheses which fit the facts much better,
but none seems yet to have been suggested.
In the rest of this chapter the tidal and electrical hypotheses of
stellar action on the sun will be taken up in detail. The tidal
hypothesis is considered because in discussions of the effect of the
planets it has hitherto held almost the entire field. The electrical
hypothesis will be considered because it appears to be the best yet
suggested, although it still seems doubtful whether electrical effects
can be of appreciable importance over such vast distances as are
inevitably involved. The discussion of both hypotheses will necessarily
be somewhat technical, and will appeal to the astronomer more than to
the layman. It does not form a necessary part of this book, for it has
no bearing on our main thesis of the effect of the sun on the earth. It
is given here because ultimately the question of changes in solar
activity during geological times must be faced.
In the astronomical portion of the following discussion we shall follow
Jeans[114] in his admirable attempt at a mathematical analysis of the
motions of the universe. Jeans divides the heavenly bodies into five
main types. (1) Spiral nebulae, which are thought by some astronomers to
be systems like our own in the making, and by others to be independent
universes lying at vast distances beyond the limits of our Galactic
universe, as it is called from the Galaxy or Milky Way. (2) Nebulae of a
smaller type, called planetary. These lie within the Galactic portion of
the universe and seem to be early stages of what may some day be stars
or solar systems. (3) Binary or multiple stars, which are
extraordinarily numerous. In some parts of the heavens they form 50 or
even 60 per cent of the stars and in the galaxy as a whole they seem to
form "fully one third." (4) Star clusters. These consist of about a
hundred groups of stars in each of which the stars move together in the
same direction with approximately the same velocity. These, like the
spiral nebulae, are thought by some astronomers to lie outside the limits
of the galaxy, but this is far from certain. (5) The solar system.
According to Jeans this seems to be unique. It does not fit into the
general mathematical theory by which he explains spiral nebulae,
planetary nebulae, binary stars, and star clusters. It seems to demand a
special explanation, such as is furnished by tidal disruption due to the
passage of the sun close to another star.
The part of Jeans' work which specially concerns us is his study of the
probability that some other star will approach the sun closely enough to
have an appreciable gravitative or electrical effect, and thus cause
disturbances in the solar atmosphere. Of course both the star and the
sun are moving, but to avoid circumlocution we shall speak of such
mutual approaches simply as approaches of the sun. For our present
purpose the most fundamental fact may be summed up in a quotation from
Jeans in which he says that most stars "show evidence of having
experienced considerable disturbance by other systems; there is no
reason why our solar system should be expected to have escaped the
common fate." Jeans gives a careful calculation from which it is
possible to derive some idea of the probability of any given degree of
approach of the sun and some other star. Of course all such calculations
must be based on certain assumptions. The assumptions made by Jeans are
such as to make the probability of close approaches as great as
possible. For example, he allows only 560 million years for the entire
evolution of the sun, whereas some astronomers and geologists would put
the figure ten or more times as high. Nevertheless, Jeans' assumptions
at least show the order of magnitude which we may expect on the basis of
reasonable astronomical conclusions.
According to the planetary hypothesis of sunspots, the difference in the
effect of Jupiter when it is nearest and farthest from the sun is the
main factor in starting the sunspot cycle and hence the corresponding
terrestrial cycle. The climatic difference between sunspot maxima and
minima, as measured by temperature, apparently amounts to at least a
twentieth and perhaps a tenth of the difference between the climate of
the last glacial epoch and the present. We may suppose, then, that a
body which introduced a gravitative or electrical factor twenty times as
great as the difference in Jupiter's effect at its maximum and minimum
distances from the sun would cause a glacial epoch if the effect lasted
long enough. Of course the other planets combine their effects with that
of Jupiter, but for the sake of simplicity we will leave the others out
of account. The difference between Jupiter's maximum and minimum tidal
effect on the sun amounts to 29 per cent of the planet's average effect.
The corresponding difference, according to the electrical hypothesis, is
about 19 per cent, for electrostatic action varies as the square of the
distance instead of as the cube. Let us assume that a body exerting four
times Jupiter's present tidal effect and placed at the average distance
of Jupiter from the sun would disturb the sun's atmosphere twenty times
as much as the present difference between sunspot maxima and minima, and
thus, perhaps, cause a glacial period on the earth.
On the basis of this assumption our first problem is to estimate the
frequency with which a star, visible or dark, is likely to approach near
enough to the sun to produce a _tidal_ effect four times that of
Jupiter. The number of visible stars is known or at least well
estimated. As to dark stars, which have grown cool, Arrhenius believed
that they are a hundred times as numerous as bright stars; few
astronomers believe that there are less than three or four times as
many. Dr. Shapley of the Harvard Observatory states that a new
investigation of the matter suggests that eight or ten is probably a
maximum figure. Let us assume that nine is correct. The average visible
star, so far as measured, has a mass about twice that of the sun, or
about 2100 times that of Jupiter. The distances of the stars have been
measured in hundreds of cases and thus we can estimate how many stars,
both visible and invisible, are on an average contained in a given
volume of space. On this basis Jeans estimates that there is only one
chance in thirty billion years that a visible star will approach within
2.8 times the distance of Neptune from the sun, that is, within about
eight billion miles. If we include the invisible stars the chances
become one in three billion years. In order to produce four times the
tidal effect of Jupiter, however, the average star would have to
approach within about four billion miles of the sun, and the chances of
that are only one in twelve billion years. The disturbing star would be
only 40 per cent farther from the sun than Neptune, and would almost
pass within the solar system.
Even though Jeans holds that the frequency of the mutual approach of the
sun and a star was probably much greater in the distant past than at
present, the figures just given lend little support to the tidal
hypothesis. In fact, they apparently throw it out of court. It will be
remembered that Jeans has made assumptions which give as high a
frequency of stellar encounters as is consistent with the astronomical
facts. We have assumed nine dark stars for every bright one, which may
be a liberal estimate. Also, although we have assumed that a disturbance
of the sun's atmosphere sufficient to cause a glacial period would arise
from a tidal effect only twenty times as great as the difference in
Jupiter's effect when nearest the sun and farthest away, in our
computations this has actually been reduced to thirteen. With all these
favorable assumptions the chances of a stellar approach of the sort here
described are now only one in twelve billion years. Yet within a hundred
million years, according to many estimates of geological time, and
almost certainly within a billion, there have been at least half a dozen
glaciations.
Our use of Jeans' data interposes another and equally insuperable
difficulty to any tidal hypothesis. Four billion miles is a very short
distance in the eyes of an astronomer. At that distance a star twice the
size of the sun would attract the outer planets more strongly than the
sun itself, and might capture them. If a star should come within four
billion miles of the sun, its effect in distorting the orbits of all the
planets would be great. If this had happened often enough to cause all
the glaciations known to geologists, the planetary orbits would be
strongly elliptical instead of almost circular. The consideration here
advanced militate so strongly against the tidal hypothesis of solar
disturbances that it seems scarcely worth while to consider it further.
Let us turn now to the electrical hypothesis. Here the conditions are
fundamentally different from those of the tidal hypothesis. In the first
place the electrostatic effect of a body has nothing to do with its
mass, but depends on the area of its surface; that is, it varies as the
square of the radius. Second, the emission of electrons varies
exponentially. If hot glowing stars follow the same law as black bodies
at lower temperatures, the emission of electrons, like the emission of
other kinds of energy, varies as the fourth power of the absolute
temperature. In other words, suppose there are two black bodies,
otherwise alike, but one with a temperature of 27 deg. C. or 300 deg. on the
absolute scale, and the other with 600 deg. on the absolute scale. The
temperature of one is twice as high as that of the other, but the
electrostatic effect will be sixteen times as great.[115] Third, the
number of electrons that reach a given body varies inversely as the
square of the distance, instead of as the cube which is the case with
tide-making forces.
In order to use these three principles in calculating the effect of the
stars we must know the diameters, distances, temperature, and number of
the stars. The distances and number may safely be taken as given by
Jeans in the calculations already cited. As to the diameters, the
measurements of the stars thus far made indicate that the average mass
is about twice that of the sun. The average density, as deduced by
Shapley[116] from the movements of double stars, is about one-eighth the
solar density. This would give an average diameter about two and a half
times that of the sun. For the dark stars, we shall assume for
convenience that they are ten times as numerous as the bright ones. We
shall also assume that their diameter is half that of the sun, for being
cool they must be relatively dense, and that their temperature is the
same as that which we shall assume for Jupiter.
As to Jupiter we shall continue our former assumption that a body with
four times the effectiveness of that planet, which here means with twice
as great a radius, would disturb the sun enough to cause glaciation. It
would produce about twenty times the electrostatic effect which now
appears to be associated with the difference in Jupiter's effect at
maximum and minimum. The temperature of Jupiter must also be taken into
account. The planet is supposed to be hot because its density is low,
being only about 1.25 that of water. Nevertheless, it is probably not
luminous, for as Moulton[117] puts it, shadows upon it are black and its
moons show no sign of illumination except from the sun. Hence a
temperature of about 600 deg.C., or approximately 900 deg. on the absolute
scale, seems to be the highest that can reasonably be assigned to the
cold outer layer whence electrons are emitted. As to the temperature of
the sun, we shall adopt the common estimate of about 6300 deg.C. on the
absolute scale. The other stars will be taken as averaging the same,
although of course they vary greatly.
When Jeans' method of calculating the probability of a mutual approach
of the sun and a star is applied to the assumptions given above, the
results are as shown in Table 5. On that basis the dark stars seem to be
of negligible importance so far as the electrical hypothesis is
concerned. Even though they may be ten times as numerous as the bright
ones there appears to be only one chance in 130 billion years that one
of them will approach the sun closely enough to cause the assumed
disturbance of the solar atmosphere. On the other hand, if all the
visible stars were the size of the sun, and as hot as that body, their
electrical effect would be fourfold that of our assumed dark star
because of their size, and 2401 times as great because of their
temperature, or approximately 10,000 times as great. Under such
conditions the theoretical chance of an approach that would cause
glaciation is one in 130 million years. If the average visible star is
somewhat cooler than the sun and has a radius about two and one-half
times as great, as appears to be the fact, the chances rise to one in
thirty-eight million years. A slight and wholly reasonable change in our
assumptions would reduce this last figure to only five or ten million.
For instance, the earth's mean temperature during the glacial period has
been assumed as 10 deg.C. lower than now, but the difference may have been
only 6 deg.. Again, the temperature of the outer atmosphere of Jupiter where
the electrons are shot out may be only 500 deg. or 700 deg. absolute, instead of
900 deg.. Or the diameter of the average star may be five or ten times that
of the sun, instead of only two and one-half times as great. All this,
however, may for the present be disregarded. The essential point is that
even when the assumptions err on the side of conservatism, the results
are of an order of magnitude which puts the electrical hypothesis within
the bounds of possibility, whereas similar assumptions put the tidal
hypothesis, with its single approach in twelve billion years, far beyond
those limits.
The figures for Betelgeuse in Table 5 are interesting. At a meeting of
the American Association for the Advancement of Science in December,
1920, Michelson reported that by measurements of the interference of
light coming from the two sides of that bright star in Orion, the
observers at Mount Wilson had confirmed the recent estimates of three
other authorities that the star's diameter is about 218 million miles,
or 250 times that of the sun. If other stars so much surpass the
estimates of only a decade or two ago, the average diameter of all the
visible stars must be many times that of the sun. The low figure for
Betelgeuse in section D of the table means that if all the stars were as
large as Betelgeuse, several might often be near enough to cause
profound disturbances of the solar atmosphere. Nevertheless, because of
the low temperature of the giant red stars of the Betelgeuse type, the
distance at which one of them would produce a given electrical effect is
only about five times the distance at which our assumed average star
would produce the same effect. This, to be sure, is on the assumption
that the radiation of energy from incandescent bodies varies according
to temperature in the same ratio as the radiation from black bodies.
Even if this assumption departs somewhat from the truth, it still seems
almost certain that the lower temperature of the red compared with the
high temperature of the white stars must to a considerable degree reduce
the difference in electrical effect which would otherwise arise from
their size.
TABLE 5
THEORETICAL PROBABILITY OF STELLAR APPROACHES
---------------------------------------------------------------------
| 1 | 2 | 3 | 4 |
| | |_Average | |
| _Dark Stars_ | _Sun_ | Star_ |_Betelgeuse_|
---------------------------------------------------------------------
A. Approximate | | | | |
radius in miles | 430,000 | 860,000 | 2,150,000|218,000,000 |
| | | | |
B. Assumed | | | | |
temperature above| | | | |
absolute zero. | 900 deg. C. | 6300 deg. C.| 5400 deg. C.| 3150 deg. C. |
| | | | |
C. Approximate | | | | |
theoretical | | | | |
distance at which| | | | |
star would cause | | | | |
solar disturbance| | | | |
great enough to | | | | |
cause glaciation | | | | |
(billions[118] | | | | |
of miles). | 1.2 | 120 | 220 | 3200 |
| | | | |
D. Average | | | | |
interval between | | | | |
approaches | | | | |
close enough to | | | | |
cause glaciation | | | | |
if all stars |130,000,000,000| | | |
were of given |[119] | | | |
type. Years. | |130,000,000|38,000,000| 700,000 |
---------------------------------------------------------------------
Thus far in our attempt to estimate the distance at which a star might
disturb the sun enough to cause glaciation on the earth, we have
considered only the star's size and temperature. No account has been
taken of the degree to which its atmosphere is disturbed. Yet in the
case of the sun this seems to be one of the most important factors. The
magnetic field of sunspots is sometimes 50 or 100 times as strong as
that of the sun in general. The strength of the magnetic field appears
to depend on the strength of the electrical currents in the solar
atmosphere. But the intensity of the sunspots and, by inference, of the
electrical currents, may depend on the electrical action of Jupiter and
the other planets. If we apply a similar line of reasoning to the stars,
we are at once led to question whether the electrical activity of double
stars may not be enormously greater than that of isolated stars like the
sun.
If this line of reasoning is correct, the atmosphere of every double
star must be in a state of commotion vastly greater than that of the
sun's atmosphere even when it is most disturbed. For example, suppose
the sun were accompanied by a companion of equal size at a distance of
one million miles, which would make it much like many known double
stars. Suppose also that in accordance with the general laws of physics
the electrical effect of the two suns upon one another is proportional
to the fourth power of the temperature, the square of the radius, and
the inverse square of the distance. Then the effect of each sun upon the
other would be sixty billion (6 x 10^{10}) times as great as the present
electrical effect of Jupiter upon the sun. Just what this would mean as
to the net effect of a pair of such suns upon the electrical potential
of other bodies at a distance we can only conjecture. The outstanding
fact is that the electrical conditions of a double star must be
radically different and vastly more intense than those of a single star
like the sun.
This conclusion carries weighty consequences. At present twenty or more
stars are known to be located within about 100 trillion miles of the sun
(five parsecs, as the astronomers say), or 16.5 light years. According
to the assumptions employed in Table 5 an average single star would
influence the sun enough to cause glaciation if it came within
approximately 200 billion miles. If the star were double, however, it
might have an electrical capacity enormously greater than that of the
sun. Then it would be able to cause glaciation at a correspondingly
great distance. Today Alpha Centauri, the nearest known star about
twenty-five trillion miles, or 4.3 light years from the sun, and Sirius,
the brightest star in the heavens, is about fifty trillion miles away,
or 8.5 light years. If these stars were single and had a diameter three
times that of the sun, and if they were of the same temperature as has
been assumed for Betelgeuse, which is about fifty times as far away as
Alpha Centauri, the relative effects of the three stars upon the sun
would be, approximately, Betelgeuse 700, Alpha Centauri 250, Sirius 1.
But Alpha Centauri is triple and Sirius double, and both are much hotter
than Betelgeuse. Hence Alpha Centauri and even Sirius may be far more
effective than Betelgeuse.
The two main components of Alpha Centauri are separated by an average
distance of about 2,200,000,000 miles, or somewhat less than that of
Neptune from the sun. A third and far fainter star, one of the faintest
yet measured, revolves around them at a great distance. In mass and
brightness the two main components are about like the sun, and we will
assume that the same is true of their radius. Then, according to the
assumptions made above, their effect in disturbing one another
electrically would be about 10,000 times the total effect of Jupiter
upon the sun, or 2500 times the effect that we have assumed to be
necessary to produce a glacial period. We have already seen in Table 5
that, according to our assumptions, a single star like the sun would
have to approach within 120 billion miles of the solar system, or within
2 per cent of a light year, in order to cause glaciation. By a similar
process of reasoning it appears that if the mutual electrical excitation
of the two main parts of Alpha Centauri, regardless of the third part,
is proportional to the apparent excitation of the sun by Jupiter, Alpha
Centauri would be 5000 times as effective as the sun. In other words, if
it came within 8,500,000,000,000 miles of the sun, or 1.4 light years,
it would so change the electrical conditions as to produce a glacial
epoch. In that case Alpha Centauri is now so near that it introduces a
disturbing effect equal to about one-sixth of the effect needed to cause
glaciation on the earth. Sirius and perhaps others of the nearer and
brighter or larger stars may also create appreciable disturbances in the
electrical condition of the sun's atmosphere, and may have done so to a
much greater degree in the past, or be destined to do so in the future.
Thus an electrical hypothesis of solar disturbances seems to indicate
that the position of the sun in respect to other stars may be a factor
of great importance in determining the earth's climate.
FOOTNOTES:
[Footnote 112: H. H. Turner: On a Long Period in Chinese Earthquake
Records; Mon. Not. Royal Astron. Soc., Vol. 79, 1919, pp. 531-539; Vol.
80, 1920, pp. 617-619; Long Period Terms in the Growth of Trees; _idem_,
pp.793-808.]
[Footnote 113: Harlow Shapley: Note on a Possible Factor in Geologic
Climates; Jour. Geol., Vol. 29, No. 4, May, 1921; Novae and Variable
Stars, Pub. Astron. Soc. Pac., No. 194, Aug., 1921.]
[Footnote 114: J. H. Jeans: Problems of Cosmogony and Stellar Dynamics,
Cambridge, 1919.]
[Footnote 115: This fact is so important and at the same time so
surprising to the layman, that a quotation from The Electron Theory of
Matter by O. W. Richardson, 1914, pp. 326 and 334 is here added.
"It is a very familiar fact that when material bodies are heated they
emit electromagnetic radiations, in the form of thermal, luminous, and
actinic rays, in appreciable quantities. Such an effect is a natural
consequence of the electron and kinetic theories of matter. On the
kinetic theory, temperature is a measure of the violence of the motion
of the ultimate particles; and we have seen that on the electron theory,
electromagnetic radiation is a consequence of their acceleration. The
calculation of this emission from the standpoint of the electron theory
alone is a very complex problem which takes us deeply into the structure
of matter and which has probably not yet been satisfactorily resolved.
Fortunately, we can find out a great deal about these phenomena by the
application of general principles like the conservation of energy and
the second law of thermodynamics without considering special assumptions
about the ultimate constitution of matter. It is to be borne in mind
that the emission under consideration occurs at all temperatures
although it is more marked the higher the temperature.... The energy per
unit volume, _in vacuo_, of the radiation in equilibrium in an enclosure
at the absolute temperature, T, is equal to a universal constant, A,
multiplied by the fourth power of the absolute temperature. Since the
intensity of the radiation is equal to the energy per unit volume
multiplied by the velocity of light, it follows that the former must
also be proportional to the fourth power of the absolute temperature.
Moreover, if E is the total emission from unit area of a perfectly black
body, we see from p. 330 that E=A'T^{4}, where A' is a new universal
constant. This result is usually known as Stefan's Law. It was suggested
by Stefan in the inaccurate form that the total radiant energy of
emission from bodies varies as the fourth power of the absolute
temperature, as a generalization from the results of experiments. The
credit for showing that it is a consequence of the existence of
radiation pressure combined with the principles of thermodynamics is due
to Bartoli and Boltzmann."]
[Footnote 116: Quoted by Moulton in his Introduction to Astronomy.]
[Footnote 117: Introduction to Astronomy.]
[Footnote 118: The term billions, here and elsewhere, is used in the
American sense, 10^{9}.]
[Footnote 119: The assumed number of stars here is ten times as great as
in the other parts of this line.]
CHAPTER XV
THE SUN'S JOURNEY THROUGH SPACE
Having gained some idea of the nature of the electrical hypothesis of
solar disturbances and of the possible effect of other bodies upon the
sun's atmosphere, let us now compare the astronomical data with those of
geology. Let us take up five chief points for which the geologist
demands an explanation, and which any hypothesis must meet if it is to
be permanently accepted. These are (1) the irregular intervals at which
glacial periods occur; (2) the division of glacial periods into epochs
separated sometimes by hundreds of thousands of years; (3) the length of
glacial periods and epochs; (4) the occurrence of glacial stages and
historic pulsations in the form of small climatic waves superposed upon
the larger waves of glacial epochs; (5) the occurrence of climatic
conditions much milder than those of today, not only in the middle
portion of the great geological eras, but even in some of the recent
inter-glacial epochs.
1. The irregular duration of the interval from one glacial epoch to
another corresponds with the irregular distribution of the stars. If
glaciation is indirectly due to stellar influences, the epochs might
fall close together, or might be far apart. If the average interval were
ten million years, one interval might be thirty million or more and the
next only one or two hundred thousand. According to Schuchert, the known
periods of glacial or semi-glacial climate have been approximately as
follows:
LIST OF GLACIAL PERIODS
1. Archeozoic.
(1/4 of geological time or perhaps much more)
No known glacial periods.
2. Proterozoic.
(1/4 of geological time)
a. Oldest known glacial period near base of Proterozoic in
Canada. Evidence widely distributed.
b. Indian glacial period; time unknown.
c. African glacial period; time unknown.
d. Glaciation near end of Proterozoic in Australia, Norway,
and China.
3. Paleozoic.
(1/4 of geological time)
a. Late Ordovician(?). Local in Arctic Norway.
b. Silurian. Local in Alaska.
c. Early Devonian. Local in South Africa.
d. Early Permian. World-wide and very severe.
4. Mesozoic and Cenozoic.
(1/4 of geological time)
a-b. None definitely determined during Mesozoic, although
there appears to have been periods of cooling (a) in the
late Triassic, and (b) in the late Cretacic, with at least
local glaciation in early Eocene.
c. Severe glacial period during Pleistocene.
This table suggests an interesting inquiry. During the last few decades
there has been great interest in ancient glaciation and geologists have
carefully examined rocks of all ages for signs of glacial deposits. In
spite of the large parts of the earth which are covered with deposits
belonging to the Mesozoic and Cenozoic, which form the last quarter of
geological time, the only signs of actual glaciation are those of the
great Pleistocene period and a few local occurrences at the end of the
Mesozoic or beginning of the Cenozoic. Late in the Triassic and early in
the Jurassic, the climate appears to have been rigorous, although no
tillites have been found to demonstrate glaciation. In the preceding
quarter, that is, the Paleozoic, the Permian glaciation was more severe
than that of the Pleistocene, and the Devonian than that of the Eocene,
while the Ordovician evidences of low temperature are stronger than
those at the end of the Triassic. In view of the fact that rocks of
Paleozoic age cover much smaller areas than do those of later age, the
three Paleozoic glaciations seem to indicate a relative frequency of
glaciation. Going back to the Proterozoic, it is astonishing to find
that evidence of two highly developed glacial periods, and possibly
four, has been discovered. Since the Indian and the African glaciations
of Proterozoic times are as yet undated, we cannot be sure that they are
not of the same date as the others. Nevertheless, even two is a
surprising number, for not only are most Proterozoic rocks so
metamorphosed that possible evidences of glacial origin are destroyed,
but rocks of that age occupy far smaller areas than either those of
Paleozoic or, still more, Mesozoic and Cenozoic age. Thus the record of
the last three-quarters of geological time suggests that if rocks of all
ages were as abundant and as easily studied as those of the later
periods, the frequency of glacial periods would be found to increase as
one goes backward toward the beginnings of the earth's history. This is
interesting, for Jeans holds that the chances that the stars would
approach one another were probably greater in the past than at present.
This conclusion is based on the assumption that our universe is like the
spiral nebulae in which the orbits of the various members are nearly
circular during the younger stages. Jeans considers it certain that in
such cases the orbits will gradually become larger and more elliptical
because of the attraction of one body for another. Thus as time goes on
the stars will be more widely distributed and the chances of approach
will diminish. If this is correct, the agreement between astronomical
theory and geological conclusions suggests that the two are at least not
in opposition.
The first quarter of geological time as well as the last three must be
considered in this connection. During the Archeozoic, no evidence of
glaciation has yet been discovered. This suggests that the geological
facts disprove the astronomical theory. But our knowledge of early
geological times is extremely limited, so limited that lack of evidence
of glaciation in the Archeozoic may have no significance. Archeozoic
rocks have been studied minutely over a very small percentage of the
earth's land surface. Moreover, they are highly metamorphosed so that,
even if glacial tills existed, it would be hard to recognize them.
Third, according to both the nebular and the planetesimal hypotheses, it
seems possible that during the earliest stages of geological history the
earth's interior was somewhat warmer than now, and the surface may have
been warmed more than at present by conduction, by lava flows, and by
the fall of meteorites. If the earth during the Archeozoic period
emitted enough heat to raise its surface temperature a few degrees, the
heat would not prevent the development of low forms of life but might
effectively prevent all glaciation. This does not mean that it would
prevent changes of climate, but merely changes so extreme that their
record would be preserved by means of ice. It will be most interesting
to see whether future investigations in geology and astronomy indicate
either a semi-uniform distribution of glacial periods throughout the
past, or a more or less regular decrease in frequency from early times
down to the present.
2. The Pleistocene glacial period was divided into at least four epochs,
while in the Permian at least one inter-glacial epoch seems certain, and
in some places the alternation between glacial and non-glacial beds
suggests no less than nine. In the other glaciations the evidence is not
yet clear. The question of periodicity is so important that it
overthrows most glacial hypotheses. Indeed, had their authors known the
facts as established in recent years, most of the hypotheses would never
have been advanced. The carbon dioxide hypothesis is the only one which
was framed with geologically rapid climatic alternations in mind. It
certainly explains the facts of periodicity better than does any of its
predecessors, but even so it does not account for the intimate way in
which variations of all degrees from those of the weather up to glacial
epochs seem to grade into one another.
According to our stellar hypothesis, occasional groups of glacial epochs
would be expected to occur close together and to form long glacial
periods. This is because many of the stars belong to groups or clusters
in which the stars move in parallel paths. A good example is the cluster
in the Hyades, where Boss has studied thirty-nine stars with special
care.[120] The stars are grouped about a center about 130 light years
from the sun. The stars themselves are scattered over an area about
thirty light years in diameter. They average about the same distance
apart as do those near the sun, but toward the center of the group they
are somewhat closer together. The whole thirty-nine sweep forward in
essentially parallel paths. Boss estimates that 800,000 years ago the
cluster was only half as far from the sun as at present, but probably
that was as near as it has been during recent geological times. All of
the thirty-nine stars of this cluster, as Moulton[121] puts it, "are
much greater in light-giving power than the sun. The luminosities of
even the five smallest are from five to ten times that of the sun, while
the largest are one hundred times greater in light-giving power than our
own luminary. Their masses are probably much greater than that of the
sun." If the sun were to pass through such a cluster, first one star and
then another might come so near as to cause a profound disturbance in
the sun's atmosphere.
3. Another important point upon which a glacial hypothesis may come to
grief is the length of the periods or rather of the epochs which compose
the periods. During the last or Pleistocene glacial period the evidence
in America and Europe indicates that the inter-glacial epochs varied in
length and that the later ones were shorter than the earlier. Chamberlin
and Salisbury, from a comparison of various authorities, estimate that
the intervals from one glacial epoch to another form a declining series,
which may be roughly expressed as follows: 16-8-4-2-1, where unity is
the interval from the climax of the late Wisconsin, or last glacial
epoch, to the present. Most authorities estimate the culmination of the
late Wisconsin glaciation as twenty or thirty thousand years ago. Penck
estimates the length of the last inter-glacial period as 60,000 years
and the preceding one as 240,000.[122] R. T. Chamberlin, as already
stated, finds that the consensus of opinion is that inter-glacial epochs
have averaged five times as long as glacial epochs. The actual duration
of the various glaciations probably did not vary in so great a ratio as
did the intervals from one glaciation to another. The main point,
however, is the irregularity of the various periods.
The relation of the stellar electrical hypothesis to the length of
glacial epochs may be estimated from column C, in Table 5. There we see
that the distances at which a star might possibly disturb the sun enough
to cause glaciation range all the way from 120 billion miles in the case
of a small star like the sun, to 3200 billion in the case of Betelgeuse,
while for double stars the figure may rise a hundred times higher. From
this we can calculate how long it would take a star to pass from a point
where its influence would first amount to a quarter of the assumed
maximum to a similar point on the other side of the sun. In making these
calculations we will assume that the relative rate at which the star and
the sun approach each other is about twenty-two miles per second, or 700
million miles per year, which is the average rate of motion of all the
known stars. According to the distances in Table 5 this gives a range
from about 500 years up to about 10,000, which might rise to a million
in the case of double stars. Of course the time might be relatively
short if the sun and a rapidly moving star were approaching one another
almost directly, or extremely long if the sun and the star were moving
in almost the same direction and at somewhat similar rates,--a condition
more common than the other. Here, as in so many other cases, the
essential point is that the figures which we thus obtain seem to be of
the right order of magnitude.
4. Post-glacial climatic stages are so well known that in Europe they
have definite names. Their sequence has already been discussed in
Chapter XII. Fossils found in the peat bogs of Denmark and Scandinavia,
for example, prove that since the final disappearance of the continental
ice cap at the close of the Wisconsin there has been at least one period
when the climate of Europe was distinctly milder than now. Directly
overlying the sheets of glacial drift laid down by the ice there is a
flora corresponding to that of the present tundras. Next come remains of
a forest vegetation dominated by birches and poplars, showing that the
climate was growing a little warmer. Third, there follow evidences of a
still more favorable climate in the form of a forest dominated by pines;
fourth, one where oak predominates; and fifth, a flora similar to that
of the Black Forest of Germany, indicating that in Scandinavia the
temperature was then decidedly higher than today. This fifth flora has
retreated southward once more, having been driven back to its present
latitude by a slight recurrence of a cool stormy climate.[123] In
central Asia evidence of post-glacial stages is found not only in five
distinct moraines but in a corresponding series of elevated strands
surrounding salt lakes and of river terraces in non-glaciated arid
regions.[124]
In historic as well as prehistoric times, as we have already seen, there
have been climatic fluctuations. For instance, the twelfth or thirteenth
century B. C. appears to have been almost as mild as now, as does the
seventh century B. C. On the other hand about 1000 B. C., at the time of
Christ, and in the fourteenth century there were times of relative
severity. Thus it appears that both on a large and on a small scale
pulsations of climate are the rule. Any hypothesis of climatic changes
must satisfy the periods of these pulsations. These conditions furnish a
problem which makes difficulty for almost all hypotheses of climatic
change. According to the present hypothesis, earth movements such as are
discussed in Chapter XII may cooeperate with two astronomical factors.
One is the constant change in the positions of the stars, a change which
we have already called kaleidoscopic, and the other is the fact that a
large proportion of the stars are double or multiple. When one star in a
group approaches the sun closely enough to cause a great solar
disturbance, numerous others may approach or recede and have a minor
effect. Thus, whenever the sun is near groups of stars we should expect
that the earth would show many minor climatic pulsations and stages
which might or might not be connected with glaciation. The historic
pulsations shown in the curve of tree growth in California, Fig. 4, are
the sort of changes that would be expected if movements of the stars
have an effect on the solar atmosphere.
Not only are fully a third of all the visible stars double, as we have
already seen, but at least a tenth of these are known to be triple or
multiple. In many of the double stars the two bodies are close together
and revolve so rapidly that whatever periodicity they might create in
the sun's atmosphere would be very short. In the triplets, however, the
third star is ordinarily at least ten times as far from the other two as
they are from each other, and its period of rotation sometimes runs into
hundreds or thousands of years. An actual multiple star in the
constellation Polaris will serve as an example. The main star is
believed by Jeans to consist of two parts which are almost in contact
and whirl around each other with extraordinary speed in four days. If
this is true they must keep each other's atmospheres in a state of
intense commotion. Much farther away a third star revolves around this
pair in twelve years. At a much greater distance a fourth star revolves
around the common center of gravity of itself and the other three in a
period which may be 20,000 years. Still more complicated cases probably
exist. Suppose such a system were to traverse a path where it would
exert a perceptible influence on the sun for thirty or forty thousand
years. The varying movements of its members would produce an intricate
series of cycles which might show all sorts of major and minor
variations in length and intensity. Thus the varied and irregular stages
of glaciation and the pulsations of historic times might be accounted
for on the hypothesis of the proximity of the sun to a multiple star, as
well as on that of the less pronounced approach and recession of a
number of stars. In addition to all this, an almost infinitely complex
series of climatic changes of long and short duration might arise if the
sun passed through a nebula.
5. We have seen in Chapter VIII that the contrast between the somewhat
severe climate of the present and the generally mild climate of the past
is one of the great geological problems. The glacial period is not a
thing of the distant past. Geologists generally recognize that it is
still with us. Greenland and Antarctica are both shrouded in ice sheets
in latitudes where fossil floras prove that at other periods the climate
was as mild as in England or even New Zealand. The present glaciated
regions, be it noted, are on the polar borders of the world's two most
stormy oceanic areas, just where ice would be expected to last longest
according to the solar cyclonic hypothesis. In contrast with the
semi-glacial conditions of the present, the last inter-glacial epoch was
so mild that not only men but elephants and hippopotamuses flourished in
central Europe, while at earlier times in the middle of long eras, such
as the Paleozoic and Mesozoic, corals, cycads, and tree ferns flourished
within the Arctic circle.
If the electro-stellar hypothesis of solar disturbances proves well
founded, it may explain these peculiarities. Periods of mild climate
would represent a return of the sun and the earth to their normal
conditions of quiet. At such times the atmosphere of the sun is assumed
to be little disturbed by sunspots, faculae, prominences, and other
allied evidences of movements; and the rice-grain structure is perhaps
the most prominent of the solar markings. The earth at such times is
supposed to be correspondingly free from cyclonic storms. Its winds are
then largely of the purely planetary type, such as trade winds and
westerlies. Its rainfall also is largely planetary rather than cyclonic.
It falls in places such as the heat equator where the air rises under
the influence of heat, or on the windward slopes of mountains, or in
regions where warm winds blow from the ocean over cold lands.
According to the electro-stellar hypothesis, the conditions which
prevailed during hundreds of millions of years of mild climate mean
merely that the solar system was then in parts of the heavens where
stars--especially double stars--were rare or small, and electrical
disturbances correspondingly weak. Today, on the other hand, the sun is
fairly near a number of stars, many of which are large doubles. Hence it
is supposed to be disturbed, although not so much as at the height of
the last glacial epoch.
After the preceding parts of this book had been written, the assistance
of Dr. Schlesinger made it possible to test the electro-stellar
hypothesis by comparing actual astronomical dates with the dates of
climatic or solar phenomena. In order to make this possible, Dr.
Schlesinger and his assistants have prepared Table 6, giving the
position, magnitude, and motions of the thirty-eight nearest stars, and
especially the date at which each was nearest the sun. In column 10
where the dates are given, a minus sign indicates the past and a plus
sign the future. Dr. Shapley has kindly added column 12, giving the
absolute magnitudes of the stars, that of the sun being 4.8, and column
13, showing their luminosity or absolute radiation, that of the sun
being unity. Finally, column 14 shows the effective radiation received
by the sun from each star when the star is at a minimum distance. Unity
in this case is the effect of a star like the sun at a distance of one
light year.
It is well known that radiation of all kinds, including light, heat, and
electrical emissions, varies in direct proportion to the exposed
surface, that is, as the square of the radius of a sphere, and inversely
as the square of the distance. From black bodies, as we have seen, the
total radiation varies as the fourth power of the absolute temperature.
It is not certain that either light or electrical emissions from
incandescent bodies vary in quite this same proportion, nor is it yet
certain whether luminous and electrical emissions vary exactly together.
Nevertheless they are closely related. Since the light coming from each
star is accurately measured, while no information is available as to
electrical emissions, we have followed Dr. Shapley's suggestion and used
the luminosity of the stars as the best available measure of total
radiation. This is presumably an approximate measure of electrical
activity, provided some allowance be made for disturbances by outside
bodies such as companion stars. Hence the inclusion of column 14.
TABLE 6
THIRTY-EIGHT STARS HAVING LARGEST KNOWN PARALLAXES
Star
Code
1 Groombr. 34
2 ++[Greek: e] Cassiop.
3
4 ++[Greek: k] Tucanae
5 [Greek: t] Ceti
6 [Greek: d]_2 Eridani
7 ++[Greek: e] Eridani
8 ++40(0)^2 Eridani
9 Cordoba Z. 243
10 Weisse 592
11 ++[Greek: a] Can. Maj. (Sirius)
12 ++[Greek: a] Can. Min. (Procyon)
13 ++Fedorenko 1457-8
14 Groombr. 1618
15 Weisse 234
16 Lalande 21185
17 Lalande 21258
18
19 Lalande 25372
20 ++[Greek: a] Centauri
21 ++[Greek: x] Bootes
22 ++Lalande 27173
23 Weisse 1259
24 Lacaille 7194
25 ++[Greek: b] 416
26 Argel -0.17415-6
27 Barnard's star
28 ++70p Ophiuchi
29 ++[Greek: S] 2398
30 [Greek: s] Draconis
31 ++[Greek: a] Aquilae (Altair)
32 ++61 Cygni
33 Lacaille 8760
34 [Greek: e] Indi
35 ++Krueger 60
36 Lacaille 9352
37 Lalande 46650
38 C. G. A. 32416
(++ Double star.)
(1) (2) (3) (4) (5) (6)
Right Declination Visual Spectrum Proper Radial
Star Ascension [Greek: d] Mag. m Motion Velocity
code [Greek: a] 1900 km. per
1900 sec.
------------------------------------------------------------------
1 0^h 12^m.7 +43 deg.27' 8.1 Ma 2".89 + 3
2 43 .0 +57 17 3.6 F8 1 .24 + 10
3 43 .9 +4 55 12.3 F0 3 .01 .....
4 1 12 .4 -69 24 5.0 F8 .39 + 12
5 39 .4 -16 28 3.6 K0 1 .92 - 16
------------------------------------------------------------------
6 3 15 .9 -43 27 4.3 G5 3 .16 + 87
7 28 .2 - 9 48 3.8 K0 .97 + 16
8 4 10 .7 - 7 49 4.5 G5 4 .08 - 42
9 5 7 .7 -44 59 9.2 K2 8 .75 +242
10 26 .4 - 3 42 8.8 K2 2 .22 .....
------------------------------------------------------------------
11 6 40 .7 -16 35 -1.6 A0 1 .32 - 8
12 7 34 .1 + 5 29 0.5 F5 1 .24 - 4
13 9 7 .6 +53 7 7.9 Ma 1 .68 + 10
14 10 5 .3 +49 58 6.8 K5p 1 .45 - 30
15 14 .2 +20 22 9.0 ... .49 .....
------------------------------------------------------------------
16 57 .9 +36 38 7.6 Mb 4 .78 - 87
17 11 0 .5 +44 2 8.5 K5 4 .52 + 65
18 12 .0 -57 2 12.0 ... 2 .69 .....
19 13 40 .7 +15 26 8.5 K5 2 .30 .....
20 14 32 .8 -60 25 0.2 G 3 .68 + 22
------------------------------------------------------------------
21 14 46 .8 +19 31 4.6 K5p .17 + 4
22 51 .6 -20 58 5.8 Kp 1 .96 + 20
23 16 41 .4 +33 41 8.4 ... .37 .....
24 17 11 .5 -46 32 5.7 K .97 .....
25 12 .1 -34 53 5.9 K5 1 .19 - 4
------------------------------------------------------------------
26 37 .0 +68 26 9.1 K 1 .33 .....
27 52 .9 + 4 25 9.7 Mb 10 .30 - 80
28 18 0 .4 + 2 31 4.3 K 1 .13 .....
29 41 .7 +59 29 8.8 K 2 .31 .....
30 19 32 .5 +69 29 4.8 G5 1 .84 + 26
------------------------------------------------------------------
31 45 .9 + 8 36 1.2 A5 .66 - 33
32 21 2 .4 +38 15 5.6 K5 5 .20 - 64
33 11 .4 -39 15 6.6 G 3 .53 + 13
34 55 .7 -57 12 4.8 K5 4 .70 - 39
35 22 24 .4 +57 12 9.2 ... .87 .....
------------------------------------------------------------------
36 59 .4 -36 26 7.1 K 6 .90 + 12
37 23 44 .0 + 1 52 8.7 Ma 1 .39 .....
38 59 .5 -37 51 8.2 G 6 .05 + 26
(7) (9) (11) (13) (14)
Present Minimum Magnitude Luminosity Effective
Parallax Distance at Min. Dist. | radiation
[Greek: p] Light Yrs. | | at
| | | | minimum
| (8) | (10) | (12) | distance
Star | Maximum | Time of | Absolute | from sun
Code | Parallax | Minimum | Magnitude | |
| | | Distance | | | |
-----------------------------------------------------------------------
1 ".28 ".28 11.6 -4000 8.1 10.3 0.0063 0.000051
2 .18 .19 17.1 -47000 3.5 4.9 0.91 0.003110
3 .24 .... .... ...... .... 14.2 0.00017 ........
4 .16 .23 14.2 -264000 4.2 6.0 0.33 0.001610
5 .32 .37 8.8 +46000 3.3 6.1 0.30 0.003840
-----------------------------------------------------------------------
6 .16 .22 14.8 -33000 3.6 5.3 0.63 0.002960
7 .31 .46 7.1 -106000 3.0 6.3 0.25 0.004970
8 .21 .23 14.2 +19000 4.3 6.1 0.30 0.001470
9 .32 .68 4.8 -10000 7.6 11.7 0.0017 0.000074
10 .17 .... .... ...... .... 9.9 0.009 ........
-----------------------------------------------------------------------
11 .37 .41 8.0 +65000 -1.8 1.2 27.50 0.429000
12 .31 .32 10.2 +34000 0.5 3.0 5.25 0.051300
13 .16 .16 20.4 -24000 7.9 8.9 0.023 0.000055
14 .18 .23 14.2 +69000 6.3 8.1 0.048 0.000238
15 .19 .... .... ...... .... 10.4 0.0057 ........
-----------------------------------------------------------------------
16 .41 .76 4.3 +20000 6.2 10.7 0.0044 0.000238
17 .19 .22 14.8 -20000 8.2 9.9 0.009 0.000041
18 .34 .... .... ...... .... 14.7 0.00011 ........
19 .19 .... .... ...... .... 9.9 0.009 ........
20 .76 1.03 3.2 -28000 -0.5 4.6 1.20 0.117500
-----------------------------------------------------------------------
21 .17 .22 14.8 -598000 4.0 5.8 0.40 0.001815
22 .18 .19 17.1 -36000 5.6 7.1 0.12 0.000412
23 .18 .... .... ...... .... 9.7 0.011 ........
24 .19 .... .... ...... .... 7.1 0.12 ........
25 .17 .17 19.2 +21000 5.7 7.1 0.12 0.000329
-----------------------------------------------------------------------
26 .22 .... .... ...... .... 10.8 0.004 ........
27 .53 .70 4.7 +10000 9.1 13.3 0.0025 0.000114
28 .19 .... .... ...... .... 5.7 0.44 ........
29 .29 .... .... ...... .... 11.1 0.0030 ........
30 .20 .23 14.2 -49000 4.5 6.3 0.25 0.001238
-----------------------------------------------------------------------
31 .21 .51 6.4 +117000 -0.7 2.8 6.30 0.153600
32 .30 .38 8.6 +19000 5.1 8.0 0.053 0.000715
33 .25 .26 12.6 -11000 6.6 8.6 0.030 0.000189
34 .28 .31 10.5 +17000 4.6 7.0 0.13 0.001230
35 .26 .... .... ....... .... 11.3 0.0025 ........
-----------------------------------------------------------------------
36 .29 .29 11.2 -3000 7.1 9.4 0.014 0.000111
37 .17 .... .... ....... .... 9.9 0.009 ........
38 .22 .22 14.8 -7000 8.2 9.9 0.009 0.000041
-----------------------------------------------------------------------
On the basis of column 14 and of the movements and distances of the
stars as given in the other columns Fig. 10 has been prepared. This
gives an estimate of the approximate electrical energy received by the
sun from the nearest stars for 70,000 years before and after the
present. It is based on the twenty-six stars for which complete data are
available in Table 6. The inclusion of the other twelve would not alter
the form of the curve, for even the largest of them would not change any
part by more than about half of 1 per cent, if as much. Nor would the
curve be visibly altered by the omission of all except four of the
twenty-six stars actually used. The four that are important, and their
relative luminosity when nearest the sun, are Sirius 429,000, Altair
153,000, Alpha Centauri 117,500, and Procyon 51,300. The figure for the
next star is only 4970, while for this star combined with the other
twenty-one that are unimportant it is only 24,850.
Figure 10 is not carried more than 70,000 years into the past or into
the future because the stars near the sun at more remote times are not
included among the thirty-eight having the largest known parallaxes.
That is, they have either moved away or are not yet near enough to be
included. Indeed, as Dr. Schlesinger strongly emphasizes, there may be
swiftly moving, bright or gigantic stars which are now quite far away,
but whose inclusion would alter Fig. 10 even within the limits of the
140,000 years there shown. It is almost certain, however, that the most
that these would do would be to raise, but not obliterate, the minima on
either side of the main maximum.
[Illustration: _Fig. 10. Climatic changes of 140,000 years as inferred
from the stars._]
In preparing Fig. 10 it has been necessary to make allowance for double
stars. Passing by the twenty-two unimportant stars, it appears that the
companion of Sirius is eight or ten magnitudes smaller than that star,
while the companions of Procyon and Altair are five or more magnitudes
smaller than their bright comrades. This means that the luminosity of
the faint components is at most only 1 per cent of that of their bright
companions and in the case of Sirius not a hundredth of 1 per cent.
Hence their inclusion would have no visible effect on Fig. 10. In Alpha
Centauri, on the other hand, the two components are of almost the same
magnitude. For this reason the effective radiation of that star as given
in column 14 is doubled in Fig. 10, while for another reason it is
raised still more. The other reason is that if our inferences as to the
electrical effect of the sun on the earth and of the planets on the sun
are correct, double stars, as we have seen, must be much more effective
electrically than single stars. By the same reasoning two bright stars
close together must excite one another much more than a bright star and
a very faint one, even if the distances in both cases are the same. So,
too, other things being equal, a triple star must be more excited
electrically than a double star. Hence in preparing Fig. 10 all double
stars receive double weight and each part of Alpha Centauri receives an
additional 50 per cent because both parts are bright and because they
have a third companion to help in exciting them.
According to the electro-stellar hypothesis, Alpha Centauri is more
important climatically than any other star in the heavens not only
because it is triple and bright, but because it is the nearest of all
stars, and moves fairly rapidly. Sirius and Procyon move slowly in
respect to the sun, only about eleven and eight kilometers per second
respectively, and their distances at minimum are fairly large, that is,
8 and 10.2 light years. Hence their effect on the sun changes slowly.
Altair moves faster, about twenty-six kilometers per second, and its
minimum distance is 6.4 light years, so that its effect changes fairly
rapidly. Alpha Centauri moves about twenty-four kilometers per second,
and its minimum distance is only 3.2 light years. Hence its effect
changes very rapidly, the change in its apparent luminosity as seen from
the sun amounting at maximum to about 30 per cent in 10,000 years
against 14 per cent for Altair, 4 for Sirius, and 2 for Procyon. The
vast majority of the stars change so much more slowly than even Procyon
that their effect is almost uniform. All the stars at a distance of more
than perhaps twenty or thirty light years may be regarded as sending to
the sun a practically unchanging amount of radiation. It is the bright
stars within this limit which are important, and their importance
increases with their proximity, their speed of motion, and the
brightness and number of their companions. Hence Alpha Centauri causes
the main maximum in Fig. 10, while Sirius, Altair, and Procyon combine
to cause a general rise of the curve from the past to the future.
Let us now interpret Fig. 10 geologically. The low position of the curve
fifty to seventy thousand years ago suggests a mild inter-glacial
climate distinctly less severe than that of the present. Geologists say
that such was the case. The curve suggests a glacial epoch culminating
about 28,000 years ago. The best authorities put the climax of the last
glacial epoch between twenty-five and thirty thousand years ago. The
curve shows an amelioration of climate since that time, although it
suggests that there is still considerable severity. The retreat of the
ice from North America and Europe, and its persistence in Greenland and
Antarctica agree with this. And the curve indicates that the change of
climate is still persisting, a conclusion in harmony with the evidence
as to historic changes.
If Alpha Centauri is really so important, the effect of its variations,
provided it has any, ought perhaps to be evident in the sun. The
activity of the star's atmosphere presumably varies, for the orbits of
the two components have an eccentricity of 0.51. Hence during their
period of revolution, 81.2 years, the distance between them ranges from
1,100,000,000 to 3,300,000,000 miles. They were at a minimum distance in
1388, 1459, 1550, 1631, 1713, 1794, 1875, and will be again in 1956. In
Fig. 11, showing sunspot variations, it is noticeable that the years
1794 and 1875 come just at the ends of periods of unusual solar
activity, as indicated by the heavy horizontal line. A similar period of
great activity seems to have begun about 1914. If its duration equals
the average of its two predecessors, it will end about 1950. Back in the
fourteenth century a period of excessive solar activity, which has
already been described, culminated from 1370 to 1385, or just before the
two parts of Alpha Centauri were at a minimum distance. Thus in three
and perhaps four cases the sun has been unusually active during a time
when the two parts of the star were most rapidly approaching each other
and when their atmospheres were presumably most disturbed and their
electrical emanations strongest.
[Illustration: _Fig. 11. Sunspot curve showing cycles, 1750 to 1920._
_Note._ The asterisks indicate two absolute minima of sunspots in 1810
and 1913, and the middle years (1780 and 1854) of two periods when the
sunspot maxima never fell below 95. If Alpha Centauri has an effect on
the sun's atmosphere, the end of another such period would be expected
not far from 1957.]
The fact that Alpha Centauri, the star which would be expected most
strongly to influence the sun, and hence the earth, was nearest the sun
at the climax of the last glacial epoch, and that today the solar
atmosphere is most active when the star is presumably most disturbed may
be of no significance. It is given for what it is worth. Its importance
lies not in the fact that it proves anything but that no contradiction
is found when we test the electro-stellar hypothesis by facts which were
not thought of when the hypothesis was framed. A vast amount of
astronomical work is still needed before the matter can be brought to
any definite conclusion. In case the hypothesis stands firm, it may be
possible to use the stars as a help in determining the exact chronology
of the later part of geological times. If the hypothesis is disproved,
it will merely leave the question of solar variations where it is today.
It will not influence the main conclusions of this book as to the causes
and nature of climatic changes. Its value lies in the fact that it calls
attention to new lines of research.
FOOTNOTES:
[Footnote 120: Lewis Boss: Convergent of a Moving Cluster in Taurus;
Astronom. Jour., Vol. 26, No. 4, 1908, pp. 31-36.]
[Footnote 121: F. R. Moulton: in Introduction to Astronomy, 1916.]
[Footnote 122: A. Penck: Die Alpen im Eiszeitalter, Leipzig, 1909.]
[Footnote 123: R. D. Salisbury: Physical Geography of the Pleistocene,
in Outlines of Geologic History, by Willis and Salisbury, 1910, pp.
273-274.]
[Footnote 124: Davis, Pumpelly, and Huntington: Explorations in
Turkestan, Carnegie Inst. of Wash., No. 26, 1905.
In North America the stages have been the subject of intensive studies
on the part of Taylor, Leverett, Goldthwait, and many others.]
CHAPTER XVI
THE EARTH'S CRUST AND THE SUN
Although the problems of this book may lead far afield, they ultimately
bring us back to the earth and to the present. Several times in the
preceding pages there has been mention of the fact that periods of
extreme climatic fluctuations are closely associated with great
movements of the earth's crust whereby mountains are uplifted and
continents upheaved. In attempting to explain this association the
general tendency has been to look largely at the past instead of the
present. Hence it has been almost impossible to choose among three
possibilities, all beset with difficulties. First, the movements of the
crust may have caused the climatic fluctuations; second, climatic
changes may cause crustal movements; and third, variations in solar
activity or in some other outside agency may give rise to both types of
terrestrial phenomena.
The idea that movements of the earth's crust are the main cause of
geological changes of climate is becoming increasingly untenable as the
complexity and rapidity of climatic changes become more clear,
especially during post-glacial times. It implies that the earth's
surface moves up and down with a speed and facility which appear to be
out of the question. If volcanic activity be invoked the problem becomes
no clearer. Even if volcanic dust should fill the air frequently and
completely, neither its presence nor absence would produce such peculiar
features as the localization of glaciers, the distribution of loess, and
the mild climate of most parts of geological time. Nevertheless, because
of the great difficulties presented by the other two possibilities many
geologists still hold that directly or indirectly the greater climatic
changes have been mainly due to movements of the earth's crust and to
the reaction of the crustal movements on the atmosphere.
The possibility that climatic changes are in themselves a cause of
movements of the earth's crust seems so improbable that no one appears
to have investigated it with any seriousness. Nevertheless, it is worth
while to raise the question whether climatic extremes may cooeperate with
other agencies in setting the time when the earth's crust shall be
deformed.
As to the third possibility, it is perfectly logical to ascribe both
climatic changes and crustal deformation to some outside agency, solar
or otherwise, but hitherto there has been so little evidence on this
point that such an ascription has merely begged the question. If
heavenly bodies should approach the earth closely enough so that their
gravitational stresses caused crustal deformation, all life would
presumably be destroyed. As to the sun, there has hitherto been no
conclusive evidence that it is related to crustal movements, although
various writers have made suggestions along this line. In this chapter
we shall carry these suggestions further and shall see that they are at
least worthy of study.
As a preliminary to this study it may be well to note that the
coincidence between movements of the earth's crust and climatic changes
is not so absolute as is sometimes supposed. For example, the profound
crustal changes at the end of the Mesozoic were not accompanied by
widespread glaciation so far as is yet known, although the temperature
appears to have been lowered. Nor was the violent volcanic and
diastrophic activity in the Miocene associated with extreme climates.
Indeed, there appears to have been little contrast from zone to zone,
for figs, bread fruit trees, tree ferns, and other plants of low
latitudes grew in Greenland. Nevertheless, both at the end of the
Mesozoic and in the Miocene the climate may possibly have been severe
for a time, although the record is lost. On the other hand, Kirk's
recent discovery of glacial till in Alaska between beds carrying an
undoubted Middle Silurian fauna indicates glaciation at a time when
there was little movement of the crust so far as yet appears.[125] Thus
we conclude that while climatic changes and crustal movements usually
occur together, they may occur separately.
According to the solar-cyclonic hypothesis such a condition is to be
expected. If the sun were especially active when the terrestrial
conditions prohibited glaciation, changes of climate would still occur,
but they would be milder than under other circumstances, and would leave
little record in the rocks. Or there might be glaciation in high
latitudes, such as that of southern Alaska in the Middle Silurian, and
none elsewhere. On the other hand, when the sun was so inactive that no
great storminess occurred, the upheaval of continents and the building
of mountains might go on without the formation of ice sheets, as
apparently happened at the end of the Mesozoic. The lack of absolute
coincidence between glaciation and periods of widespread emergence of
the lands is evident even today, for there is no reason to suppose that
the lands are notably lower or less extensive now than they were during
the Pleistocene glaciation. In fact, there is much evidence that many
areas have risen since that time. Yet glaciation is now far less
extensive than in the Pleistocene. Any attempt to explain this
difference on the basis of terrestrial changes is extremely difficult,
for the shape and altitude of continents and mountains have not changed
much in twenty or thirty thousand years. Yet the present moderately mild
epoch, like the puzzling inter-glacial epochs of earlier times, is
easily explicable on the assumption that the sun's atmosphere may
sometimes vary in harmony with crustal activity, but does not
necessarily do so at all times.
Turning now to the main problem of how climatic changes may be connected
with movements of the earth's crust, let us follow our usual method and
examine what is happening today. Let us first inquire whether
earthquakes, which are one of the chief evidences that crustal movements
are actually taking place in our own times, show any connection with
sunspots. In order to test this, we have compared _Milne's Catalogue of
Destructive Earthquakes_ from 1800 to 1899, with Wolf's sunspot numbers
for the same period month by month. The earthquake catalogue, as its
compiler describes it, "is an attempt to give a list of earthquakes
which have announced changes of geological importance in the earth's
crust; movements which have probably resulted in the creation or the
extension of a line of fault, the vibrations accompanying which could,
with proper instruments, have been recorded over a continent or the
whole surface of our world. Small earthquakes have been excluded, while
the number of large earthquakes both for ancient and modern times has
been extended. As an illustration of exclusion, I may mention that
between 1800 and 1808, which are years taken at random, I find in
Mallet's catalogue 407 entries. Only thirty-seven of these, which were
accompanied by structural damage, have been retained. Other catalogues
such as those of Perry and Fuchs have been treated similarly."[126]
If the earthquakes in such a carefully selected list bear a distinct
relation to sunspots, it is at least possible and perhaps probable that
a similar relation may exist between solar activity and geological
changes in the earth's crust. The result of the comparison of
earthquakes and sunspots is shown in Table 7. The first column gives the
sunspot numbers; the second, the number of months that had the
respective spot numbers during the century from 1800 to 1899. Column C
shows the total number of earthquakes during the months having any
particular degree of spottedness; while D, which is the significant
column, gives the average number of destructive earthquakes per month
under each of the six conditions of solar spottedness. The regularity of
column D is so great as to make it almost certain that we are here
dealing with a real relationship. Column F, which shows the average
number of earthquakes in the month succeeding any given condition of the
sun, is still more regular except for the last entry.
TABLE 7
DESTRUCTIVE EARTHQUAKES FROM 1800 TO 1899 COMPARED WITH SUNSPOTS
A: _Sunspot numbers_
B: _Number of months per Wolf's Table_
C: _Number of earthquakes_
D: _Average number of earthquakes per month_
E: _Number of earthquakes in succeeding month_
F: _Average number of earthquakes in succeeding month_
A B C D E F
0-15 344 522 1.52 512 1.49
15-30 194 306 1.58 310 1.60
30-50 237 433 1.83 439 1.85
50-70 195 402 2.06 390 2.00
70-100 135 286 2.12 310 2.30
over 100 95 218 2.30 175 1.84
The chance that six numbers taken at random will arrange themselves in
any given order is one in 720. In other words, there is one chance in
720 that the regularity of column D is accidental. But column F is as
regular as column D except for the last entry. If columns D and E were
independent there would be one chance in about 500,000 that the six
numbers in both columns would fall in the same order, and one chance in
14,400 that five numbers in each would fall in the same order. But the
two columns are somewhat related, for although the after-shocks of a
great earthquake are never included in Milne's table, a world-shaking
earthquake in one region during a given month probably creates
conditions that favor similar earthquakes elsewhere during the next
month. Hence the probability that we are dealing with a purely
accidental arrangement in Table 7 is less than one in 14,400 and greater
than one in 500,000. It may be one in 20,000 or 100,000. In any event it
is so slight that there is high probability that directly or indirectly
sunspots and earthquakes are somehow connected.
In ascertaining the relation between sunspots and earthquakes it would
be well if we could employ the strict method of correlation
coefficients. This, however, is impossible for the entire century, for
the record is by no means homogeneous. The earlier decades are
represented by only about one-fourth as many earthquakes as the later
ones, a condition which is presumably due to lack of information. This
makes no difference with the method employed in Table 7, since years
with many and few sunspots are distributed almost equally throughout the
entire nineteenth century, but it renders the method of correlation
coefficients inapplicable. During the period from 1850 onward the record
is much more nearly homogeneous, though not completely so. Even in these
later decades, however, allowance must be made for the fact that there
are more earthquakes in winter than in summer, the average number per
month for the fifty years being as follows:
Jan. 2.8 May 2.4 Sept. 2.5
Feb. 2.4 June 2.3 Oct. 2.6
Mar. 2.5 July 2.4 Nov. 2.7
Apr. 2.4 Aug. 2.4 Dec. 2.8
The correlation coefficient between the departures from these monthly
averages and the corresponding departures from the monthly averages of
the sunspots for the same period, 1850-1899, are as follows:
Sunspots and earthquakes of same month: +0.042, or 1.5 times the
probable error.
Sunspots of a given month and earthquakes of that month and the
next: +0.084, or 3.1 times the probable error.
Sunspots of three consecutive months and earthquakes of three
consecutive months allowing a lag of one month, i.e., sunspots of
January, February, and March compared with earthquakes of February,
March, and April; sunspots of February, March, and April with
earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the
probable error.
These coefficients are all small, but the number of individual cases,
600 months, is so large that the probable error is greatly reduced,
being only +-0.027 or +-0.028. Moreover, the nature of our data is such
that even if there is a strong connection between solar changes and
earth movements, we should not expect a large correlation coefficient.
In the first place, as already mentioned, the earthquake data are not
strictly homogeneous. Second, an average of about two and one-half
strong earthquakes per month is at best only a most imperfect indication
of the actual movement of the earth's crust. Third, the sunspots are
only a partial and imperfect measure of the activity of the sun's
atmosphere. Fourth, the relation between solar activity and earthquakes
is almost certainly indirect. In view of all these conditions, the
regularity of Table 7 and the fact that the most important correlation
coefficient rises to more than four times the probable error makes it
almost certain that the solar and terrestrial phenomena are really
connected.
We are now confronted by the perplexing question of how this connection
can take place. Thus far only three possibilities present themselves,
and each is open to objections. The chief agencies concerned in these
three possibilities are heat, electricity, and atmospheric pressure.
Heat may be dismissed very briefly. We have seen that the earth's
surface becomes relatively cool when the sun is active. Theoretically
even the slightest change in the temperature of the earth's surface must
influence the thermal gradient far into the interior and hence cause a
change of volume which might cause movements of the crust. Practically
the heat of the surface ceases to be of appreciable importance at a
depth of perhaps twenty feet, and even at that depth it does not act
quickly enough to cause the relatively prompt response which seems to be
characteristic of earthquakes in respect to the sun.
The second possibility is based on the relationship between solar and
terrestrial electricity. When the sun is active the earth's atmospheric
electrical potential is subject to slight variations. It is well known
that when two opposing points of an ionized solution are oppositely
charged electrically, a current passes through the liquid and sets up
electrolysis whereby there is a segregation of materials, and a
consequent change in the volume of the parts near the respective
electrical poles. The same process takes place, although less freely, in
a hot mass such as forms the interior of the earth. The question arises
whether internal electrical currents may not pass between the two
oppositely charged poles of the earth, or even between the great
continental masses and the regions of heavier rock which underlie the
oceans. Could this lead to electrolysis, hence to differentiation in
volume, and thus to movements of the earth's crust? Could the results
vary in harmony with the sun? Bowie[127] has shown that numerous
measurements of the strength and direction of the earth's gravitative
pull are explicable only on the assumption that the upheaval of a
continent or a mountain range is due in part not merely to pressure, or
even to flowage of the rocks beneath the crust, but also to an actual
change in volume whereby the rocks beneath the continent attain
relatively great volume and those under the oceans a small volume in
proportion to their weight. The query arises whether this change of
volume may be related to electrical currents at some depth below the
earth's surface.
The objections to this hypothesis are numerous. First, there is little
evidence of electrolytic differentiation in the rocks. Second, the outer
part of the earth's crust is a very poor conductor so that it is
doubtful whether even a high degree of electrification of the surface
would have much effect on the interior. Third, electrolysis due to any
such mild causes as we have here postulated must be an extremely slow
process, too slow, presumably, to have any appreciable result within a
month or two. Other objections join with these three in making it seem
improbable that the sun's electrical activity has any direct effect upon
movements of the earth's crust.
The third, or meteorological hypothesis, which makes barometric pressure
the main intermediary between solar activity and earthquakes, seems at
first sight almost as improbable as the thermal and electrical
hypotheses. Nevertheless, it has a certain degree of observational
support of a kind which is wholly lacking in the other two cases. Among
the extensive writings on the periodicity of earthquakes one main fact
stands out with great distinctness: earthquakes vary in number according
to the season. This fact has already been shown incidentally in the
table of earthquake frequency by months. If allowance is made for the
fact that February is a short month, there is a regular decrease in the
frequency of severe earthquakes from December and January to June. Since
most of Milne's earthquakes occurred in the northern hemisphere, this
means that severe earthquakes occur in winter about 20 per cent oftener
than in summer.
The most thorough investigation of this subject seems to have been that
of Davisson.[128] His results have been worked over and amplified by
Knott,[129] who has tested them by Schuster's exact mathematical
methods. His results are given in Table 8.[130] Here the northern
hemisphere is placed first; then come the East Indies and the Malay
Archipelago lying close to the equator; and finally the southern
hemisphere. In the northern hemisphere practically all the maxima come
in the winter, for the month of December appears in fifteen cases out of
the twenty-five in column D, while January, February, or November
appears in six others. It is also noticeable that in sixteen cases out
of twenty-five the ratio of the actual to the expected amplitude in
column G is four or more, so that a real relationship is indicated,
while the ratio falls below three only in Japan and Zante. The
equatorial data, unlike those of the northern hemisphere, are
indefinite, for in the East Indies no month shows a marked maximum and
the expected amplitude exceeds the actual amplitude. Even in the Malay
Archipelago, which shows a maximum in May, the ratio of actual to
expected amplitude is only 2.6. Turning to the southern hemisphere, the
winter months of that hemisphere are as strongly marked by a maximum as
are the winter months of the northern hemisphere. July or August appears
in five out of six cases. Here the ratio between the actual and expected
amplitudes is not so great as in the northern hemisphere. Nevertheless,
it is practically four in Chile, and exceeds five in Peru and Bolivia,
and in the data for the entire southern hemisphere.
TABLE 8
SEASONAL MARCH OF EARTHQUAKES
AFTER DAVISSON AND KNOTT
A: _Region_
B: _Limiting Dates_
C: _Number of Shocks_
D: _Maximum Month_
E: _Amplitude_
F: _Expected Amplitude_
G: _Ratio of Actual to Expected Amplitude_
A B C D E F G
Northern Hemisphere 223-1850 5879 Dec. 0.110 0.023 4.8
Northern Hemisphere 1865-1884 8133 Dec. 0.290 0.020 14.5
Europe 1865-1884 5499 Dec. 0.350 0.024 14.6
Europe 306-1843 1961 Dec. 0.220 0.040 5.5
Southeast Europe 1859-1887 3470 Dec. 0.210 0.030 7.0
Vesuvius District 1865-1883 513 Dec. 0.250 0.078 3.2
Italy:
Old Tromometre 1872-1887 61732 Dec. 0.490 0.007 70.0
Old Tromometre 1876-1887 38546 Dec. 0.460 0.009 49.5
Normal Tromometre 1876-1887 38546 Dec. 0.490 0.009 52.8
Balkan, etc. 1865-1884 624 Dec. 0.270 0.071 3.8
Hungary, etc. 1865-1884 384 Dec. 0.310 0.090 3.4
Italy 1865-1883 2350 Dec.(Sept.)0.140 0.037 3.8
Grecian Archip. 1859-1881 3578 Dec.-Jan. 0.164 0.030 5.5
Austria 1865-1884 461 Jan. 0.370 0.083 4.4
Switzerland, etc. 1865-1883 524 Jan. 0.560 0.077 7.3
Asia 1865-1884 458 Feb. 0.330 0.083 4.0
North America 1865-1884 552 Nov. 0.350 0.075 4.7
California 1850-1886 949 Oct. 0.300 0.058 5.2
Japan 1878-1881 246 Dec. 0.460 0.113 4.1
Japan 1872-1880 367 Dec.-Jan. 0.256 0.093 2.8
Japan 1876-1891 1104 Feb. 0.190 0.053 3.6
Japan 1885-1889 2997 Oct. 0.080 0.032 2.5
Zante 1825-1863 1326 Aug. 0.100 0.049 2.0
Italy, North 1865-1883 1513 Sept.(Nov.) 0.210 0.046 4.6
of Naples
East Indies 1873-1881 515 Aug., Oct., 0.071? 0.078 0.9
or Dec.?
Malay Archip. 1865-1884 598 May 0.190 0.072 2.6
New Zealand 1869-1879 585 Aug.-Sept. 0.203 0.073 2.8
Chile 1873-1881 212 July 0.480 0.122 3.9
Southern Hemisphere 1865-1884 751 July 0.370 0.065 5.7
New Zealand 1868-1890 641 March, May 0.050 0.070 0.7
Chile 1865-1883? 316 July, Dec. 0.270 0.100 2.7
Peru, Bolivia 1865-1884 350 July 0.480 0.095 5.1
The whole relationship between earthquakes and the seasons in the
northern and southern hemispheres is summed up in Fig. 12 taken from
Knott. The northern hemisphere shows a regular diminution in earthquake
frequency from December until June, and an increase the rest of the
year. In the southern hemisphere the course of events is the same so far
as summer and winter are concerned, for August with its maximum comes in
winter, while February with its minimum comes in summer. In the southern
hemisphere the winter month of greatest seismic activity has over 100
per cent more earthquakes than the summer month of least activity. In
the northern hemisphere this difference is about 80 per cent, but this
smaller figure occurs partly because the northern data include certain
interesting and significant regions like Japan and China where the usual
conditions are reversed.[131] If equatorial regions were included in
Fig. 12, they would give an almost straight line.
The connection between earthquakes and the seasons is so strong that
almost no students of seismology question it, although they do not agree
as to its cause. A meteorological hypothesis seems to be the only
logical explanation.[132] Wherever sufficient data are available,
earthquakes appear to be most numerous when climatic conditions cause
the earth's surface to be most heavily loaded or to change its load most
rapidly. The main factor in the loading is apparently atmospheric
pressure. This acts in two ways. First, when the continents become cold
in winter the pressure increases. On an average the air at sea level
presses upon the earth's surface at the rate of 14.7 pounds per square
inch, or over a ton per square foot, and only a little short of thirty
million tons per square mile. An average difference of one inch between
the atmospheric pressure of summer and winter over ten million square
miles of the continent of Asia, for example, means that the continent's
load in winter is about ten million million tons heavier than in summer.
Second, the changes in atmospheric pressure due to the passage of storms
are relatively sharp and sudden. Hence they are probably more effective
than the variations in the load from season to season. This is suggested
by the rapidity with which the terrestrial response seems to follow the
supposed solar cause of earthquakes. It is also suggested by the fact
that violent storms are frequently followed by violent earthquakes.
"Earthquake weather," as Dr. Schlesinger suggests, is a common phrase in
the typhoon region of Japan, China, and the East Indies. During tropical
hurricanes a change of pressure amounting to half an inch in two hours
is common. On September 22, 1885, at False Point Lighthouse on the Bay
of Bengal, the barometer fell about an inch in six hours, then nearly an
inch and a half in not much over two hours, and finally rose fully two
inches inside of two hours. A drop of two inches in barometric pressure
means that a load of about two million tons is removed from each square
mile of land; the corresponding rise of pressure means the addition of a
similar load. Such a storm, and to a less degree every other storm,
strikes a blow upon the earth's surface, first by removing millions of
tons of pressure and then by putting them on again.[133] Such storms, as
we have seen, are much more frequent and severe when sunspots are
numerous than at other times. Moreover, as Veeder[134] long ago showed,
one of the most noteworthy evidences of a connection between sunspots
and the weather is a sudden increase of pressure in certain widely
separated high pressure areas. In most parts of the world winter is not
only the season of highest pressure and of most frequent changes of
Veeder's type, but also of severest storms. Hence a meteorological
hypothesis would lead to the expectation that earthquakes would occur
more frequently in winter than in summer. On the Chinese coast, however,
and also on the oceanic side of Japan, as well as in some more tropical
regions, the chief storms come in summer in the form of typhoons. These
are the places where earthquakes also are most abundant in summer. Thus,
wherever we turn, storms and the related barometric changes seem to be
most frequent and severe at the very times when earthquakes are also
most frequent.
[Illustration: _Fig. 12. Seasonal distribution of earthquakes. (After
Davisson and Knott.)_
solid line ---- Northern Hemisphere.
dashed line .... Southern Hemisphere.]
Other meteorological factors, such as rain, snow, winds, and currents,
probably have some effect on earthquakes through their ability to load
the earth's crust. The coming of vegetation may also help. These
agencies, however, appear to be of small importance compared with the
storms. In high latitudes and in regions of abundant storminess most of
these factors generally combine with barometric pressure to produce
frequent changes in the load of the earth's crust, especially in winter.
In low latitudes, on the other hand, there are few severe storms, and
relatively little contrast in pressure and vegetation from season to
season; there is no snow; and the amount of ground water changes little.
With this goes the twofold fact that there is no marked seasonal
distribution of earthquakes, and that except in certain local volcanic
areas, earthquakes appear to be rare. In proportion to the areas
concerned, for example, there is little evidence of earthquakes in
equatorial Africa and South America.
The question of the reality of the connection between meteorological
conditions and crustal movements is so important that every possible
test should be applied. At the suggestion of Professor Schlesinger we
have looked up a very ingenious line of inquiry. During the last decades
of the nineteenth century, a long series of extremely accurate
observations of latitude disclosed a fact which had previously been
suspected but not demonstrated, namely, that the earth wabbles a little
about its axis. The axis itself always points in the same direction, and
since the earth slides irregularly around it the latitude of all parts
of the earth keeps changing. Chandler has shown that the wabbling thus
induced consists of two parts. The first is a movement in a circle with
a radius of about fifteen feet which is described in approximately 430
days. This so-called Eulerian movement is a normal gyroscopic motion
like the slow gyration of a spinning top. This depends on purely
astronomical causes, and no terrestrial cause can stop it or eliminate
it. The period appears to be constant, but there are certain puzzling
irregularities. The usual amplitude of this movement, as
Schlesinger[135] puts it, "is about 0".27, but twice in recent years it
has jumped to 0".40. Such a change could be accounted for by supposing
that the earth had received a severe blow or a series of milder blows
tending in the same direction." These blows, which were originally
suggested by Helmert are most interesting in view of our suggestion as
to the blows struck by storms.
The second movement of the pole has a period of a year, and is roughly
an ellipse whose longest radius is fourteen feet and the shortest, four
feet; or, to put it technically, there is an annual term with a maximum
amplitude of about 0".20. This, however, varies irregularly. The result
is that the pole seems to wander over the earth's surface in the spiral
fashion illustrated in Fig. 13. It was early suggested that this
peculiar wandering of the pole in an annual period must be due to
meteorological causes. Jeffreys[136] has investigated the matter
exhaustively. He assumes certain reasonable values for the weight of air
added or subtracted from different parts of the earth's surface
according to the seasons. He also considers the effect of precipitation,
vegetation, and polar ice, and of variations of temperature and
atmospheric pressure in their relation to movements of the ocean. Then
he proceeds to compare all these with the actual wandering of the pole
from 1907 to 1913. While it is as yet too early to say that any special
movement of the pole was due to the specific meteorological conditions
of any particular year, Jeffreys' work makes it clear that
meteorological causes, especially atmospheric pressure, are sufficient
to cause the observed irregular wanderings. Slight wanderings may arise
from various other sources such as movements of the rocks when
geological faults occur or the rush of a great wave due to a submarine
earthquake. So far as known, however, all these other agencies cause
insignificant displacements compared with those arising from movements
of the air. This fact coupled with the mathematical certainty that
meteorological phenomena must produce some wandering of the pole, has
caused most astronomers to accept Jeffreys' conclusion. If we follow
their example we are led to conclude that changes in atmospheric
pressure and in the other meteorological conditions strike blows which
sometimes shift the earth several feet from its normal position in
respect to the axis.
[Illustration: _Fig. 13. Wandering of the pole from 1890 to 1898._
(_After Moulton._)]
If the foregoing reasoning is correct, the great and especially the
sudden departures from the smooth gyroscopic circle described by the
pole in the Eulerian motion would be expected to occur at about the same
time as unusual earthquake activity. This brings us to an interesting
inquiry carried out by Milne[137] and amplified by Knott.[138] Taking
Albrecht's representation of the irregular spiral-like motion of the
pole, as given in Fig. 13, they show that there is a preponderance of
severe earthquakes at times when the direction of motion of the earth in
reference to its axis departs from the smooth Eulerian curve. A summary
of their results is given in Table 9. The table indicates that during
the period from 1892 to 1905 there were nine different times when the
curve of Fig. 13 changed its direction or was deflected by less than 10 deg.
during a tenth of a year. In other words, during those periods it did
not curve as much as it ought according to the Eulerian movement. At
such times there were 179 world-shaking earthquakes, or an average of
about 19.9 per tenth of a year. According to the other lines of Table 9,
in thirty-two cases the deflection during a tenth of a year was between
10 deg. and 25 deg., while in fifty-six cases it was from 25 deg. to 40 deg.. During
these periods the curve remained close to the Eulerian path and the
world-shaking earthquakes averaged only 8.2 and 12.9. Then, when the
deflection was high, that is, when meteorological conditions threw the
earth far out of its Eulerian course, the earthquakes were again
numerous, the number rising to 23.4 when the deflection amounted to more
than 55 deg..
TABLE 9
DEFLECTION OF PATH OF POLE COMPARED WITH EARTHQUAKES
_No. of _No. of _Average No.
_Deflection_ Deflections_ Earthquakes_ of Earthquakes_
0-10 deg. 9 179 19.9
10-25 deg. 32 263 8.2
25-40 deg. 56 722 12.9
40-55 deg. 19 366 19.3
over 55 deg. 7 164 23.4
In order to test this conclusion in another way we have followed a
suggestion of Professor Schlesinger. Under his advice the Eulerian
motion has been eliminated and a new series of earthquake records has
been compared with the remaining motions of the poles which presumably
arise largely from meteorological causes. For this purpose use has been
made of the very full records of earthquakes published under the
auspices of the International Seismological Commission for the years
1903 to 1908, the only years for which they are available. These include
every known shock of every description which was either recorded by
seismographs or by direct observation in any part of the world. Each
shock is given the same weight, no matter what its violence or how
closely it follows another. The angle of deflection has been measured as
Milne measured it, but since the Eulerian motion is eliminated, our zero
is approximately the normal condition which would prevail if there were
no meteorological complications. Dividing the deflections into six equal
groups according to the size of the angle, we get the result shown in
Table 10.
TABLE 10
EARTHQUAKES IN 1903-1908 COMPARED WITH DEPARTURES OF THE PROJECTED
CURVE OF THE EARTH'S AXIS FROM THE EULERIAN POSITION
_Average angle of deflection_ _Average daily number
(_10 periods of 1/10 year each_) of earthquakes_
-10.5 deg. 8.31
11.5 deg. 8.35
25.8 deg. 8.23
40.2 deg. 8.14
54.7 deg. 8.86
90.3 deg. 11.81
Here where some twenty thousand earthquakes are employed the result
agrees closely with that of Milne for a different series of years and
for a much smaller number of earthquakes. So long as the path of the
pole departs less than about 45 deg. from the smooth gyroscopic Eulerian
path, the number of earthquakes is almost constant, about eight and a
quarter per day. When the angle becomes large, however, the number
increases by nearly 50 per cent. Thus the work of Milne, Knott, and
Jeffreys is confirmed by a new investigation. Apparently earthquakes and
crustal movements are somehow related to sudden changes in the load
imposed on the earth's crust by meteorological conditions.
This conclusion is quite as surprising to the authors as to the
reader--perhaps more so. At the beginning of this investigation we had
no faith whatever in any important relation between climate and
earthquakes. At its end we are inclined to believe that the relation is
close and important.
It must not be supposed, however, that meteorological conditions are the
_cause_ of earthquakes and of movements of the earth's crust. Even
though the load that the climatic agencies can impose upon the earth's
crust runs into millions of tons per square mile, it is a trifle
compared with what the crust is able to support. There is, however, a
great difference between the cause and the occasion of a phenomenon.
Suppose that a thick sheet of glass is placed under an increasing
strain. If the strain is applied slowly enough, even so rigid a material
as glass will ultimately bend rather than break. But suppose that while
the tension is high the glass is tapped. A gentle tap may be followed by
a tiny crack. A series of little taps may be the signal for small cracks
to spread in every direction. A few slightly harder taps may cause the
whole sheet to break suddenly into many pieces. Yet even the hardest tap
may be the merest trifle compared with the strong force which is keeping
the glass in a state of strain and which would ultimately bend it if
given time.