Astronomy for Everybody/Part 3/Chapter 6

If the moon moved exactly in the plane of the ecliptic she would pass over the face of the sun at every new moon. But, owing to the inclination of her orbit, as described in the preceding chapter, she will actually do so only when the direction of the sun happens to be near one of the moon's nodes. When this is the case we may see an eclipse of the sun if we are only on the right part of the earth.

Supposing the moon to pass over the sun, the first question is whether it can wholly hide the sun from our eyes. This depends not on the actual size of the two

bodies but on their apparent size. We know that the sun has about four hundred times the diameter of the moon. But it is also four hundred times as far from us as the moon. The curious result of this Is that the two bodies appear of nearly the same size to our eyes. Sometimes ​the moon appears a little the larger, and sometimes the sun. In the former case the moon may entirely hide the sun; in the latter case she cannot do so.

One important difference between an eclipse of the moon and of the sun is that the former is always the same wherever it is visible, while an eclipse of the sun depends upon the position of the observer. The most interesting eclipses are those in which the centre of the moon passes exactly over that of the sun. These are called central

eclipses. To see one, the observer must station himself at a point through which the line joining the centres shall pass. Then if the apparent size of the moon exceeds that of the sun, the former will completely hide the sun from view. The eclipse is then said to be total.

If the sun appears the larger, a ring of its light will surround the dark body of the moon at the moment of central eclipse. The latter is then called annular (Latin annulus, a ring).

The line of centres of the two bodies sweeps along the surface of the earth, and its course may be shown by a line marked on a map. Such maps, showing the regions ​and lines of eclipses are published in the astronomical ephemerides. An eclipse may be total or annular in a region a few miles north or south of this central line, but never for so far as one hundred miles. Outside this limit an observer will see only a partial eclipse, that is, one in which the moon partly covers the sun. In yet more distant regions of the earth there will be no eclipse at all.

A total eclipse is one of the most impressive sights that nature offers to the eye of man. To see it to the best advantage one should be in an elevated position commanding the widest possible view of the surrounding country, especially in the direction from which the shadow of the moon is to come. The first indication of anything unusual is to be seen, not on the earth or in the air, but on the disk of the sun. At the predicted moment a little notch will be seen to form somewhere on the western edge of the sun's outline. It increases minute by minute, gradually eating away, as it were, the visible sun. No wonder that imperfectly civilised people, when they saw the great luminary thus diminishing in size, fancied that a dragon was devouring its substance.

For some time, perhaps an hour, nothing will be noticed but the continued progress of the advancing moon. It will be interesting if, during this time, the observer is in the neighbourhood of a tree that will permit the sun's rays to reach the ground through the small openings in its foliage. The little images of the sun which form here and there on the ground will then have ​the form of the partially eclipsed sun. Soon the latter appears as the new moon, only instead of increasing, the crescent form grows thinner minute by minute. Even then, so well has the eye accommodated itself to the diminishing light, there may be little noticeable darkness until the crescent has grown very thin. If the observer has a telescope with a dark glass for viewing the sun, he will now have an excellent opportunity of seeing the mountains on the moon. The unbroken limb of the sun will keep its usual soft and uniform outline. But the inside of the crescent, the edge of which is formed by the surface of the moon, will be rough and jagged in outline.

As the crescent is about to disappear the advancing mountains on the rugged surface of the moon will reach the sun's edge, leaving nothing of the latter but a row of broken fragments or points of light, shining between the hollows on the lunar surface. They last but a second or two and then vanish.

Now is seen the glory of the spectacle. The sky is clear and the sun in mid-heaven, and yet no sun is visible. Where the latter ought to be the densely black globe of the moon hangs, as it were, in mid-air. It is surrounded by an effulgence radiating a saintly glory. This is the sun's corona, already mentioned in our chapter on the sun. Though bright enough to the unaided vision, it is seen to the best advantage with a telescope of very low magnifying power. Even a common opera glass may suffice. With a telescope of high power only a portion of the corona is visible, and thus the finest part of the ​effect is lost. A common spy-glass, magnifying ten or twelve times, is better, so far as effect is concerned, than the largest telescope. Such an instrument will show not only the corona itself but the so-called "prominences"—fantastic cloud-like forms of rosy colour rising here and there, seemingly from the dark body of the moon.

It is remarkable that though the ancients were familiar with the fact of eclipses, and the more enlightened of them perfectly understood their causes, some even the laws of their recurrence, there are very few actual accounts of these phenomena in the writings of the ancient historians. The old Chinese annals now and then record the fact that an eclipse of the sun occurred at a certain time in some province or near some city of the empire. But no particulars are given. Quite recently the Assyriologists have deciphered from ancient tablets a statement that an eclipse of the sun was seen at Nineveh, B. C. 768, June 15. Our astronomical tables show that there actually was a total eclipse of the sun on this day, during which the shadow passed a hundred miles or so north of Nineveh.

Perhaps the most celebrated of the ancient eclipses, and the one that has given rise to most discussion, is that known as the eclipse of Thales. Its principal historical basis is a statement of Herodotus that in a battle between the Lydians and the Medes the day was suddenly turned into night. The armies thereupon ceased battle and were more eager to come to terms of peace with each other. It ​is added that Thales, the Milesian, had predicted to the Ionians this change of day, even the very year in which it should occur. Our astronomical tables show that there actually was a total eclipse of the sun in the year B. C. 585, which was near enough to the time of the battle to be the one alluded to, but it is now known that the path of the shadow did not quite reach the seat of hostilities till after sunset. Some doubt therefore still rests on the subject.

There is a curious law of the recurrence of eclipses which has been known from ancient times. It is based on the fact that the sun and moon return to nearly the same positions, relative to the node and perigee of the moon's orbit, after a period of six thousand five hundred and eighty-five days eight hours, or eighteen years and twelve days. This period is called the Saros. Eclipses of every sort repeat themselves at the end of a Saros. For example, the eclipse of May, 1900, may be regarded as a repetition of those which occurred in the years 1846, 1864, and 1882. But when such an eclipse recurs it is not visible in the same part of the earth, because of the excess of eight hours in the period. During this eight hours the earth performs one third of a rotation on its axis, which brings a different region under the sun. Each eclipse is visible in a region about one third of the way round the world, or one hundred and twenty degrees of longitude, west of where it occurred before. Only after three periods will the recurrence be near the same region. But in the meantime the moon's line of motion will have ​changed so that the path of its shadow will pass farther north or south than before.

There are two series of eclipses remarkable for the long duration of the total phase. To one of these the eclipse of 1868, hereafter mentioned, belongs. This recurred in 1886, and will recur again in 1904. Unfortunately, at the first recurrence, the shadow was cast almost entirely on the Atlantic and Pacific Oceans, so that it was not favourable for observation by astronomers. That of 1904, September 9, will be yet more unfortunate for us, because the shadow will pass only over the Pacific Ocean. Possibly, however, it may touch some island where observations may be made. The recurrence of 1922, September 1, will be visible in northern Australia, where the duration of totality will be about four minutes.

To the other and yet more remarkable series belonged the eclipse of May 7, 1883, and that of May 11, 1901. At the successive recurrences of this eclipse the duration of totality will be longer and longer through the twentieth century. In 1937, 1955, and 1978 it will exceed seven minutes, so that so far as duration is concerned, our successors will see eclipses more remarkable than any their ancestors have enjoyed for many centuries.

About 1863-64 the spectroscope began to be applied to researches on the heavenly bodies. Mr. (now Sir William) Huggins, of London, was a pioneer in observing the spectra of the stars and nebulæ. For several years it did not seem that much was to be learned in this ​way about the sun. The year 1868 at length arrived. On August eighteenth there was to be a remarkable total eclipse of the sun, visible in India. The shadow was one hundred and forty miles broad; the duration of the total phase was more than six minutes. The French sent Mr. Janssen, one of their leading spectroscopists, to observe the eclipse in India and see what he could find out. Wonderful was his report. The red prominences which had perplexed scientists for two centuries were found to be immense masses of glowing hydrogen, rising here and there from various parts of the sun, of a size compared with which our earth was a mere speck. This was not all. After the sunlight reappeared, Janssen began to watch these objects in his spectroscope. He followed them as more and more of the sun came out, and continued to see them until after the eclipse was over. They could be observed at any time when the air was sufficiently clear and the sun high in the sky.

By a singular coincidence this same discovery was made independently in London without any eclipse. Mr. J. Norman Lockyer was then rising into prominence as an enthusiastic worker with the spectroscope. It occurred independently to him and to Mr. Huggins that the heat in the neighbourhood of the sun was so intense that any matter that existed there would probably take the form of a gas shining by its own light. Both of these investigators endeavoured to get a sight of the prominences in this way; but it was not until October twentieth, two months after the Indian eclipse, that Mr. Lockyer succeeded in having an instrument of sufficient ​power completed. Then, at the first opportunity, he found that he could see the prominences without an eclipse!

At that time communication with India was by mail, so that for the news of Mr. Janssen's discovery astronomers had to wait until a ship arrived. By a singular coincidence his report and Mr. Lockyer's communication announcing his own discovery reached the French Academy of Sciences at the same meeting. This eminent body, with pardonable enthusiasm, caused a medal to be struck in commemoration of the new method of research, in which the profiles of Lockyer and Janssen appeared together as co-discoverers. Since that time the prominences are regularly mapped out from day to day by spectroscopic observers in various parts of the world.

The greatest beauty of a total eclipse is due to the sun's corona. The exact nature of this appendage is still in doubt. Indeed, until photography was called to the aid of the astronomer its structure was unknown. It was described by observers simply as a soft light surrounding the sun; but when it is photographed and carefully examined it is found to be of a radial, hairy structure which the reader can easily see from the frontispiece of the book. It extends out farthest in the direction of the sun's equator and least at the poles. The rays which chance to be exactly at the poles go straight out from the sun. But those on each side are found to curve toward the equator, while farther from the equator they are lost in the more powerful effulgence going out from the region of the solar spots. Near the poles the ​forms are remarkably like those which iron filings assume when scattered on paper above a magnet. It is therefore a question whether there is not here something in the nature of a magnetic force. But in the region called the sun's equator this analogy ceases to hold. In describing the sun we mentioned the much greater activity in the regions of greater spottedness than elsewhere. It now seems as if the forces which throw out the corona are also greatest where the sun's activity is greatest.

The probability now seems to be that the corona is composed of matter thrown up from the sun, and kept from falling back again by the repulsion of the solar rays, and that it bears a certain resemblance to the tail of a comet.

A very important question is whether the corona shines mostly by reflected light, or by its own light, due to the high temperature which it must have so near the sun. No doubt its light arises from both sources, but it is not yet known in what proportion. The fact is that its spectrum shows some bright lines. These can be due only to the light of the matter itself. Some observers have supposed that they also saw dark lines in the spectrum. This, however, has not been proved. On the whole the probability seems to be that the corona shines mostly by its own light.