Part 14 (1/2)
CHAPTER XXIX
ECLIPSES OF THE MOON
Of all the weird happenings of the nighttime sky, eclipses of the moon are the most impressive. Rarely is there a year without one. What is the cause? Simply the earth getting in between sun and moon, and thereby shutting off the sunlight which at all other times enables us to see the moon. As the earth is a dark body it must cast a black shadow on the side away from the sun, and it is the moon's pa.s.sing into this shadow or some part of it that causes a lunar eclipse.
Sun and earth being so different in size, the earth's shadow must stretch away from it into s.p.a.ce, growing smaller and smaller, until at length it comes to an end--the apex of a cone 857,000 miles long. If we cut off this shadow at the moon's distance from the earth, we find it about 6,000 miles in diameter at that point; and this accounts for the fact that the curvature on the side of the moon, when the eclipse is coming on and where it is dropping into the shadow, is always much less rapid than the curvature of the moon's own disk is.
When an eclipse is approaching, the eastern limb will be duskily darkened for half an hour or more, because the moon must first pa.s.s through the outer penumbra, or half-shadow which everywhere surrounds the true shadow itself. If the moon hits only the upper or lower part of the shadow, the eclipse will be only partial, and during the progress of the eclipse it will seem as if the uneclipsed part had swung or twisted around in the sky, from the western limb of the moon to the eastern. But when the moon pa.s.ses through the middle regions of the shadow, the eclipse is always total, and direct sunlight is wholly cut off from every part of the moon's face, for a greater or less length of time, according to the part of the shadow through which it pa.s.ses. When pa.s.sing centrally through the shadow, the total eclipse will last about two hours, as the moon's diameter is about one-third of the breadth of the shadow; and the eclipse will be partial about two hours longer, an hour at beginning and an hour at the end, because the moon moves over her own breadth in about an hour.
While the moon is wholly immersed in the shadow, her body is nevertheless visible, as a dull tarnished copper disk; and this is caused by the reddish sunlight which grazes the earth all around and is refracted or bent by our atmosphere into the shadow itself. If this belt or ring of terrestrial atmosphere happens to be everywhere filled with dense clouds, as was the case in 1886, even the familiar copper moon of a total lunar eclipse disappears completely in the black sky.
Quite different from a solar eclipse, all the phases of a lunar eclipse are visible at the same time on the earth wherever the moon is above the horizon. Eclipses of the moon are therefore seen with great frequency at any given place as compared with solar eclipses, which are restricted to relatively narrow areas of the earth's surface. Nor are lunar eclipses of very much significance to the astronomer, mainly because of the slowness and indefiniteness of the phenomena. It is a good time to observe occultations of faint stars at the moon's edge or limb, and several such programs have been carried out by cooperation of observatories in widely separate regions of the world: the object being improvement in our knowledge of the distance of the moon, and in the accuracy of the mathematical tables of her motion. Search by photography for a possible satellite, or moon of the moon, has been made on several occasions, though without success.
A lunar eclipse was first observed and photographed from an aeroplane, May 2, 1920. At the request of the writer, two aviators of the United States navy ascended to a height of 15,000 feet above Rockaway, and secured many advantages accruing from great elevation in viewing a celestial phenomenon of this character.
CHAPTER x.x.x
TOTAL ECLIPSES OF THE SUN
Primitive peoples indulged in every variety of explanation of mysterious happenings in the sky. To the Chinese and all through India, a total eclipse of the sun is caused by ”a certain dragon with very black claws,” who, except for their frightening him away by every conceivable sort of hideous noise, would most certainly ”eat up the sun.” The eclipse always goes off, the sun has never been eaten yet. Can you convince a Chinaman that Rahu, the Dragon, wouldn't have eaten up the sun, if his unearthly din hadn't frightened him away?
In j.a.pan the eclipse drops poison from the sky into wells, so the j.a.panese cover them up. Fontenelle relates that in the middle of the seventeenth century a mult.i.tude of people shut themselves up in cellars in Paris during a total eclipse.
In the Shu-king, an ancient Chinese work, occurs the earliest record of a total eclipse of the sun, in the year B. C. 2158. The Nineveh eclipse of B. C. 763 is perhaps the first of the ancient eclipses of which we possess a really clear description on the a.s.syrian eponym tablets in the British Museum. It is the eclipse possibly referred to in the Book of Amos, viii.
But of all the ancient eclipses none perhaps exceeds in interest the famous eclipse of Thales, B. C. 585, May 28. It is the first eclipse to have been predicted, probably by means of the saros, or 18-year period of eclipses, which is useful as an approximate method even at the present day. But the accident of a war between the Lydians and the Medes has added greatly to the historic interest, because the combatants were so terrified by the sudden turning of day into night that they at once concluded a peace cemented by two marriages.
Very many of the ancient eclipses have been of great use to the historian in verifying dates, and mathematical astronomers have employed them in correcting the lunar tables, or intricate mathematical data by which the motion of the moon is predicted.
Coming down to the middle of the sixth century, we find the first eclipse recorded in England, in the ”Saxon Chronicle,” A. D. 538. During the epoch of the Arabian Nights several eclipses were witnessed at Bagdad, A. D. 829 to 928, and many a century later by Ibu-Jounis, court astronomer of Hakem, the Caliph of Egypt. Nothing is more interesting than to search the quaint records of these ancient eclipses. One occurring in 1560, when Tycho Brahe was but fourteen, had much to do with turning his permanent interest toward mathematics and astronomy.
The eclipse of 1612 was the first ”seen through a tube,” the telescope having been invented only a few years before. ”Paradise Lost” was completed about 1665, and the censors.h.i.+p was still in existence; and it is matter of record that the oft-quoted pa.s.sage,
”As when the Sun, new risen, Looks through the horizontal misty air, Shorn of his beams; or from behind the Moon, In dim eclipse, disastrous twilight sheds On half the nations, and with fear of change Perplexes monarchs.”
_P. L._, i. 594
was strongly urged as sufficient reason for suppressing the entire epic.
London was favored with the outflas.h.i.+ng corona, May 3, 1715, and a pamphlet was issued in prediction, ent.i.tled ”The Black Day, or a Prospect of Doomsday.”
The first American eclipse expedition was on occasion of the totality of Oct. 27, 1780, sent out by Harvard College and the American Academy of Arts and Sciences under Professor Samuel Williams to Pen.o.bscot. There was a fine total eclipse from Albany to Boston on June 16, 1806, and many important observations of it were made in this country.
But it was not till the European eclipse of 1842 that research got fully under way, because the germ of the new astronomy, particularly as applied to the sun, had begun its development; and the significance of the corona was obvious, if it could be proved a true appendage of the sun. Photography had not long been discovered, and the corona of 1851 was the first to be automatically registered on a daguerreotype. In 1860 it was proved that prominences and corona both belong to the sun and not to the moon.
The great Indian eclipse of 1868 brought the important discovery that the prominences can be observed at any time without an eclipse by means of the spectroscope. In 1869 bright lines were found in the spectrum of the corona, one line in the green indicating the presence of an element not then known on the earth and hence called coronium. In 1870 the reversing layer or stratum of the sun was discovered. In 1878 a vast ecliptic extension of the streams of the corona many millions of miles both east and west of the sun was first seen. This is now known to be the type of corona characteristic of minimum spots on the sun. In 1882 the spectrum of the corona was first photographed and in 1889 excellent detail photographs of the corona were taken. In 1893 it was shown that the corona quite certainly rotates bodily with the sun. In 1896 actual spectrum photographs of the reversing layer established its existence beyond doubt--”flash spectrum” it is often called. In 1898 the long ecliptic streamers of the corona were successfully photographed for the first time. In 1900 the depth of the reversing layer was found to average 500 miles, the heat of the corona was first measured by the bolometer, and many observations showed that the coronal streamers, in part at least, partake of the nature of electric discharges.