THE SUN
110. DEPENDENCE OF THE EARTH UPON THE SUN.--There is no better
introduction to the study of the sun than Byron's Ode to Darkness,
beginning with the lines--
"I dreamed a dream
That was not all a dream.
The bright sun was extinguished,"
and proceeding to depict in vivid words the consequences of this
extinction. The most matter-of-fact language of science agrees with the
words of the poet in declaring the earth's dependence upon the sun for
all those varied forms of energy which make it a fit abode for living
beings. The winds blow and the rivers run; the crops grow, are gathered
and consumed, by virtue of the solar energy. Factory, locomotive, beast,
bird, and the human body furnish types of machines run by energy derived
from the sun; and the student will find it an instructive exercise to
search for kinds of terrestrial energy which are not derived either
directly or indirectly from the sun. There are a few such, but they are
neither numerous nor important.
111. THE SUN'S DISTANCE FROM THE EARTH.--To the astronomer the sun
presents problems of the highest consequence and apparently of very
diverse character, but all tending toward the same goal: the framing of
a mechanical explanation of the sun considered as a machine; what it is,
and how it does its work. In the forefront of these problems stand those
numerical determinations of distance, size, mass, density, etc., which
we have already encountered in connection with the moon, but which must
here be dealt with in a different manner, because the immensely greater
distance of the sun makes impossible the resort to any such simple
method as the triangle used for determining the moon's distance. It
would be like determining the distance of a steeple a mile away by
observing its direction first from one eye, then from the other; too
short a base for the triangle. In one respect, however, we stand upon a
better footing than in the case of the moon, for the mass of the earth
has already been found (Chapter IV) as a fractional part of the sun's
mass, and we have only to invert the fraction in order to find that the
sun's mass is 329,000 times that of the earth and moon combined, or
333,000 times that of the earth alone.
If we could rely implicitly upon this number we might make it determine
for us the distance of the sun through the law of gravitation as
follows: It was suggested in § 38 that Newton proved Kepler's three laws
to be imperfect corollaries from the law of gravitation, requiring a
little amendment to make them strictly correct, and below we give in the
form of an equation Kepler's statement of the Third Law together with
Newton's amendment of it. In these equations--
_T_ = Periodic time of any planet;
_a_ = One half the major axis of its orbit;
_m_ = Its mass;
_M_ = The mass of the sun;
_k_ = The gravitation constant corresponding to the particular set of
units in which _T_, _a_, _m_, and _M_ are expressed.
(Kepler) a^{3}/T^{2} = h;
(Newton) a^{3}/T^{2} = k (M + m).
Kepler's idea was: For every planet which moves around the sun, _a^{3}_
divided by _T^{2}_ always gives the same quotient, _h_; and he did not
concern himself with the significance of this quotient further than to
note that if the particular _a_ and _T_ which belong to any
planet--e. g., the earth--be taken as the units of length and time, then
the quotient will be 1. Newton, on the other hand, attached a meaning to
the quotient, and showed that it is equal to the product obtained by
multiplying the sum of the two masses, planet and sun, by a number which
is always the same when we are dealing with the action of gravitation,
whether it be between the sun and planet, or between moon and earth, or
between the earth and a roast of beef in the butcher's scales, provided
only that we use always the same units with which to measure times,
distances, and masses.
Numerically, Newton's correction to Kepler's Third Law does not amount
to much in the motion of the planets. Jupiter, which shows the greatest
effect, makes the circuit of his orbit in 4,333 days instead of 4,335,
which it would require if Kepler's law were strictly true. But in
another respect the change is of the utmost importance, since it enables
us to extend Kepler's law, which relates solely to the sun and its
planets, to other attracting bodies, such as the earth, moon, and stars.
Thus for the moon's motion around the earth we write--
(240,000^{3})/(27.32^{2}) = k (1 + 1/81),
from which we may find that, with the units here employed, the earth's
mass as the unit of mass, the mean solar day as the unit of time, and
the mile as the unit of distance--
k = 1830 × 10^{10}.
If we introduce this value of _k_ into the corresponding equation, which
represents the motion of the earth around the sun, we shall have--
a^{3}/(365.25)^{2} = 1830 × 10^{10} (333,000 + 1),
where the large number in the parenthesis represents the number of times
the mass of the sun is greater than the mass of the earth. We shall find
by solving this equation that _a_, the mean distance of the sun from the
earth, is very approximately 93,000,000 miles.
113. ANOTHER METHOD OF DETERMINING THE SUN'S DISTANCE.--This will be
best appreciated by a reference to Fig. 17. It appears here that the
earth makes its nearest approach to the orbit of Mars in the month of
August, and if in any August Mars happens to be in opposition, its
distance from the earth will be very much less than the distance of the
sun from the earth, and may be measured by methods not unlike those
which served for the moon. If now the orbits of Mars and the earth were
circles having their centers at the sun this distance between them,
which we may represent by _D_, would be the difference of the radii of
these orbits--
D = a´´ - a´,
where the accents ´´, ´ represent Mars and the earth respectively.
Kepler's Third Law furnishes the relation--
(a´´)^{3}/(T´´)^{2} = (a´)^{3}/(T´)^{2};
and since the periodic times of the earth and Mars, _T´_, _T´´_, are
known to a high degree of accuracy, these two equations are sufficient
to determine the two unknown quantities, _a´_, _a´´_--i. e., the
distance of the sun from Mars as well as from the earth. The first of
these equations is, of course, not strictly true, on account of the
elliptical shape of the orbits, but this can be allowed for easily
enough.
In practice it is found better to apply this method of determining the
sun's distance through observations of an asteroid rather than
observations of Mars, and great interest has been aroused among
astronomers by the discovery, in 1898, of an asteroid, or planet, Eros,
which at times comes much closer to the earth than does Mars or any
other heavenly body except the moon, and which will at future
oppositions furnish a more accurate determination of the sun's distance
than any hitherto available. Observations for this purpose are being
made at the present time (October, 1900).
Many other methods of measuring the sun's distance have been devised by
astronomers, some of them extremely ingenious and interesting, but every
one of them has its weak point--e. g., the determination of the mass of
the earth in the first method given above and the measurement of _D_ in
the second method, so that even the best results at present are
uncertain to the extent of 200,000 miles or more, and astronomers,
instead of relying upon any one method, must use all of them, and take
an average of their results. According to Professor Harkness, this
average value is 92,796,950 miles, and it seems certain that a line of
this length drawn from the earth toward the sun would end somewhere
within the body of the sun, but whether on the nearer or the farther
side of the center, or exactly at it, no man knows.
114. PARALLAX AND DISTANCE.--It is quite customary among astronomers to
speak of the sun's parallax, instead of its distance from the earth,
meaning by parallax its difference of direction as seen from the center
and surface of the earth--i. e., the angle subtended at the sun by a
radius of the earth placed at right angles to the line of sight. The
greater the sun's distance the smaller will this angle be, and it
therefore makes a substitute for the distance which has the advantage of
being represented by a small number, 8".8, instead of a large one.
The books abound with illustrations intended to help the reader
comprehend how great is a distance of 93,000,000 miles, but a single one
of these must suffice here. To ride 100 miles a day 365 days in the year
would be counted a good bicycling record, but the rider who started at
the beginning of the Christian era and rode at that rate toward the sun
from the year 1 A. D. down to the present moment would not yet have
reached his destination, although his journey would be about three
quarters done. He would have crossed the orbit of Venus about the time
of Charlemagne, and that of Mercury soon after the discovery of America.
115. SIZE AND DENSITY OF THE SUN.--Knowing the distance of the sun, it
is easy to find from the angle subtended by its diameter (32 minutes of
arc) that the length of that diameter is 865,000 miles. We recall in
this connection that the diameter of the moon's _orbit_ is only 480,000
miles, but little more than half the diameter of the sun, thus affording
abundant room inside the sun, and to spare, for the moon to perform the
monthly revolution about its orbit, as shown in Fig. 65.
[Illustration: FIG. 65.--The sun's size.--YOUNG.]
In the same manner in which the density of the moon was found from its
mass and diameter, the student may find from the mass and diameter of
the sun given above that its mean density is 1.4 times that of water.
This is about the same as the density of gravel or soft coal, and is
just about one quarter of the average density of the earth.
We recall that the small density of the moon was accounted for by the
diminished weight of objects upon it, but this explanation can not hold
in the case of the sun, for not only is the density less but the force
of gravity (weight) is there 28 times as great as upon the earth. The
athlete who here weighs 175 pounds, if transported to the surface of the
sun would weigh more than an elephant does here, and would find his
bones break under his own weight if his muscles were strong enough to
hold him upright. The tremendous pressure exerted by gravity at the
surface of the sun must be surpassed below the surface, and as it does
not pack the material together and make it dense, we are driven to one
of two conclusions: Either the stuff of which the sun is made is
altogether unlike that of the earth, not so readily compressed by
pressure, or there is some opposing influence at work which more than
balances the effect of gravity and makes the solar stuff much lighter
than the terrestrial.
116. MATERIAL OF WHICH THE SUN IS MADE.--As to the first of these
alternatives, the spectroscope comes to our aid and shows in the sun's
spectrum (Fig. 50) the characteristic line marked _D_, which we know
always indicates the presence of sodium and identifies at least one
terrestrial substance as present in the sun in considerable quantity.
The lines marked _C_ and _F_ are produced by hydrogen, which is one of
the constituents of water, _E_ shows calcium to be present in the sun,
_b_ magnesium, etc. In this way it has been shown that about one half of
our terrestrial elements, mainly the metallic ones, are present as gases
on or near the sun's surface, but it must not be inferred that elements
not found in this way are absent from the sun. They may be there,
probably are there, but the spectroscopic proof of their presence is
more difficult to obtain. Professor Rowland, who has been prominent in
the study of the solar spectrum, says: "Were the whole earth heated to
the temperature of the sun, its spectrum would probably resemble that of
the sun very closely."
Some of the common terrestrial elements found in the sun are:
Aluminium.
Calcium.
Carbon.
Copper.
Hydrogen.
Iron.
Lead.
Nickel.
Potassium.
Silicon.
Silver.
Sodium.
Tin.
Zinc.
Oxygen (?)
Whatever differences of chemical structure may exist between the sun and
the earth, it seems that we must regard these bodies as more like than
unlike to each other in substance, and we are brought back to the second
of our alternatives: there must be some influence opposing the force of
gravity and making the substance of the sun light instead of heavy, and
we need not seek far to find it in--
117. THE HEAT OF THE SUN.--That the sun is hot is too evident to require
proof, and it is a familiar fact that heat expands most substances and
makes them less dense. The sun's heat falling upon the earth expands it
and diminishes its density in some small degree, and we have only to
imagine this process of expansion continued until the earth's diameter
becomes 58 per cent larger than it now is, to find the earth's density
reduced to a level with that of the sun. Just how much the temperature
of the earth must be raised to produce this amount of expansion we do
not know, neither do we know accurately the temperature of the sun, but
there can be no doubt that heat is the cause of the sun's low density
and that the corresponding temperature is very high.
Before we inquire more closely into the sun's temperature, it will be
well to draw a sharp distinction between the two terms heat and
temperature, which are often used as if they meant the same thing. Heat
is a form of energy which may be found in varying degree in every
substance, whether warm or cold--a block of ice contains a considerable
amount of heat--while temperature corresponds to our sensations of warm
and cold, and measures the extent to which heat is concentrated in the
body. It is the amount of heat per molecule of the body. A barrel of
warm water contains more heat than the flame of a match, but its
temperature is not so high. Bearing in mind this distinction, we seek to
determine not the amount of heat contained in the sun but the sun's
temperature, and this involves the same difficulty as does the question,
What is the temperature of a locomotive? It is one thing in the fire box
and another thing in the driving wheels, and still another at the
headlight; and so with the sun, its temperature is certainly different
in different parts--one thing at the center and another at the surface.
Even those parts which we see are covered by a veil of gases which
produce by absorption the dark lines of the solar spectrum, and
seriously interfere both with the emission of energy from the sun and
with our attempts at measuring the temperature of those parts of the
surface from which that energy streams.
In view of these and other difficulties we need not be surprised that
the wildest discordance has been found in estimates of the solar
temperature made by different investigators, who have assigned to it
values ranging from 1,400° C. to more than 5,000,000° C. Quite recently,
however, improved methods and a better understanding of the problem have
brought about a better agreement of results, and it now seems probable
that the temperature of the visible surface of the sun lies somewhere
between 5,000° and 10,000° C., say 15,000° of the Fahrenheit scale.
118. DETERMINING THE SUN'S TEMPERATURE.--One ingenious method which has
been used for determining this temperature is based upon the principle
stated above, that every object, whether warm or cold, contains heat and
gives it off in the form of radiant energy. The radiation from a body
whose temperature is lower than 500° C. is made up exclusively of energy
whose wave length is greater than 7,600 tenth meters, and is therefore
invisible to the eye, although a thermometer or even the human hand can
often detect it as radiant heat. A brick wall in the summer sunshine
gives off energy which can be felt as heat but can not be seen. When
such a body is further heated it continues to send off the same kinds
(wave lengths) of energy as before, but new and shorter waves are added
to its radiation, and when it begins to emit energy of wave length 7,500
or 7,600 tenth meters, it also begins to shine with a dull-red light,
which presently becomes brighter and less ruddy and changes to white as
the temperature rises, and waves of still shorter length are thereby
added to the radiation. We say, in common speech, the body becomes first
red hot and then white hot, and we thus recognize in a general way that
the kind or color of the radiation which a body gives off is an index to
its temperature. The greater the proportion of energy of short wave
lengths the higher is the temperature of the radiating body. In sunlight
the maximum of brilliancy to the eye lies at or near the wave length,
5,600 tenth meters, but the greatest intensity of radiation of all kinds
(light included) is estimated to fall somewhere between green and blue
in the spectrum at or near the wave length 5,000 tenth meters, and if we
can apply to this wave length Paschen's law--temperature reckoned in
degrees centigrade from the absolute zero is always equal to the
quotient obtained by dividing the number 27,000,000 by the wave length
corresponding to maximum radiation--we shall find at once for the
absolute temperature of the sun's surface 5,400° C.
Paschen's law has been shown to hold true, at least approximately, for
lower temperatures and longer wave lengths than are here involved, but
as it is not yet certain that it is strictly true and holds for all
temperatures, too great reliance must not be attached to the numerical
result furnished by it.
[Illustration: FIG. 66.--The sun, August 11, 1894. Photographed at the
Goodsell Observatory.]
[Illustration: FIG. 67.--The sun, August 14, 1894. Photographed at the
Goodsell Observatory.]
119. THE SUN'S SURFACE.--A marked contrast exists between the faces of
sun and moon in respect of the amount of detail to be seen upon them,
the sun showing nothing whatever to correspond with the mountains,
craters, and seas of the moon. The unaided eye in general finds in the
sun only a blank bright circle as smooth and unmarked as the surface of
still water, and even the telescope at first sight seems to show but
little more. There may usually be found upon the sun's face a certain
number of black patches called _sun spots_, such as are shown in Figs.
66 to 69, and occasionally these are large enough to be seen through a
smoked glass without the aid of a telescope. When seen near the edge of
the sun they are quite frequently accompanied, as in Fig. 69, by vague
patches called _faculæ_ (Latin, _facula_ = a little torch), which look a
little brighter than the surrounding parts of the sun. So, too, a good
photograph of the sun usually shows that the central parts of the disk
are rather brighter than the edge, as indeed we should expect them to
be, since the absorption lines in the sun's spectrum have already taught
us that the visible surface of the sun is enveloped by invisible vapors
which in some measure absorb the emitted light and render it feebler at
the edge where it passes through a greater thickness of this envelope
than at the center. See Fig. 70, where it is shown that the energy
coming from the edge of the sun to the earth has to traverse a much
longer path inside the vapors than does that coming from the center.
[Illustration: FIG. 68.--The sun, August 18, 1894. Photographed at the
Goodsell Observatory.]
Examine the sun spots in the four photographs, Figs. 66 to 69, and note
that the two spots which appear at the extreme left of the first
photograph, very much distorted and foreshortened by the curvature of
the sun's surface, are seen in a different part of the second picture,
and are not only more conspicuous but show better their true shape.
[Illustration: PLATE II. THE EQUATORIAL CONSTELLATIONS]
120. THE SUN'S ROTATION.--The changed position of these spots shows that
the sun rotates about an axis at right angles to the direction of the
spot's motion, and the position of this axis is shown in the figure by a
faint line ruled obliquely across the face of the sun nearly north and
south in each of the four photographs. This rotation in the space of
three days has carried the spots from the edge halfway to the center of
the disk, and the student should note the progress of the spots in the
two later photographs, that of August 21st showing them just ready to
disappear around the farther edge of the sun.
[Illustration: FIG. 69.--The sun, August 21, 1894. Photographed at the
Goodsell Observatory.]
Plot accurately in one of these figures the positions of the spots as
shown in the other three, and observe whether the path of the spots
across the sun's face is a straight line. Is there any reason why it
should not be straight?
These four pictures may be made to illustrate many things about the sun.
Thus the sun's axis is not parallel to that of the earth, for the
letters _N S_ mark the direction of a north and south line across the
face of the sun, and this line, of course, is parallel to the earth's
axis, while it is evidently not parallel to the sun's axis. The group of
spots took more than ten days to move across the sun's face, and as at
least an equal time must be required to move around the opposite side of
the sun, it is evident that the period of the sun's rotation is
something more than 20 days. It is, in fact, rather more than 25 days,
for this same group of spots reappeared again on the left-hand edge of
the sun on September 5th.
[Illustration: FIG. 70.--Absorption at the sun's edge.]
121. SUN SPOTS.--Another significant fact comes out plainly from the
photographs. The spots are not permanent features of the sun's face,
since they changed their size and shape very appreciably in the few days
covered by the pictures. Compare particularly the photographs of August
14th and August 18th, where the spots are least distorted by the
curvature of the sun's surface. By September 16th this group of spots
had disappeared absolutely from the sun's face, although when at its
largest the group extended more than 80,000 miles in length, and several
of the individual spots were large enough to contain the earth if it had
been dropped upon them. From Fig. 67 determine in miles the length of
the group on August 14th. Fig. 71 shows an enlarged view of these spots
as they appeared on August 17th, and in this we find some details not so
well shown in the preceding pictures. The larger spots consist of a
black part called the _nucleus_ or _umbra_ (Latin, shadow), which is
surrounded by an irregular border called the _penumbra_ (partial
shadow), which is intermediate in brightness between the nucleus and
the surrounding parts of the sun. It should not be inferred from the
picture that the nucleus is really black or even dark. It shines, in
fact, with a brilliancy greater than that of an electric lamp, but the
background furnished by the sun's surface is so much brighter that by
contrast with it the nucleus and penumbra appear relatively dark.
[Illustration: FIG. 71.--Sun spots, August 17, 1894. Goodsell
Observatory.]
[Illustration: FIG. 72.--Sun spot of March 5, 1873.--From LANGLEY, The
New Astronomy. By permission of the publishers.]
The bright shining surface of the sun, the background for the spots, is
called the _photosphere_ (Greek, light sphere), and, as Fig. 71 shows,
it assumes under a suitable magnifying power a mottled aspect quite
different from the featureless expanse shown in the earlier pictures.
The photosphere is, in fact, a layer of little clouds with darker
spaces between them, and the fine detail of these clouds, their
complicated structure, and the way in which, when projected against the
background of a sun spot, they produce its penumbra, are all brought out
in Fig. 72. Note that the little patch in one corner of this picture
represents North and South America drawn to the same scale as the sun
spots.
[Illustration: FIG. 73.--Spectroheliograph, showing distribution of
faculæ upon the sun.--HALE.]
[Illustration: FIG. 74.--Eclipse of July 20, 1878.--TROUVELOT.]
122. FACULÆ.--We have seen in Fig. 69 a few of the bright spots called
faculæ. At the telescope or in the ordinary photograph these can be seen
only at the edge of the sun, because elsewhere the background furnished
by the photosphere is so bright that they are lost in it. It is
possible, however, by an ingenious application of the spectroscope to
break up the sunlight into a spectrum in such a way as to diminish the
brightness of this background, much more than the brightness of the
faculæ is diminished, and in this way to obtain a photograph of the
sun's surface which shall show them wherever they occur, and such a
photograph, showing faintly the spectral lines, is reproduced in Fig.
73. The faculæ are the bright patches which stretch inconspicuously
across the face of the sun, in two rather irregular belts with a
comparatively empty lane between them. This lane lies along the sun's
equator, and it is upon either side of it between latitudes 5° and 40°
that faculæ seem to be produced. It is significant of their connection
with sun spots that the spots occur in these particular zones and are
rarely found outside them.
[Illustration: FIG. 75.--Eclipse of April 16, 1893.--SCHAEBERLE.]
123. INVISIBLE PARTS OF THE SUN. THE CORONA.--Thus far we have been
dealing with parts of the sun that may be seen and photographed under
all ordinary conditions. But outside of and surrounding these parts is
an envelope, or rather several envelopes, of much greater extent than
the visible sun. These envelopes are for the most part invisible save at
those times when the brighter central portions of the sun are hidden in
a total eclipse.
[Illustration: FIG. 76.--Eclipse of January 21, 1898.--CAMPBELL.]
Fig. 74 is from a drawing, and Figs. 75 and 76 are from eclipse
photographs showing this region, in which the most conspicuous object
is the halo of soft light called the _corona_, that completely surrounds
the sun but is seen to be of differing shapes and differing extent at
the several eclipses here shown, although a large part of these apparent
differences is due to technical difficulties in photographing, and
reproducing an object with outlines so vague as those of the corona. The
outline of the corona is so indefinite and its outer portions so faint
that it is impossible to assign to it precise dimensions, but at its
greatest extent it reaches out for several millions of miles and fills a
space more than twenty times as large as the visible part of the sun.
Despite its huge bulk, it is of most unsubstantial character, an airy
nothing through which comets have been known to force their way around
the sun from one side to the other, literally for millions of miles,
without having their course influenced or their velocity checked to any
appreciable extent. This would hardly be possible if the density even at
the bottom of the corona were greater than that of the best vacuum which
we are able to produce in laboratory experiments. It seems odd that a
vacuum should give off so bright a light as the coronal pictures show,
and the exact character of that light and the nature of the corona are
still subjects of dispute among astronomers, although it is generally
agreed that, in part at least, its light is ordinary sunlight faintly
reflected from the widely scattered molecules composing the substance of
the corona. It is also probable that in part the light has its origin in
the corona itself. A curious and at present unconfirmed result announced
by one of the observers of the eclipse of May 28, 1900, is that _the
corona is not hot_, its effective temperature being lower than that of
the instrument used for the observation.
[Illustration: FIG. 77.--Solar prominence of March 25, 1895.--HALE.]
124. THE CHROMOSPHERE.--Between the corona and the photosphere there is
a thin separating layer called the _chromosphere_ (Greek, color sphere),
because when seen at an eclipse it shines with a brilliant red light
quite unlike anything else upon the sun save the _prominences_ which are
themselves only parts of the chromosphere temporarily thrown above its
surface, as in a fountain a jet of water is thrown up from the basin and
remains for a few moments suspended in mid-air. Not infrequently in such
a fountain foreign matter is swept up by the rush of the water--dirt,
twigs, small fish, etc.--and in like manner the prominences often carry
along with them parts of the underlying layers of the sun, photosphere,
faculæ, etc., which reveal their presence in the prominence by adding
their characteristic lines to the spectrum, like that of the
chromosphere, which the prominence presents when they are absent. None
of the eclipse photographs (Figs. 74 to 76) show the chromosphere,
because the color effect is lacking in them, but a great curving
prominence may be seen near the bottom of Fig. 75, and smaller ones at
other parts of the sun's edge.
[Illustration: FIG. 78.--A solar prominence.--HALE.]
125. PROMINENCES.--Fig. 77 shows upon a larger scale one of these
prominences rising to a height of 160,000 miles above the photosphere;
and another photograph, taken 18 minutes later, but not reproduced here,
showed the same prominence grown in this brief interval to a stature of
280,000 miles. These pictures were not taken during an eclipse, but in
full sunlight, using the same spectroscopic apparatus which was employed
in connection with the faculæ to diminish the brightness of the
background without much enfeebling the brilliancy of the prominence
itself. The dark base from which the prominence seems to spring is not
the sun's edge, but a part of the apparatus used to cut off the direct
sunlight.
Fig. 78 contains a series of photographs of another prominence taken
within an interval of 1 hour 47 minutes and showing changes in size and
shape which are much more nearly typical of the ordinary prominence than
was the very unusual change in the case of Fig. 77.
[Illustration: FIG. 79.--Contrasted forms of solar
prominences.--ZOELLNER.]
The preceding pictures are from photographs, and with them the student
may compare Fig. 79, which is constructed from drawings made at the
spectroscope by the German astronomer Zoellner. The changes here shown
are most marked in the prominence at the left, which is shaped like a
broken tree trunk, and which appears to be vibrating from one side to
the other like a reed shaken in the wind. Such a prominence is
frequently called an _eruptive_ one, a name suggested by its appearance
of having been blown out from the sun by something like an explosion,
while the prominence at the right in this series of drawings, which
appears much less agitated, is called by contrast with the other a
_quiescent_ prominence. These quiescent prominences are, as a rule, much
longer-lived than the eruptive ones. One more picture of prominences
(Fig. 80) is introduced to show the continuous stretch of chromosphere
out of which they spring.
[Illustration: FIG. 80.--Prominences and chromosphere.--HALE.]
Prominences are seen only at the edge of the sun, because it is there
alone that the necessary background can be obtained, but they must occur
at the center of the sun and elsewhere quite as well as at the edge, and
it is probable that quiescent prominences are distributed over all parts
of the sun's surface, but eruptive prominences show a strong tendency
toward the regions of sun spots and faculæ as if all three were
intimately related phenomena.
126. THE SUN AS A MACHINE.--Thus far we have considered the anatomy of
the sun, dissecting it into its several parts, and our next step should
be a consideration of its physiology, the relation of the parts to each
other, and their function in carrying on the work of the solar organism,
but this step, unfortunately, must be a lame one. The science of
astronomy to-day possesses no comprehensive and well-established theory
of this kind, but looks to the future for the solution of this the
greatest pending problem of solar physics. Progress has been made
toward its solution, and among the steps of this progress that we shall
have to consider, the first and most important is the conception of the
sun as a kind of heat engine.
In a steam engine coal is burned under the boiler, and its chemical
energy, transformed into heat, is taken up by the water and delivered,
through steam as a medium, to the engine, which again transforms and
gives it out as mechanical work in the turning of shafts, the driving of
machinery, etc. Now, the function of the sun is exactly opposite to that
of the engine and boiler: it gives out, instead of receiving, radiant
energy; but, like the engine, it must be fed from some source; it can
not be run upon nothing at all any more than the engine can run day
after day without fresh supplies of fuel under its boiler. We know that
for some thousands of years the sun has been furnishing light and heat
to the earth in practically unvarying amount, and not to the earth
alone, but it has been pouring forth these forms of energy in every
direction, without apparent regard to either use or economy. Of all the
radiant energy given off by the sun, only two parts out of every
thousand million fall upon any planet of the solar system, and of this
small fraction the earth takes about one tenth for the maintenance of
its varied forms of life and action. Astronomers and physicists have
sought on every hand for an explanation of the means by which this
tremendous output of energy is maintained century after century without
sensible diminution, and have come with almost one mind to the
conclusion that the gravitative forces which reside in the sun's own
mass furnish the only adequate explanation for it, although they may be
in some small measure re-enforced by minor influences, such as the fall
of meteoric dust and stones into the sun.
Every boy who has inflated a bicycle tire with a hand pump knows that
the pump grows warm during the operation, on account of the compression
of the air within the cylinder. A part of the muscular force (energy)
expended in working the pump reappears in the heat which warms both air
and pump, and a similar process is forever going on in the sun, only in
place of muscular force we must there substitute the tremendous
attraction of gravitation, 28 times as great as upon the earth. "The
matter in the interior of the sun must be as a shuttlecock between the
stupendous pressure and the enormously high temperature," the one
tending to compress and the other to expand it, but with this important
difference between them: the temperature steadily tends to fall as the
heat energy is wasted away, while the gravitative force suffers no
corresponding diminution, and in the long run must gain the upper hand,
causing the sun to shrink and become more dense. It is this progressive
shrinking and compression of its molecules into a smaller space which
supplies the energy contained in the sun's output of light and heat.
According to Lord Kelvin, each centimeter of shrinkage in the sun's
diameter furnishes the energy required to keep up its radiation for
something more than an hour, and, on account of the sun's great
distance, the shrinkage might go on at this rate for many centuries
without producing any measurable effect in the sun's appearance.
127. GASEOUS CONSTITUTION OF THE SUN.--But Helmholtz's dynamical theory
of the maintenance of the sun's heat, which we are here considering,
includes one essential feature that is not sufficiently stated above. In
order that the explanation may hold true, it is necessary that the sun
should be in the main a gaseous body, composed from center to
circumference of gases instead of solid or liquid parts. Pumping air
warms the bicycle pump in a way that pumping water or oil will not.
The high temperature of the sun itself furnishes sufficient reason for
supposing the solar material to be in the gaseous state, but the gas
composing those parts of the sun below the photosphere must be very
different in some of its characteristics from the air or other gases
with which we are familiar at the earth, since its average density is
1,000 times as great as that of air, and its consistence and mechanical
behavior must be more like that of honey or tar than that of any gas
with which we are familiar. It is worth noting, however, that if a hole
were dug into the crust of the earth to a depth of 15 or 20 miles the
air at the bottom of the hole would be compressed by that above it to a
density comparable with that of the solar gases.
128. THE SUN'S CIRCULATION.--It is plain that under the conditions which
exist in the sun the outer portions, which can radiate their heat freely
into space, must be cooler than the inner central parts, and this
difference of temperature must set up currents of hot matter drifting
upward and outward from within the sun and counter currents of cooler
matter settling down to take its place. So, too, there must be some
level at which the free radiation into outer space chills the hot matter
sufficiently to condense its less refractory gases into clouds made up
of liquid drops, just as on a cloudy day there is a level in our own
atmosphere at which the vapor of water condenses into liquid drops which
form the thin shell of clouds that hovers above the earth's surface,
while above and below is the gaseous atmosphere. In the case of the sun
this cloud layer is always present and is that part which we have
learned to call the photosphere. Above the photosphere lies the
chromosphere, composed of gases less easily liquefied, hydrogen is the
chief one, while between photosphere and chromosphere is a thin layer of
metallic vapors, perhaps indistinguishable from the top crust of the
photosphere itself, which by absorbing the light given off from the
liquid photosphere produces the greater part of the Fraunhofer lines in
the solar spectrum.
From time to time the hot matter struggling up from below breaks through
the photosphere and, carrying with it a certain amount of the metallic
vapors, is launched into the upper and cooler regions of the sun,
where, parting with its heat, it falls back again upon the photosphere
and is absorbed into it. It is altogether probable that the corona is
chiefly composed of fine particles ejected from the sun with velocities
sufficient to carry them to a height of millions of miles, or even
sufficient to carry them off never to return. The matter of the corona
must certainly be in a state of the most lively agitation, its particles
being alternately hurled up from the photosphere and falling back again
like fireworks, the particles which make up the corona of to-day being
quite a different set from those of yesterday or last week. It seems
beyond question that the prominences and faculæ too are produced in some
way by this up-and-down circulation of the sun's matter, and that any
mechanical explanation of the sun must be worked out along these lines;
but the problem is an exceedingly difficult one, and must include and
explain many other features of the sun's activity of which only a few
can be considered here.
129. THE SUN-SPOT PERIOD.--Sun spots come and go, and at best any
particular spot is but short-lived, rarely lasting more than a month or
two, and more often its duration is a matter of only a few days. They
are not equally numerous at all times, but, like swarms of locusts, they
seem to come and abound for a season and then almost to disappear, as if
the forces which produced them were of a periodic character alternately
active and quiet. The effect of this periodic activity since 1870 is
shown in Fig. 81, where the horizontal line is a scale of times, and the
distance of the curve above this line for any year shows the relative
number of spots which appeared upon the sun in that year. This indicates
very plainly that 1870, 1883, and 1893 were years of great sun-spot
activity, while 1879 and 1889 were years in which few spots appeared.
The older records, covering a period of two centuries, show the same
fluctuations in the frequency of sun spots and from these records
curves (which may be found in Young's, The Sun) have been plotted,
showing a succession of waves extending back for many years.
[Illustration: FIG. 81.--The curve of sun-spot frequency.]
The sun-spot period is the interval of time from the crest or hollow of
one wave to the corresponding part of the next one, and on the average
this appears to be a little more than eleven years, but is subject to
considerable variation. In accordance with this period there is drawn in
broken lines at the right of Fig. 81 a predicted continuation of the
sun-spot curve for the first decade of the twentieth century. The
irregularity shown by the three preceding waves is such that we must not
expect the actual course of future sun spots to correspond very closely
to the prediction here made; but in a general way 1901 and 1911 will
probably be years of few sun spots, while they will be numerous in 1905,
but whether more or less numerous than at preceding epochs of greatest
frequency can not be foretold with any approach to certainty so long as
we remain in our present ignorance of the causes which make the sun-spot
period.
Determine from Fig. 81 as accurately as possible the length of the
sun-spot period. It is hard to tell the exact position of a crest or
hollow of the curve. Would it do to draw a horizontal line midway
between top and bottom of the curve and determine the length of the
period from its intersections with the curve--e. g., in 1874 and 1885?
[Illustration: FIG. 82.--Illustrating change of the sun-spot zones.]
130. THE SUN-SPOT ZONES.--It has been already noted that sun spots are
found only in certain zones of latitude upon the sun, and that faculæ
and eruptive prominences abound in these zones more than elsewhere,
although not strictly confined to them. We have now to note a
peculiarity of these zones which ought to furnish a clew to the sun's
mechanism, although up to the present time it has not been successfully
traced out. Just before a sun-spot minimum the few spots which appear
are for the most part clustered near the sun's equator. As these spots
die out two new groups appear, one north the other south of the sun's
equator and about 25° or 30° distant from it, and as the period advances
toward a maximum these groups shift their positions more and more toward
the equator, thus approaching each other but leaving between them a
vacant lane, which becomes steadily narrower until at the close of the
period, when the next minimum is at hand, it reaches its narrowest
dimensions, but does not altogether close up even then. In Fig. 82 these
relations are shown for the period falling between 1879 and 1890, by
means of the horizontal lines; for each year one line in the northern
and one in the southern hemisphere of the sun, their lengths being
proportional to the number of spots which appeared in the corresponding
hemisphere during the year, and their positions on the sun's disk
showing the average latitude of the spots in question. It is very
apparent from the figure that during this decade the sun's southern
hemisphere was much more active than the northern one in the production
of spots, and this appears to be generally the case, although the
difference is not usually as great as in this particular decade.
131. INFLUENCE OF THE SUN-SPOT PERIOD.--Sun spots are certainly less hot
than the surrounding parts of the sun's surface, and, in view of the
intimate dependence of the earth upon the solar radiation, it would be
in no way surprising if their presence or absence from the sun's face
should make itself felt in some degree upon the earth, raising and
lowering its temperature and quite possibly affecting it in other ways.
Ingenious men have suggested many such kinds of influence, which,
according to their investigations, appear to run in cycles of eleven
years. Abundant and scanty harvests, cyclones, tornadoes, epidemics,
rainfall, etc., are among these alleged effects, and it is possible that
there may be a real connection between any or all of them and the
sun-spot period, but for the most part astronomers are inclined to hold
that there is only one case in which the evidence is strong enough to
really establish a connection of this kind. The magnetic condition of
the earth and its disturbances, which are called magnetic storms, do
certainly follow in a very marked manner the course of sun-spot
activity, and perhaps there should be added to this the statement that
auroras (northern lights) stand in close relation to these magnetic
disturbances and are most frequent at the times of sun-spot maxima.
Upon the sun, however, the influence of the spot period is not limited
to things in and near the photosphere, but extends to the outermost
limits of the corona. Determine from Fig. 81 the particular part of the
sun-spot period corresponding to the date of each picture of the corona
and note how the pictures which were taken near times of sun-spot minima
present a general agreement in the shape and extent of the corona, while
the pictures taken at a time of maximum activity of the sun spots show a
very differently shaped and much smaller corona.
132. THE LAW OF THE SUN'S ROTATION.--We have seen in a previous part of
the chapter how the time required by the sun to make a complete rotation
upon its axis may be determined from photographs showing the progress of
a spot or group of spots across its disk, and we have now to add that
when this is done systematically by means of many spots situated in
different solar latitudes it leads to a very peculiar and extraordinary
result. Each particular parallel of latitude has its own period of
rotation different from that of its neighbors on either side, so that
there can be no such thing as a fixed geography of the sun's surface.
Every part of it is constantly taking up a new position with respect to
every other part, much as if the Gulf of Mexico should be south of the
United States this year, southeast of it next year, and at the end of a
decade should have shifted around to the opposite side of the earth from
us. A meridian of longitude drawn down the Mississippi Valley remains
always a straight line, or, rather, great circle, upon the surface of
the earth, while Fig. 83 shows what would become of such a meridian
drawn through the equatorial parts of the sun's disk. In the first
diagram it appears as a straight line running down the middle of the
sun's disk. Twenty-five days later, when the same face of the sun comes
back into view again, after making a complete revolution about the axis,
the equatorial parts will have moved so much faster and farther than
those in higher latitudes that the meridian will be warped as in the
second diagram, and still more warped after another and another
revolution, as shown in the figure.
[Illustration: FIG. 83.--Effect of the sun's peculiar rotation in
warping a meridian, originally straight.]
At least such is the case if the spots truly represent the way in which
the sun turns round. There is, however, a possibility that the spots
themselves drift with varying speeds across the face of the sun, and
that the differences which we find in their rates of motion belong to
them rather than to the photosphere. Just what happens in the regions
near the poles is hard to say, for the sun spots only extend about
halfway from the equator to the poles, and the spectroscope, which may
be made to furnish a certain amount of information bearing upon the
case, is not as yet altogether conclusive, nor are the faculæ which have
also been observed for this purpose.
The simple theory that the solar phenomena are caused by an interchange
of hotter and cooler matter between the photosphere and the lower strata
of the sun furnishes in its present shape little or no explanation of
such features as the sun-spot period, the variations in the corona, the
peculiar character of the sun's rotation, etc., and we have still
unsolved in the mechanical theory of the sun one of the noblest problems
of astronomy, and one upon which both observers and theoretical
astronomers are assiduously working at the present time. A close watch
is kept upon sun spots and prominences, the corona is observed at every
total eclipse, and numerous are the ingenious methods which are being
suggested and tried for observing it without an eclipse in ordinary
daylight. Attempts, more or less plausible, have been made and are now
pending to explain photosphere, spots and the reversing layer by means
of the refraction of light within the sun's outer envelope of gases, and
it seems altogether probable, in view of these combined activities, that
a considerable addition to our store of knowledge concerning the sun may
be expected in the not distant future.