Astronomy of To-day: A Popular Introduction in Non-Technical Language. Cecil Goodrich Julius Dolmage
solar disc, and the eclipse is entirely at an end.
The corona, the chromosphere, and the prominences are the most important of these accompaniments of the sun which a total eclipse reveals to us. Our further consideration of them must, however, be reserved for a subsequent chapter, in which the sun will be treated of at length.
Every one who has had the good fortune to see a total eclipse of the sun will, the writer feels sure, agree with the verdict of Sir Norman Lockyer that it is at once one of the "grandest and most awe-inspiring sights" which man can witness. Needless to say, such an occurrence used to cause great consternation in less civilised ages; and that it has not in modern times quite parted with its terrors for some persons, is shown by the fact that in Iowa, in the United States, a woman died from fright during the eclipse of 1869.
To the serious observer of a total solar eclipse every instant is extremely precious. Many distinct observations have to be crowded into a time all too limited, and this in an eclipse-party necessitates constant rehearsals in order that not a moment may be wasted when the longed-for totality arrives. Such preparation is very necessary; for the rarity and uncommon nature of a total eclipse of the sun, coupled with its exceeding short duration, tends to flurry the mind, and to render it slow to seize upon salient points of detail. And, even after every precaution has been taken, weather possibilities remain to be reckoned with, so that success is rather a lottery.
Above all things, therefore, a total solar eclipse is an occurrence for the proper utilisation of which personal experience is of absolute necessity. It was manifestly out of the question that such experience could be gained by any individual in early times, as the imperfection of astronomical theory and geographical knowledge rendered the predicting of the exact position of the track of totality well-nigh impossible. Thus chance alone would have enabled one in those days to witness a total phase, and the probabilities, of course, were much against a second such experience in the span of a life-time. And even in more modern times, when the celestial motions had come to be better understood, the difficulties of foreign travel still were in the way; for it is, indeed, a notable fact that during many years following the invention of the telescope the tracks were placed for the most part in far-off regions of the earth, and Europe was visited by singularly few total solar eclipses. Thus it came to pass that the building up of a body of organised knowledge upon this subject was greatly delayed.
Nothing perhaps better shows the soundness of modern astronomical theory than the almost exact agreement of the time predicted for an eclipse with its actual occurrence. Similarly, by calculating backwards, astronomers have discovered the times and seasons at which many ancient eclipses took place, and valuable opportunities have thus arisen for checking certain disputed dates in history.
It should not be omitted here that the ancients were actually able, in a rough way, to predict eclipses. The Chaldean astronomers had indeed noticed very early a curious circumstance, i.e. that eclipses tend to repeat themselves after a lapse of slightly more than eighteen years.
In this connection it must, however, be pointed out, in the first instance, that the eclipses which occur in any particular year are in no way associated with those which occurred in the previous year. In other words, the mere fact that an eclipse takes place upon a certain day this year will not bring about a repetition of it at the same time next year. However, the nicely balanced behaviour of the solar system, an equilibrium resulting from æons of orbital ebb and flow, naturally tends to make the members which compose that family repeat their ancient combinations again and again; so that after definite lapses of time the same order of things will almost exactly recur. Thus, as a consequence of their beautifully poised motions, the sun, the moon, and the earth tend, after a period of 18 years and 10⅓ days,[5] to occupy very nearly the same positions with regard to each other. The result of this is that, during each recurring period, the eclipses comprised within it will be repeated in their order.
To give examples:—
The total solar eclipse of August 30, 1905, was a repetition of that of August 19, 1887.
The partial solar eclipse of February 23, 1906, corresponded to that which took place on February 11, 1888.
The annular eclipse of July 10, 1907, was a recurrence of that of June 28, 1889.
In this way we can go on until the eighteen year cycle has run out, and we come upon a total solar eclipse predicted for September 10, 1923, which will repeat the above-mentioned ones of 1905 and 1887; and so on too with the others.
From mere observation alone, extending no doubt over many ages, those time-honoured watchers of the sky, the early Chaldeans, had arrived at this remarkable generalisation; and they used it for the rough prediction of eclipses. To the period of recurrence they give the name of "Saros."
And here we find ourselves led into one of the most interesting and fascinating by-paths in astronomy, to which writers, as a rule, pay all too little heed.
In order not to complicate matters unduly, the recurrence of solar eclipses alone will first be dealt with. This limitation will, however, not affect the arguments in the slightest, and it will be all the more easy in consequence to show their application to the case of eclipses of the moon.
The reader will perhaps have noticed that, with regard to the repetition of an eclipse, it has been stated that the conditions which bring it on at each recurrence are reproduced almost exactly. Here, then, lies the crux of the situation. For it is quite evident that were the conditions exactly reproduced, the recurrences of each eclipse would go on for an indefinite period. For instance, if the lapse of a saros period found the sun, moon, and earth again in the precise relative situations which they had previously occupied, the recurrences of a solar eclipse would tend to duplicate its forerunner with regard to the position of the shadow upon the terrestrial surface. But the conditions not being exactly reproduced, the shadow-track does not pass across the earth in quite the same regions. It is shifted a little, so to speak; and each time the eclipse comes round it is found to be shifted a little farther. Every solar eclipse has therefore a definite "life" of its own upon the earth, lasting about 1150 years, or 64 saros returns, and working its way little by little across our globe from north to south, or from south to north, as the case may be. Let us take an imaginary example. A partial eclipse occurs, say, somewhere near the North Pole, the edge of the "partial" shadow just grazing the earth, and the "track of totality" being as yet cast into space. Here we have the beginning of a series. At each saros recurrence the partial shadow encroaches upon a greater extent of earth-surface. At length, in its turn, the track of totality begins to impinge upon the earth. This track streaks across our globe at each return of the eclipse, repeating itself every time in a slightly more southerly latitude. South and south it moves, passing in turn the Tropic of Cancer, the Equator, the Tropic of Capricorn, until it reaches the South Pole; after which it touches the earth no longer, but is cast into space. The rear portion of the partial shadow, in its turn, grows less and less in extent; and it too in time finally passes off. Our imaginary eclipse series is now no more—its "life" has ended.
We have taken, as an example, an eclipse series moving from north to south. We might have taken one moving from south to north, for they progress in either direction.
From the description just given the reader might suppose that, if the tracks of totality of an eclipse series were plotted upon a chart of the world, they would lie one beneath another like a set of steps. This is, however, not the case, and the reason is easily found. It depends upon the fact that the saros does not comprise an exact number of days, but includes, as we have seen, one-third of a day in addition.
It will be granted, of course, that if the number of days was exact, the same parts of the earth would always be brought round by the axial rotation to front the sun at the moment of the recurrence of the eclipse. But as there is still one-third of a day to complete the saros period, the earth has yet to make one-third of a rotation upon its axis before the eclipse takes place. Thus at every recurrence the track of totality finds itself placed one-third of the earth's circumference to the westward. Three of the recurrences will, of course, complete the circuit of the