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press: All the separate mechanisms of that enabled Hall to weigh the planet this marvelous product of man's genius Mars, once he had discovered its tiny perform their separate tasks with perfect satellite, and Le Verrier and Adams to adaptation, each at the absolute instant point out infallibly the hidden Neptune. intended. All are interrelated by tangible and material rods and gears, so that

Method of Calculation failure occurs almost never.

But the very precision of the celestial So with the solar system: Earth goes machine, while it enables us to predict around the sun, and moon around the without chance of error the exact spot

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DIAGRAM SHOWING HOW ECLIPSES ARE CAUSED, 1. Moon's shadow cut off by earth (total solar eclipse); 2. Moon's shadow does not reach earth (annular eclipse):

3. Moon in earth's shadow (total lunar eclipse); 4. Moon's shadow just reaches earth

(total solar eclipse in middle of path, but annular at both ends).

earth, our satellite now and then coming on the earth where an eclipse will be visbetween us and the sun to screen com- ible, and the fraction of a second when pletely his effulgence in total eclipse. this obscuration will take place—yet inAnd as cog and pinion serve to keep all troduces complications of the process of parts of the printing press in unvarying prediction which would be very tedious correlation, so does the mysterious force to the uninitiated computer. Not only of gravitation, by its tense and unyielding is the earth swinging round the sun, and stresses exerted between all material the moon round the earth, but our globe atoms, hold all members of the cosmos is turning slowly once a day round an

axis which is itself far from fixed in direction among the stars. And these are but the chief of the ever-shifting elements of the eclipse machine, all of which must be expressed by mathematical equations with the last degree of precision, in both space and time.

All this theory, then, of the working

of the celestial machine is embodied in CORONA OF JULY 29, 1878.

what are called Tables of Motion of Sun Harkness, from photographs.

and Moon. From them we calculate ex

actly what region of space the earth and in constant and working relation to one moon are occupying at any time in relaanother, all wheeling through space with tion to the sun; and so the instant bea refinement of precision as exact as if comes known when the moon's long, connected by rigid rods of steel. slender shadow will impinge upon some And it is but the same law of gravitation spot of earth and so cause a total eclipse which Sir Isaac Newton first formulated, of the sun.

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COPYRIGHT, 1990, BY L'TTLE, BROWN & Co.

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The most favorable condition is shown at the upper left-hand figure of the fourway diagram—moon is nearest to earth and so its shadow is broadest-it may be 150 miles broad, or even more. Diagonally opposite is the least possible length of total eclipse, the tip of the moon's shadow just reaching the earth with totality for but a second—a touch and go.

So eccentric is the moon's path round the earth that the relation obtains as shown in the upper right wing of the diagram, where the moon is farther from the earth than the length of its shadow; so that a ring of sunlight still remains surrounding the eclipsing moon; and a ring—or annular eclipse—is all that is seen, with not a glimpse even of that striking corona which forms the chief object of study when a total eclipse favors the waiting astronomers of earth.

Advances in Scientific Methods It should now be apparent that no simple period like the Saros can be more than the crudest of methods, or could for a moment replace the refined mathematical processes of to-day. The first astronomer who really adopted a more scientific method of prediction, as

CORONA OF 1896.

made by La Grange, who seems to have first developed the calculation to a point which might be used for the circumstances of an eclipse at any special place on the earth's surface.

The final precision of prediction was reached by the great Bessel, his fundamental equations, upon which all the circumstances of visibility rest, being presented in the convenient shape of formulæ for computing the exact moment of beginning and ending at any geographical point that may be desired. To render the calculation still more facile, the official astronomers of our Government give, in the Ephemeris or Nautical Almanac, each year, all the Besselian data, or elements requisite for calculating the eclipses of the year. By means of them, an expert calculator can, in an hour or two, figure out the time that any eclipse will begin or end, at any place, with a higher degree of precision than the untrained astronomer would be able to observe it.

If one aims short of the highest precision, a fairly accurate knowledge of the regions visited by eclipses may be derived from the late Theodor von Oppolzer's Canon der Finsternisse, comprising tables of eclipses as remote as B. C. 1208. In all, there are approximate calculations for 8,000 solar eclipses, extending more than 250 years into the far-away future, and no less than 160 charts of the most important tracks on the surface of our globe.

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Future Eclipses

cent years; and the result of much study As the stupendous mechanism of the ‘of eclipses it presents. sky moves onward in unceasing round, In New England fine eclipses will be eclipse after eclipse will come true to its visible January 24th, 1925, and August time, in all the future as in the past. 31st, 1932; while our new Oriental posHere is a chart of many of them, as well sessions will be the scene of two extraas of all that have happened within re ordinarily long darkenings, of five and

seven minutes respectively, on May 9th, 1929, and June 20th, 1955. While we write of this, perhaps the longest totality that humanity will witness for a thousand years, it is worth while to call attention to its companion, the total eclipse most recently observed (previously to that of this year), on the 18th of May, 1901, in the Dutch East Indies:

By noticing the interval between them, or 54 years I month, it is evident that three Saros periods have intervened ; and this newly discovered cycle brings a return of any given eclipse most nearly to its region of visibility on the earth. Near the middle of the map are seen two similar tracks, those of the 29th May, 1919, and the 30th June, 1973. If we restrict our inquiry to a very small country, a total eclipse need not be expected to re

turn oftener than 350 years, generally Long Coronal STREAMERS OF JANUARY 22, 1898. speaking ; but there are exceptions, of From a photograph taken in India by Mrs. Maunder. course, Spain having had five total

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COPYRIGHT, 183, DY I TTLE, BROWN & Co.

year, the corona is likely to be similar in structure to that of 1882, the engraving here shown being made from the photograph taken that year in Egypt;

eclipses in 63 years; while the totalities of both 1900 and 1905 pass over Tripoli, and it is not known that so large a city has ever before been favored with an interval so brief.

In spite of our exact knowledge of just when to expect these impressive phenomena, uncivilized peoples are still in terror when eclipses appear, and, like the nations of earliest times, invent dragons and evil spirits to account for what seems so abnormal. Science has robbed us of many myths and legends and pretty fairy tales, but the actual and precise study of sun and moon is as picturesque as any possible story.

Aspect of Eclipses But while we can predict eclipses, predicting what they will look like is quite another matter. Many astronomers have essayed to forecast the figure of the corona, with little more than encouraging failure. Only the most general types of its structure can be said to be known beforehand. When the spots on the sun are most numerous, as during the present

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COOU may get through the world, but 'twill be very slow

I f you listen to all that is said as you go;
C29 You'll be worried, and fretted, and kept in a stew;
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