Compare with frontispiece.]

[Illustration: _Photo: Royal Observatory, Greenwich._

FIG. 6.--SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, May 29, 1919.TAKEN AT SOBRAL, BRAZIL.

The small Corona is also visible.]

[Illustration: FIG. 7.--THE VISIBLE SURFACE OF THE SUN

A photograph taken at the Mount Wilson Observatory of the CarnegieInstitution at Washington.]

[Illustration: FIG. 8.--THE SUN

Photographed in the light of glowing hydrogen, at the Mount WilsonObservatory of the Carnegie Institution of Washington: vortex phenomenanear the spots are especially prominent.]

The fourth and uppermost layer or region is that of the corona, ofimmense extent and fading away into the surrounding sky–this we havealready referred to. The diagram (Fig. 5) shows the dispositions ofthese various layers of the sun. It is through these several transparentlayers that we see the white light body of the sun.

§ 2

The Surface of the Sun

Here let us return to and see what more we know about thephotosphere–the sun’s surface. It is from the photosphere that we havegained most of our knowledge of the composition of the sun, which isbelieved not to be a solid body. Examination of the photosphere showsthat the outer surface is never at rest. Small bright cloudlets come andgo in rapid succession, giving the surface, through contrasts inluminosity, a granular appearance. Of course, to be visible at all at92,830,000 miles the cloudlets cannot be small. They imply enormousactivity in the photosphere. If we might speak picturesquely the sun’ssurface resembles a boiling ocean of white-hot metal vapours. We haveto-day a wonderful instrument, which will be described later, whichdilutes, as it were, the general glare of the sun, and enables us toobserve these fiery eruptions at any hour. The “oceans” of red-hot gasand white-hot metal vapour at the sun’s surface are constantly driven bygreat storms. Some unimaginable energy streams out from the body ormuscles of the sun and blows its outer layers into gigantic shreds, asit were.

The actual temperature at the sun’s surface, or what appears to us to bethe surface–the photosphere–is, of course, unknown, but carefulcalculation suggests that it is from 5,000° C. to 7,000° C. The interioris vastly hotter. We can form no conception of such temperatures as mustexist there. Not even the most obdurate solid could resist suchtemperatures, but would be converted almost instantaneously into gas.But it would not be gas as we know gases on the earth. The enormouspressures that exist on the sun must convert even gases into thicktreacly fluids. We can only infer this state of matter. It is beyond ourpower to reproduce it.

Sun-spots

It is in the brilliant photosphere that the dark areas known assun-spots appear. Some of these dark spots–they are dark only bycontrast with the photosphere surrounding them–are of enormous size,covering many thousands of square miles of surface. What they are wecannot positively say. They look like great cavities in the sun’ssurface. Some think they are giant whirlpools. Certainly they seem to begreat whirling streams of glowing gases with vapours above them andimmense upward and downward currents within them. Round the edges of thesun-spots rise great tongues of flame.

Perhaps the most popularly known fact about sun-spots is that they aresomehow connected with what we call magnetic storms on earth. Thesemagnetic storms manifest themselves in interruptions of our telegraphicand telephonic communications, in violent disturbances of the mariner’scompass, and in exceptional auroral displays. The connection between thetwo sets of phenomena cannot be doubted, even although at times theremay be a great spot on the sun without any corresponding “magneticstorm” effects on the earth.

A surprising fact about sun-spots is that they show definite periodicvariations in number. The best-defined period is one of about elevenyears. During this period the spots increase to a maximum in number andthen diminish to a minimum, the variation being more or less regular.Now this can only mean one thing. To be periodic the spots must havesome deep-seated connection with the fundamental facts of the sun’sstructure and activities. Looked at from this point of view theirimportance becomes great.

[Illustration: _Reproduction from "The Forces of Nature"_ (_Messrs.Macmillan_)

THE AURORA BOREALIS

The aurora borealis is one of the most beautiful spectacles in the sky.The colours and shape change every instant; sometimes a fan-like clusterof rays, at other times long golden draperies gliding one over theother. Blue, green, yellow, red, and white combine to give a gloriousdisplay of colour. The theory of its origin is still, in part, obscure,but there can be no doubt that the aurora is related to the magneticphenomena of the earth and therefore is connected with the electricalinfluence of the sun.]

It is from the study of sun-spots that we have learned that the sun’ssurface does not appear to rotate all at the same speed. The”equatorial” regions are rotating quicker than regions farther north orsouth. A point forty-five degrees from the equator seems to take abouttwo and a half days longer to complete one rotation than a point on theequator. This, of course, confirms our belief that the sun cannot be asolid body.

What is its composition? We know that there are present, in a gaseousstate, such well-known elements as sodium, iron, copper, zinc, andmagnesium; indeed, we know that there is practically every element inthe sun that we know to be in the earth. How do we know?

It is from the photosphere, as has been said, that we have won most ofour knowledge of the sun. The instrument used for this purpose is thespectroscope; and before proceeding to deal further with the sun and thesource of its energy it will be better to describe this instrument.

A WONDERFUL INSTRUMENT AND WHAT IT REVEALS

The spectroscope is an instrument for analysing light. So important isit in the revelations it has given us that it will be best to describeit fully. Every substance to be examined must first be made to glow,made luminous; and as nearly everything in the heavens _is_ luminous theinstrument has a great range in Astronomy. And when we speak ofanalysing light, we mean that the light may be broken up into waves ofdifferent lengths. What we call light is a series of minute waves inether, and these waves are–measuring them from crest to crest, so tosay–of various lengths. Each wave-length corresponds to a colour of therainbow. The shortest waves give us a sensation of violet colour, andthe largest waves cause a sensation of red. The rainbow, in fact, is asort of natural spectrum. (The meaning of the rainbow is that themoisture-laden air has sorted out these waves, in the sun’s light,according to their length.) Now the simplest form of spectroscope is aglass prism–a triangular-shaped piece of glass. If white light(sunlight, for example) passes through a glass prism, we see a series ofrainbow-tinted colours. Anyone can notice this effect when sunlight isshining through any kind of cut glass–the stopper of a wine decanter,for instance. If, instead of catching with the eye the coloured lightsas they emerge from the glass prism, we allow them to fall on a screen,we shall find that they pass, by continuous gradations, from red at theone end of the screen, through orange, yellow, green, blue, and indigo,to violet at the other end. _In other words, what we call white light iscomposed of rays of these several colours. They go to make up the effectwhich we call white._ And now just as water can be split up into its twoelements, oxygen and hydrogen, so sunlight can be broken up into itsprimary colours, which are those we have just mentioned.

This range of colours, produced by the spectroscope, we call the solarspectrum, and these are, from the spectroscopic point of view, primarycolours. Each shade of colour has its definite position in the spectrum.That is to say, the light of each shade of colour (corresponding to itswave-length) is reflected through a certain fixed angle on passingthrough the glass prism. Every possible kind of light has its definiteposition, and is denoted by a number which gives the wave-length of thevibrations constituting that particular kind of light.

Now, other kinds of light besides sunlight can be analysed. Lightfrom any substance which has been made incandescent may be observed withthe spectroscope in the same way, and each element can be thusseparated. It is found that each substance (in the same conditions ofpressure, etc.) gives a constant spectrum of its own. _Each metaldisplays its own distinctive colour. It is obvious, therefore, that thespectrum provides the means for identifying a particular substance._ Itwas by this method that we discovered in the sun the presence of suchwell-known elements as sodium, iron, copper, zinc, and magnesium.

[Illustration: _Yerkes Observatory._

FIG. 9.--THE GREAT SUN-SPOT OF JULY 17, 1905]

[Illustration: _From photographs taken at the Yerkes Observatory._

FIG. 10.--SOLAR PROMINENCES

These are about 60,000 miles in height. The two photographs show thevast changes occurring in ten minutes. October 10, 1910.]

[Illustration: _Photo: Mount Wilson Observatory._

FIG. 11.--MARS, October 5, 1909

Showing the dark markings and the Polar Cap.]

[Illustration: FIG. 12.--JUPITER

Showing the belts which are probably cloud formations.]

[Illustration: _Photo: Professor E. E. Barnard, Yerkes Observatory._

FIG. 13.--SATURN, November 19, 1911

Showing the rings, mighty swarms of meteorites.]

Every chemical element known, then, has a distinctive spectrum of itsown when it is raised to incandescence, and this distinctive spectrum isas reliable a means of identification for the element as a human face isfor its owner. Whether it is a substance glowing in the laboratory or ina remote star makes no difference to the spectroscope; if the light ofany substance reaches it, that substance will be recognised andidentified by the characteristic set of waves.

The spectrum of a glowing mass of gas will consist in a number of brightlines of various colours, and at various intervals; corresponding toeach kind of gas, there will be a peculiar and distinctive arrangementof bright lines. But if the light from such a mass of glowing gas bemade to pass through a cool mass of the _same_ gas it will be found thatdark lines replace the bright lines in the spectrum, the reason for thisbeing that the cool gas absorbs the rays of light emitted by the hotgas. Experiments of this kind enable us to reach the important generalstatement that every gas, when cold, absorbs the same rays of lightwhich it emits when hot.

Crossing the solar spectrum are hundreds and hundreds of dark lines.These could not at first be explained, because this fact ofdiscriminative absorption was not known. We understand now. The sun’swhite light comes from the photosphere, but between us and thephotosphere there is, as we have seen, another solar envelope ofrelatively cooler vapours–the reversing layer. Each constituentelement in this outer envelope stops its own kind of light, that is, thekind of light made by incandescent atoms of the same element in thephotosphere. The “stoppages” register themselves in the solar spectrumas dark lines placed exactly where the corresponding bright lines wouldhave been. The explanation once attained, dark lines became assignificant as bright lines. The secret of the sun’s composition wasout. We have found practically every element in the sun that we know tobe in the earth. We have identified an element in the sun before we wereable to isolate it on the earth. We have been able even to point to thecoolest places on the sun, the centres of sun-spots, where alone thetemperature seems to have fallen sufficiently low to allow chemicalcompounds to form.

It is thus we have been able to determine what the stars, comets, ornebulæ are made of.

A Unique Discovery

In 1868 Sir Norman Lockyer detected a light coming from the prominencesof the sun which was not given by any substance known on earth, andattributed this to an unknown gas which he called helium, from the Greek_helios_, the sun. _In 1895 Sir William Ramsay discovered in certainminerals the same gas identified by the spectroscope._ We can say,therefore, that this gas was discovered in the sun nearly thirty yearsbefore it was found on earth; this discovery of the long-lost heir is asthrilling a chapter in the detective story of science as any in thesensational stories of the day, and makes us feel quite certain that ourmethods really tell us of what elements sun and stars are built up. Thelight from the corona of the sun, as we have mentioned indicates a gasstill unknown on earth, which has been christened Coronium.

Measuring the Speed of Light

But this is not all; soon a new use was found for the spectroscope. Wefound that we could measure with it the most difficult of all speedsto measure, speed in the line of sight. Movement at right angles to thedirection in which one is looking is, if there is sufficient of it, easyto detect, and, if the distance of the moving body is known, easy tomeasure. But movement in the line of vision is both difficult to detectand difficult to measure. Yet, even at the enormous distances with whichastronomers have to deal, the spectroscope can detect such movement andfurnish data for its measurement. If a luminous body containing, say,sodium is moving rapidly towards the spectroscope, it will be found thatthe sodium lines in the spectrum have moved slightly from their usualdefinite positions towards the violet end of the spectrum, the amount ofthe change of position increasing with the speed of the luminous body.If the body is moving away from the spectroscope the shifting of thespectral lines will be in the opposite direction, towards the red end ofthe spectrum. In this way we have discovered and measured movements thatotherwise would probably not have revealed themselves unmistakably to usfor thousands of years. In the same way we have watched, and measuredthe speed of, tremendous movements on the sun, and so gained proof thatthe vast disturbances we should expect there actually do occur.

[Illustration: THE SPECTROSCOPE IS AN INSTRUMENT FOR ANALYSING LIGHT; ITPROVIDES THE MEANS FOR IDENTIFYING DIFFERENT SUBSTANCES

This pictorial diagram illustrates the principal of Spectrum Analysis,showing how sunlight is decomposed into its primary colours. What wecall white light is composed of seven different colours. The diagram isrelieved of all detail which would unduly obscure the simple process bywhich a ray of light is broken up by a prism into differentwave-lengths. The spectrum rays have been greatly magnified.]

IS THE SUN DYING?

§ 3

Now let us return to our consideration of the sun.

To us on the earth the most patent and most astonishing fact about thesun is its tremendous energy. Heat and light in amazing quantities pourfrom it without ceasing.

Where does this energy come from? Enormous jets of red glowing gases canbe seen shooting outwards from the sun, like flames from a fire, forthousands of miles. Does this argue fire, as we know fire on the earth?On this point the scientist is sure. The sun is not burning, andcombustion is not the source of its heat. Combustion is a chemicalreaction between atoms. The conditions that make it possible are knownand the results are predictable and measurable. But no chemical reactionof the nature of combustion as we know it will explain the sun’s energy,nor indeed will any ordinary chemical reaction of any kind. If the sunwere composed of combustible material throughout and the conditions ofcombustion as we understand them were always present, the sun would burnitself out in some thousands of years, with marked changes in its heatand light production as the process advanced. There is no evidence ofsuch changes. There is, instead, strong evidence that the sun has beenemitting light and heat in prodigious quantities, not for thousands, butfor millions of years. Every addition to our knowledge that throws lighton the sun’s age seems to make for increase rather than decrease of itsyears. This makes the wonder of its energy greater.

And we cannot avoid the issue of the source of the energy by sayingmerely that the sun is gradually radiating away an energy thatoriginated in some unknown manner, away back at the beginning of things.Reliable calculations show that the years required for the mere coolingof a globe like the sun could not possibly run to millions. In otherwords, the sun’s energy must be subject to continuous and more or lesssteady renewal. However it may have acquired its enormous energy in thepast, it must have some source of energy in the present.

The best explanation that we have to-day of this continuous accretion ofenergy is that it is due to shrinkage of the sun’s bulk under the forceof gravity. Gravity is one of the most mysterious forces of nature, butit is an obvious fact that bodies behave as if they attracted oneanother, and Newton worked out the law of this attraction. We may say,without trying to go too deeply into things, that every particle ofmatter attracts every other throughout the universe. If the diameter ofthe sun were to shrink by one mile all round, this would mean that allthe millions of tons in the outer one-mile thickness would have astraight drop of one mile towards the centre. And that is not all,because obviously the layers below this outer mile would also dropinwards, each to a less degree than the one above it. What a tremendousmovement of matter, however slowly it might take place! And what atremendous energy would be involved! Astronomers calculate that theabove shrinkage of one mile all round would require fifty years for itscompletion, assuming, reasonably, that there is close and continuousrelationship between loss of heat by radiation and shrinkage. Even ifthis were true we need not feel over-anxious on this theory; before thesun became too cold to support life many millions of years would berequired.

It was suggested at one time that falls of meteoric matter into the sunwould account for the sun’s heat. This position is hardly tenable now.The mere bulk of the meteoric matter required by the hypothesis, apartfrom other reasons, is against it. There is undoubtedly an enormousamount of meteoric matter moving about within the bounds of the solarsystem, but most of it seems to be following definite routes round thesun like the planets. The stray erratic quantities destined to meettheir doom by collision with the sun can hardly be sufficient to accountfor the sun’s heat.

Recent study of radio-active bodies has suggested another factor thatmay be working powerfully along with the force of gravitation tomaintain the sun’s store of heat. In radio-active bodies certain atomsseem to be undergoing disintegration. These atoms appear to be splittingup into very minute and primitive constituents. But since matter may besplit up into such constituents, may it not be built up from them?

The question is whether these “radio-active” elements are undergoingdisintegration, or formation, in the sun. If they are undergoingdisintegration–and the sun itself is undoubtedly radio-active–then wehave another source of heat for the sun that will last indefinitely.

THE PLANETS

LIFE IN OTHER WORLDS?

§ 1

It is quite clear that there cannot be life on the stars. Nothing solidor even liquid can exist in such furnaces as they are. Life exists onlyon planets, and even on these its possibilities are limited. Whether allthe stars, or how many of them, have planetary families like our sun, wecannot positively say. If they have, such planets would be too faint andsmall to be visible tens of trillions of miles away. Some astronomersthink that our sun may be exceptional in having planets, but theirreasons are speculative and unconvincing. Probably a large proportion atleast of the stars have planets, and we may therefore survey the globesof our own solar system and in a general way extend the results to therest of the universe.

In considering the possibility of life as we know it we may at once ruleout the most distant planets from the sun, Uranus and Neptune. They areprobably intrinsically too hot. We may also pass over the nearest planetto the sun, Mercury. We have reason to believe that it turns on its axisin the same period as it revolves round the sun, and it must thereforealways present the same side to the sun. This means that the heat on thesunlit side of Mercury is above boiling-point, while the cold on theother side must be between two and three hundred degrees belowfreezing-point.

The Planet Venus

The planet Venus, the bright globe which is known to all as the morningand evening “star,” seems at first sight more promising as regards thepossibility of life. It is of nearly the same size as the earth, and ithas a good atmosphere, but there are many astronomers who believe that,like Mercury, it always presents the same face to the sun, and it wouldtherefore have the same disadvantage–a broiling heat on the sunny sideand the cold of space on the opposite side. We are not sure. Thesurface of Venus is so bright–the light of the sun is reflected to usby such dense masses of cloud and dust–that it is difficult to traceany permanent markings on it, and thus ascertain how long it takes torotate on its axis. Many astronomers believe that they have succeeded,and that the planet always turns the same face to the sun. If it does,we can hardly conceive of life on its surface, in spite of thecloud-screen.

[Illustration: FIG. 14.--THE MOON

Showing a great plain and some typical craters. There are thousands ofthese craters, and some theories of their origin are explained on page34.]

[Illustration: FIG. 15.--MARS

1} Drawings by Prof. Lowell to accompany actual photographs of Mars showing many of the 2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory. 3 Drawing by Prof. Lowell made January 6, 1914. 4 Drawing by Prof. Lowell made January 21, 1914.

Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4depict quite different sections. Note the change in the polar snow-capsin the last two.]

[Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYS

Note the mysterious "rays" diverging from the almost perfectly circularcraters indicated by the arrows (Tycho, upper; Copernicus, lower), andalso the mountains to the right with the lunar dawn breaking on them.]

We turn to Mars; and we must first make it clear why there is so muchspeculation about life on Mars, and why it is supposed that, if there_is_ life on Mars, it must be more advanced than life on the earth.

Is there Life on Mars?

The basis of this belief is that if, as we saw, all the globes in oursolar system are masses of metal that are cooling down, the smaller willhave cooled down before the larger, and will be further ahead in theirdevelopment. Now Mars is very much smaller than the earth, and must havecooled at its surface millions of years before the earth did. Hence, ifa story of life began on Mars at all, it began long before the story oflife on the earth. We cannot guess what sort of life-forms would beevolved in a different world, but we can confidently say that they wouldtend toward increasing intelligence; and thus we are disposed to lookfor highly intelligent beings on Mars.

But this argument supposes that the conditions of life, namely air andwater, are found on Mars, and it is disputed whether they are foundthere in sufficient quantity. The late Professor Percival Lowell, whomade a lifelong study of Mars, maintained that there are hundreds ofstraight lines drawn across the surface of the planet, and he claimedthat they are beds of vegetation marking the sites of great channels orpipes by means of which the “Martians” draw water from their polarocean. Professor W. H. Pickering, another high authority, thinks thatthe lines are long, narrow marshes fed by moist winds from the poles.There are certainly white polar caps on Mars. They seem to melt in thespring, and the dark fringe round them grows broader.

Other astronomers, however, say that they find no trace of water-vapourin the atmosphere of Mars, and they think that the polar caps may besimply thin sheets of hoar-frost or frozen gas. They point out that, asthe atmosphere of Mars is certainly scanty, and the distance from thesun is so great, it may be too cold for the fluid water to exist on theplanet.

If one asks why our wonderful instruments cannot settle these points,one must be reminded that Mars is never nearer than 34,000,000 milesfrom the earth, and only approaches to this distance once in fifteen orseventeen years. The image of Mars on the photographic negative taken ina big telescope is very small. Astronomers rely to a great extent on theeye, which is more sensitive than the photographic plate. But it is easyto have differences of opinion as to what the eye sees, and so there isa good deal of controversy.

In August, 1924, the planet will again be well placed for observation,and we may learn more about it. Already a few of the much-disputedlines, which people wrongly call “canals,” have been traced onphotographs. Astronomers who are sceptical about life on Mars are oftennot fully aware of the extraordinary adaptability of life. There was atime when the climate of the whole earth, from pole to pole, wassemi-tropical for millions of years. No animal could then endure the

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