Two total eclipses happen on the average of every three years; their average duration is only two or three minutes; totality can be seen only in a narrow path about a hundred miles wide, though it may be several thousand miles long; there is usually about equal chance of cloud with clear skies; and fully three-fourths of the totality areas of the globe are unavailable because covered by water. So that even if we imagine the tracks of eclipses quite thickly populated with astronomers and telescopes, at least one every hundred miles, how much solid watching of the corona would this permit? Only a little more than one week"s time in a whole century.
The true corona is at least a triple phenomenon and a very complex one.
The photographs reveal it much as the eye sees it, with all its complexity of interlacing streamers projected into a flat, or plane, surrounding the disk of the dark moon which hides the true sun completely. But we must keep in mind the fact that the sun is a globe, not a disk, and that the streamers of the corona radiate more or less from all parts of the surface of the solar sphere, much as quills from a porcupine.
From the sun"s magnetic poles branch out the polar rays, nearly straight throughout their visible extent. Gradually as the coronal rays originate at points around the solar disk farther and farther removed from the poles, they are more and more curved. Very probably they extend into the equatorial regions, but it is not easy to trace them there because they are projected upon and confused with the filaments having their origin remote from the poles. Then there is the inner equatorial corona, apparently connected intimately with truly solar phenomena, quite as the polar rays are. The third element in the composite is the outer ecliptic corona, for the most part made up of long streamers. This is most fully developed at the time of the fewest spots on the sun. It is traceable much farther against the black sky with the naked eye than by photography. Without any doubt it is a solar appendage and possibly it may merge into the zodiacal light.
Naturally this superb spectacle must have been an amazing sight to the beholders of antiquity who were fortunate enough to see it. Historical references are rare: perhaps the earliest was by Plutarch about A. D.
100, who wrote of it, "A radiance shone round the rim, and would not suffer darkness to become deep and intense." Philostratus a century later mentions the death of the emperor Domitian at Ephesus as "announced" by a total eclipse.
Kepler thought the corona was evidence of a lunar atmosphere; indeed, it was not until the middle of the 19th century that its lack of relation to the moon was finally demonstrated. Later observers, Wyberd in 1652 and Ulloa, got the impression that the corona turned round the disk catherine-wheel fashion, "like an ignited wheel in fireworks, turning on its center." But no later observer has reported anything of the sort.
Quite the contrary, there it stands against the black sky in motionless magnificence a colorless pearly ma.s.s of wisps and streamers for the most part nebulous and ill-defined, fading out very irregularly into the black sky beyond, but with a complex interlacing of filaments, sometimes very sharply defined near the solar poles. It defies the skill of artist and draughtsman to sketch it before it is gone.
Photograph it? Yes, but there are troubles. Of course the camera work is superior to sketches by hand. As Langley used to say, "The camera has no nerves, and what it sets down we may rely on." Foremost among the photographic difficulties is the wide variation in intensity of the coronal light in different regions of the corona. If a plate is exposed long enough to get the outer corona, the exceeding brightness of the inner corona overexposes and burns out that part of the plate or film.
If the exposure is short, we get certain regions of the inner corona excellently, but the outer regions are a blank because they can be caught only by a long exposure.
So the only way is to take a series of pictures with a wide range of exposures, and then by careful and artistic handwork, combine them all into a single drawing. Wesley of London has succeeded eminently in work of this character, and his drawings of the sun"s corona, visible at total eclipses from 1871 onward, in possession of the Royal Astronomical Society, are the finest in existence. They give a vastly better idea of the corona, as the eye sees it, than any single photograph possibly can.
The early observers apparently never thought of the corona as being connected with the sun. It was a halo merely, and so drawn. Its real structure was neither known, depicted, or investigated. Sketches were structureless, as any aureola formed by stray sunlight grazing the moon might naturally be. That the rays are curved and far from radial round the sun was shown for the first time in the sketches of 1842, and in 1860 Sir Francis Galton observed that the long arms or streamers "do not radiate strictly from the center."
The inner corona had first been recorded photographically on a daguerreotype plate during the eclipse of 1851, but the lens belonged to a heliometer, and was of course uncorrected for the photographic rays.
The wet collodion plates of the eclipse of 1860, by De la Rue, proved that not only the prominences but the corona were truly solar, because his series of technically perfect pictures revealed the steady and unchanged character of these phenomena while the moon"s disk was pa.s.sing over them as totality progressed. And at the eclipse of 1869, Young put the solar theory of the corona beyond the shadow of any further doubt by examination of its light with the spectroscope and discovering a green line in the spectrum due to incandescent vapor of a substance not then identified with anything terrestrial, and therefore called coronium.
The total brilliance of the corona was very differently estimated by the earlier observers, though pretty carefully measured at later eclipses.
The standard full moon is used for reference, and at one eclipse the corona falls short of, while at another it will exceed the full moon in brightness. Variations in brilliancy are quite marked: at one eclipse it was nearly four times as bright as the full moon. Much evidence has already acc.u.mulated on this question; but whether the observed variations are real, or due mainly to the varying relative sizes of sun and moon at different eclipses, is not yet known. The coronal light is largely bluish in tint, and this is the region of the spectrum most powerfully absorbed by our atmosphere. Eclipses are observed by different expeditions located at stations where the eclipsed sun stands at very different alt.i.tudes above the horizon; besides this the localities of observation are at varied elevations above sea level; so that the varying amount of absorption of the coronal light renders the problem one of much difficulty.
The long ecliptic streamers of the corona were first seen by Newcomb and Langley during the totality of 1878. On one side of the sun there was a stupendous extension of at least twelve solar diameters, or nearly 11 millions of miles. Langley observed from the summit of Pike"s Peak, over 14,000 feet high, and was sure that he was witnessing a "real phenomenon heretofore undescribed." The vast advantage of elevation was apparent also from the fact that he held the corona for more than four minutes after true totality had ended. These streamers are characteristic of the epoch of minimum spots on the sun, as Ranyard first suggested. It was found that this type of corona had been recorded also in 1867; and it has reappeared in 1889, 1900 and 1911, and will doubtless be visible again in 1922.
How rapidly the streamers of the corona vary is not known. Occasionally an observer reports having seen the filaments vibrate rapidly as in the aurora borealis, but this is not verified by others who saw the same corona perfectly unmoving. Comparisons of photographs taken at widely separate stations during the same eclipse have shown that at least the corona remained stationary for hours at a time. Whether it may be unchanged at the end of a day, or a week, or a month, is not known; because no two total eclipses can ever happen nearer each other than within an interval of 173 days, or one-half of the eclipse year. And usually the interval between total eclipses is twice or three times this period.
Theories of what the solar corona may be are very numerous. The extreme inner corona is perhaps in part a sort of gaseous atmosphere of the sun, due to matter ejected from the sun, and kept in motion by forces of ejection, gravity, and repulsion of some sort. Meteoric matter is likely concerned in it, and Huggins suggested the debris of disintegrating comets. Schuster was in agreement with Huggins that the brighter filaments of the corona might be due to electric discharges, but it seems very unlikely that any single hypothesis can completely account for the intricate tracery of so complex a phenomenon.
[Ill.u.s.tration: SOLAR CORONA AND PROMINENCES. Photographed during a total eclipse of the sun, June 8, 1918. (_Courtesy, American Museum of Natural History._)]
[Ill.u.s.tration: VENUS, SHOWING CRESCENT PHASE OF THE PLANET. Venus is the earth"s nearest neighbor on the side toward the sun.
(_Photo, Yerkes Observatory._)]
[Ill.u.s.tration: MARS, THE PLANET NEXT BEYOND THE EARTH. The photograph shows one of the white polar caps. The caps are thought to be snow or ice and may indicate the existence of atmosphere. (_Photo, Yerkes Observatory._)]
Elaborate spectroscopic programs have been carried out at recent eclipses, affording evidence that certain regions are due to incandescent matter of lower temperature than the sun"s surface. A small part of the light of the corona is sunlight reflected from dark particles possibly meteoric, but more likely dust particles or fog of some sort. This accounts for the weakened solar spectrum with Fraunhofer absorption lines, and this part of the light is polarized.
Many have been the attempts to see, or photograph, the corona without an eclipse. None of them has, however, succeeded as yet. Huggins got very promising results nearly forty years ago, and success was thought to have been reached; but subsequent experiments on the Riffelberg in 1884 and later convinced him that his results related only to a spurious corona. In 1887 the writer made an unsuccessful attempt to visualize the corona from the summit of Fujiyama, and Hale tried both optical and photographic methods on Pike"s Peak in 1893 without success. He devised later a promising method by which the heat of the corona in different regions can be measured by the bolometer, and an outline corona afterward sketched from these results.
Still another method of attacking the problem occurred to the writer in 1919, which has not yet been carried out. It would take advantage of recent advances in aeronautics, and contemplates an artificial eclipse in the upper air by means of a black spherical balloon. This would be sent up to an alt.i.tude of perhaps 40,000 feet, where it would partake of the motion of the air current in which it came to equilibrium. Then a snapshot camera would be mounted on an aeroplane, in which the aviator would ascend to such a height that the balloon just covered the sun, as the moon does in a total eclipse. With the center of the balloon in line with the sun"s center, he would photograph the regions of the sky immediately surrounding the sun, against which the corona is projected.
As the entire apparatus would be above more than an entire half of the earth"s atmosphere, the experiment would be well worth the attempt, as pretty much everything else has been tried and found wanting. Needless to say, the importance of seeing the corona at regular intervals whenever desired, without waiting for eclipses of the sun, remains as insistent as ever.
CHAPTER x.x.xII
THE RUDDY PLANET
Mars is a planet next in order beyond the earth, and its distance from the sun averages 141-1/2 million miles. It has a relatively rapid motion among the stars, its color is reddish, and, when nearest to us, it is perhaps the most conspicuous object in the sky.
Mars appeared to the ancients just as it does to us to-day. Aristotle recorded an observation of Mars, 356 B. C., when the moon pa.s.sed over the planet, or occulted it, as our expression is. Galileo made the first observations of Mars with a telescope in 1610, and his little instrument was powerful enough to enable him to discover that the planet had phases, though it did not pa.s.s through all the phases that Mercury and Venus do. This was obvious from the fact that Mars is always at a greater distance from the sun than we are, and the phase can only be gibbous, or about like the moon when midway between full and quarter.
Many observers in the seventeenth century followed up the planet with such feeble optical power as the telescopes of that epoch provided: Fontana (who made the first sketch), Riccioli and Bianchini in Italy, Ca.s.sini in France, Huygens in Holland, and later Sir William Herschel in England.
It was Ca.s.sini who first made out the whitish spots or polar caps of Mars in 1666, but not until after Huygens had noted the fact that Mars turned round on an axis in a period but little longer than the earth"s.
Ca.s.sini followed it up later with a more accurate value; and observations in our own day, when combined with these early ones, enable us to say that the Martian day is equal to 24 hours 37 minutes 22.67 seconds, accurate probably to the hundredth part of a second.
When we know that a planet turns round on an axis, we know that it has a day. When we know the direction of the axis in s.p.a.ce or in relation to the plane of its path round the sun, we know that it has seasons: we can tell their length and when they begin and end. It did not take many years of observation to prove that the axis round which Mars turns is tilted to the plane of its path round the sun by an angle practically the same as that at which the earth"s axis is tilted. So there is the immediate inference that on Mars the order and perhaps the character of the seasons is much the same as here on the earth.
At least two things, however, tend to modify them. First, the year of Mars is not 365 days like ours, but 687 days. Each of the four seasons on Mars, therefore, is proportionally longer than our seasons are. Then comes the question of atmosphere--how much of an atmosphere does Mars really possess in proportion to ours, and how would its lesser amount modify the blending of the seasons into one another?
All discussion of Mars and the problems of existence of life upon that planet hinge upon the character and extent of Martian atmosphere. The planet seems never to be covered, as the earth usually is, with extensive areas of cloud which to an observer in s.p.a.ce would completely mask its oceans and continents. Nearly all the time Mars in his equatorial and temperate zones is quite clear of clouds. A few whitish spots are occasionally seen to change their form and position in both northern and southern lat.i.tudes, and they vary with the progress of the day on Mars, as clouds naturally would. But Schiaparelli, perhaps the best of all observers, thought them to be not low-lying clouds of the nimbus type that would produce rains, but rather a veil of fog, or perhaps a temporary condensation of vapor, as dew or h.o.a.r frost. But the strongest argument for an atmosphere is based on the temporary darkening or obscuration of well known and permanent markings on the surface of Mars. These are more or less frequently observed and clouds afford the best explanation of their occurrence.
So much for evidence supplied by the telescope alone. When, however, we employ the spectroscope in conjunction with the telescope, another sort of evidence is at hand. Several astronomers have reached the conclusion that watery vapor exists in the atmosphere of Mars, while other astronomers equipped with equal or superior apparatus, and under equally favorable or even better conditions, have reached the remarkable conclusion that the spectra of Mars and the moon are identical in every particular. From this we should be led to infer that Mars has perhaps no more atmosphere than the moon has, that is to say, none whatever that present instruments and methods of investigation have enabled us to detect.
What then, shall we conclude? Simply that the atmosphere of Mars is neither very dense nor extensive. Probably its lower strata close to the planet"s surface are about as dense as the earth"s atmosphere is at the summits of our highest mountains.
This conclusion is not unwelcome, if we keep a few fundamental facts in clear and constant view. Mars is a planet of intermediate size between the earth and the moon: twice the moon"s diameter (2,160 miles) very nearly equals the diameter of Mars (4,200 miles), and twice the diameter of Mars does not greatly exceed the earth"s diameter (7,920 miles). As to the weights or ma.s.ses of these bodies, Mars is about one-ninth, and the moon one-eightieth of the earth. The atmospheric envelope of the earth is abundant, the moon has none as far as we can ascertain; so it seems safe to infer that Mars has an atmosphere of slight density: not dense enough to be detected by spectroscopic methods, but yet dense enough to enable us to explain the varying telescopic phenomena of the planet"s disk which we should not know how to account for, if there were no atmosphere whatever. One astronomer has, indeed, gone so far as to calculate that in comparison with our planet Mars is ent.i.tled to one-twentieth as much atmosphere as we have, and that the mercurial barometer at "sea level" would run about five and a half inches, as against thirty inches on the earth.
In general, then, the climate of Mars is probably very much like that of a clear season on a very high terrestrial table land or mountain--a climate of wide extremes, with great changes of temperature from day to night. The inequality of Martian seasons is such that in his northern hemisphere the winter lasts 381 days and the summer only 306 days.
Now, the polar caps of Mars, which are reasonably a.s.sumed to be due to snow or h.o.a.r frost, attain their maximum three or four months after the winter solstice, and their minimum about the same length of time after the summer solstice. This lagging should be interpreted as an argument for a Martian atmosphere with heat-storing qualities, similar to that possessed by the earth.
Upon this characteristic, indeed, depends the climate at the surface of Mars: whether it is at all similar to our own, and whether fluid water is a possibility on Mars or not. While the cosmic relations of the planet in its...o...b..t are quite the same as ours, nevertheless the greater distance of Mars diminishes his supply of direct solar heat to about half what we receive. On the other hand, his distance from the sun during his year of motion around it varies much more widely than ours, so that he receives when nearest the sun about one-half more of solar heat than he does when farthest away.
Southern summers on Mars, therefore, must be much hotter, and southern winters colder than the corresponding seasons of his northern hemisphere. Indeed, the length of the southern summer, nearly twice that of the terrestrial season, sometimes amply suffices to melt all the polar ice and snow, as in October, 1894, when the southern polar cap of Mars dwindled rapidly and finally vanished completely.
Very interesting in this connection are the researches of Stoney on the general conditions affecting planetary atmospheres and their composition. According to the kinetic theory, if the molecules of gases which are continually in motion travel outward from the center of a planet, as they frequently must, and with velocities surpa.s.sing the limit that a planet"s gravity is capable of controlling, these molecules will effect a permanent escape from the planet, and travel through s.p.a.ce in orbits of their own.
So the moon is wholly without atmosphere because the moon"s gravity is not powerful enough to retain the molecules of its component gases. So also the earth"s atmosphere contains no helium or free hydrogen. So, too, Mars is possessed of insufficient force of gravity to retain water vapor, and the Martian atmosphere may therefore consist mainly of nitrogen, argon, and carbon dioxide.
As everyone knows, the axis of the earth if extended to the northern heavens would pa.s.s very near the north polar star, which on that account is known as Polaris. In a similar manner the axis of Mars pierces the northern heavens about midway between the two bright stars Alpha Cephei and Alpha Cygni (Deneb). The direction of this axis is pretty accurately known, because the measurement of the polar caps of the planet as they turn round from night to night, year in and year out, has enabled astronomers to a.s.sign the inclination of the axis with great precision.
These caps are a brilliant white, and they are generally supposed to be snow and ice. They wax and wane alternately with the seasons on Mars, being largest at the end of the Martian winter and smallest near the end of summer. The existence of the polar caps together with their seasonal fluctuations afford a most convincing argument for the reality of a Martian atmosphere, sufficiently dense to be capable of diffusing and transporting vapor.
The northern cap is centered on the pole almost with geometric exactness, and as far as the 85th parallel of lat.i.tude. On the other hand, the south polar cap is centered about 200 miles from the true pole, and this distance has been observed to vary from one season to another. No suggestion has been made to account for this singular variation. On one occasion it stretched down to Martian lat.i.tude 70 degrees and was over 1,200 miles in diameter.
Pickering watched the changing conditions of shrinking of the south polar cap in 1892 with a large telescope located in the Andes of Peru.
Mars was faithfully followed on every night but one from July 13 to September 9, and the apparent alterations in this cap were very marked, even from night to night. As the snows began to decrease, a long dark line made its appearance near the middle of the cap, and gradually grew until it cut the cap in two. This white polar area (and probably also the northern one in similar fashion) becomes notched on the edge with the progress of its summer season; dark interior spots and fissures form, isolated patches separate from the princ.i.p.al ma.s.s, and later seem to dissolve and disappear. Possibly if one were located on Mars and viewing our earth with a big telescope, the seasonal variation of our north and south polar caps might present somewhat similar phenomena. All the recent oppositions of Mars have been critically observed by Pickering from an excellent station in Jamaica.
Quite obviously the fluctuations of the polar caps are the key to the physiographic situation on Mars, and they are made the subject of the closest scrutiny at every recurring opposition of the planet. Several observers, Lowell in particular, record a bluish line or a sort of retreating polar sea, following up the diminishing polar cap as it shrinks with the advance of summer. It is said that no such line is visible during the formation of the polar cap with the approach of winter. All such results of critical observation, just on the limit of visibility, have to be repeated over and over again before they become part of the body of accepted scientific fact. And in many instances the only sure way is to fall back on the photographic record, which all astronomers, whether prejudiced or not, may have the opportunity to examine and draw their individual conclusions.
Already the approaching opposition of 1924, the most favorable since the invention of the telescope, is beginning to attract attention, and preparations are in progress, of new and more powerful instruments, with new and more sensitive photographic processes, by means of which many of the present riddles of Mars may be solved.