Chapter X: Front Matter (10)

Mr. Seton is a nephew of the celebrated foundress of the American branch of the institute of the Daughters of Charity, and a brother of the Rt. Rev. Monsignor Seton. He served with honor as an officer of one of our New York regiments during the late war, and since that time has especially devoted himself to the study of early New England history, which he has illustrated by his historical novels. Our first impression respecting the merits of a previous novel by Mr. Seton, in which he took great pains to depict the manners and customs of the early Puritan inhabitants of Connecticut and Massachusetts (the _Romance of the Charter Oak_), was not very favorable. We have since been disposed to think that we did not duly appreciate the skill and talent of the author, and have found other persons, well qualified to judge of such matters, who have considered the _Charter Oak_ as a remarkably successful effort of its kind. Both that novel and the present one are characterized by a marked realism, like that of a certain Dutch and Flemish school of painting. Probably they do present a more correct and faithful picture of those old times than that given by writers who have more idealism and romance in their delineation, like James F. Cooper. We confess to a taste, nevertheless, for these more romantic authors. And, speaking in cool criticism, we think a novelist, in following the highest principles and ends of his art, ought to idealize more than Mr. Seton is disposed to do. He has a broad sense of the humorous and ridiculous in commonplace characters and actions. The absurdities and trivialities of common life are too faithfully represented in his pages, and there is frequently a degree of coarseness in the description of vulgar persons which is disagreeable. Yankee children, however, devour Mr. Seton’s stories with avidity, which is a good proof of their naturalness. And, putting aside the peculiarity which we have noticed, the story lately published, _The Pride of Lexington_, is, even more than the first one, a composition of real originality and power, establishing fully the author’s ability as a historical novelist. The battles of Lexington and Bunker Hill are well described; the heroes, and especially the heroine, of the story, with the plot of private incidents and events that make the filling up of the historical scenes, are interesting; there is much genuine comic humor in the by‐play, especially in the episode of Billy Smith and the black coon, called “the parson,” and we are quite sure that the genuine, unsophisticated children of the by‐ gone generation of New England forefathers, if they get hold of _The Pride of Lexington_, will pay the author the tribute of an oft‐repeated and delighted perusal.

CONFERENCES ON THE SPIRITUAL LIFE. By the Rev. Father de Ravignan,
S.J. Translated from the French by Mrs. Abel Ram. London: R.
Washbourne. 1873. (New York: Sold by The Catholic Publication
Society.)

F. de Ravignan was undoubtedly an orator. The impression which he made upon his hearers is enough to justify us in making this assertion. The orator must be heard; when his words are written, their fire is gone, and they no longer burn. In the case of F. de Ravignan especially, there must have been much in the magnetism of the man, in his earnestness, in his deep religious feeling, in the firm conviction and strong love, shown in the manner in which he spoke; for in his printed conferences and sermons we do not find great eloquence or beauty of diction or depth of thought. There are none of those bursts of passion, of those profound thoughts and comprehensive views, in which a whole subject is condensed into a single phrase, as strong as it is striking, which we so often meet with in the conferences of Lacordaire. Nor yet is there that stately flow of language, at once simple and majestic, that evenness of style and unbroken sequence of thought, which characterize the discourses of F. Felix. And yet neither Lacordaire nor Felix excited greater enthusiasm or made a profounder impression in the pulpit of Notre Dame than De Ravignan.

If he had not the depth and comprehensiveness of thought of the one, or the sonorous diction and lofty manner of the other, he must have been, in some respects at least, a greater orator than either. The conferences contained in the volume now before us were preached to the “Enfants de Marie,” in the Convent of the Sacred Heart, in Paris, during the years 1855, 1856, and 1857. They were not written out by F. de Ravignan, but were compiled by one of his hearers from notes taken at the time of their delivery, and are, we think, equally as good as the conferences preached in Notre Dame from 1837 to 1846, which were published in four volumes shortly after his death. They are simply familiar discourses to ladies in the world on the most important subjects connected with their duties as Christians; in which we find all the best qualities that distinguished F. Ravignan as a preacher—sincere piety and much earnestness, united with delicacy and refinement both of thought and language. He does not inveigh against the vices of society, but rather seeks to describe the beauties of the Christian life; to show its dignity and responsibilities, its perfect harmony with the highest aspirations of the soul and the soundest dictates of reason.

The name of F. de Ravignan will of itself be sufficient to obtain a wide circulation for this English version of his conferences.

ECCLESIASTICAL ANTIQUITIES OF LONDON. By Alex. Wood, M.A. Oxon.
London: Burns & Oates. 1874. (New York: Sold by The Catholic
Publication Society.)

This book is quite a storehouse of curious and valuable information—just the kind of matter that would be overlooked by the civil historian, and which the reverent chronicler (alas! an almost extinct species, now) alone would be apt to take cognizance of.

It doubtless surprised many intelligent readers to find what interesting facts even a cursory investigation would bring to light, while reading what our “Looker‐Back” saw while in London. This work is a treat of a similar character. It is constructed on the plan of an itinerary, and divided into nine “walks,” in which the most notable localities are looked at from an archæological point of view, re‐peopled by the actors on the stage at the respective dates, and reanimated by the deeds then being performed.

NOTES OF THE WANDERING JEW; or, The Jesuits and their Opponents.
Edited by John Fairplay, Esq. Dublin: McGlashan & Gill. 1873. (New
York: Sold by The Catholic Publication Society.)

We are doubtless indebted to the famous romance of Eugene Sue for these notes of the Wandering Jew, in which this extraordinary personage, after his ceaseless journeyings for more than eighteen hundred years, finally turns up as an author, and, surprising as it may seem, a defender of the Jesuits.

The first part of the little volume is devoted to S. Ignatius. The Wandering Jew had seen him on two occasions—first in Spain, in his hot youth, with his light, graceful form clad in a page’s rich attire, with the plumed cap and velvet mantle, the hawk upon his wrist, the hounds following at his heels, whilst his foot seemed hardly to touch the ground as he walked; and again, at Rome, he saw him in his old age, arrayed in the flowing gown of the priest, with the calm of deliberate wisdom on his high forehead, advancing with a sweet and awful majesty to the altar.

“I loved and revered him then,” says the Jew, “albeit a stranger to his communion; and I cannot recall the memory of that marked and expressive countenance, whether in the gallant boy or the venerable and saintly old man, without feeling some interest in the fate of that illustrious order which he alone created, and which still bears the impress of his character and genius.”

The remaining chapters are devoted to _The Spiritual Exercises_, “The Constitutions of the Order,” “The Missions and Schools of the Jesuits,” and, finally, to answering some of the charges which Protestants and infidels have brought against the Society. There is a very good chapter on the _Provincial Letters_, in which Pascal, with a wit and power of sarcasm surpassed only by the artful unfairness with which he treats the subject, has sought to make the whole order responsible for the extravagant opinions of some few Spanish and Flemish Jesuits.

The author, who is evidently not a Catholic, has written with great fairness and good sense, and we most willingly recommend his book to our readers.

THE RED FLAG, AND OTHER POEMS. By the Hon. Roden Noel. London:
Strahan & Co. 1872.

We have been asked to notice this book. But how are Catholics to regard it with favor, when, before they have read far in the poem of “The Red Flag,” they come upon a passage containing an insult too gross and slanderous, we should have thought, for even Exeter Hall? We forbear to quote the words. Suffice it to say that the author, ignoring the martyred archbishop and priests, represents the church as gloating over the execution of the communists in Paris.

Affectation, verboseness, and sensuous description characterize these poems as works of art; while the metre of “The Red Flag” is in the worst taste, and the lyrics are spoilt by all sorts of quirks and the clumsiest divisions of stanzas.

The Catholic Publication Society has in press, and will soon publish, _The Life of St. John of the Cross_, 1 vol. 12mo, and _The Farm of Muiceron and Madame Agnes_, in 1 vol. 8vo.

THE CATHOLIC WORLD. VOL. XIX., NO. 110.—MAY, 1874.

The Coming Transit Of Venus.

This year, 1874, bids fair to be memorable in the annals of astronomy. A subject which has long occupied our students of that venerable and now gigantic science is gradually passing from their closets and their scientific discussions into reviews and newspapers, and is forcing itself on the attention of the world at large. At first sight the matter seems a very trivial one. On the 8th of next December, keen eyes in certain parts of the world may, if the sky be clear, and if they look closely, notice that a small, dark spot, a mere speck, will flit across the face of the sun. Examined through a telescope, it is seen to have an appreciable diameter—about 1’. It is not half as interesting to look at as ordinary solar spots, with their jagged edges, their umbra and penumbra, their changing forms, and their whirling faculæ. It has not, as they seem to have, some vague connection with the magnetic disturbances, the auroral lights, or any other atmospheric changes of this sublunary world of ours. It simply passes across the sun in something less than six hours, leaving no trace behind, and producing, so far as would appear, no appreciable effect of any kind. It occurs but rarely—twice in a century; in some centuries, not at all. Small as it is, it can be foretold and calculated beforehand. Except as a verification of such calculations, ordinary minds might think it singularly unimportant—scarcely more important than the gleam in the heavens at night of an occasional and isolated falling star, which glides along its shining path for an instant, and then disappears never more to be seen.

Yet for the last ten—we might, with more truth, say for fifty—years back, the best astronomers have been preparing to observe, with unequalled care, the passage of that little black spot. Some have again and again gone over the records of the observations made in 1761 and 1769, when it was last seen, criticising what was then done, distinguishing what was well done from what they judge to have been faulty, and tracing these faults back to their sources—either to the imperfection of the instruments used, to personal errors, or to mistakes or omissions of the observers themselves. In the observations now to be made, all these sources of error will, as far as possible, be excluded. Others have spent years in patiently going over the long calculations connected with those observations, detecting and eliminating any errors they find, and introducing such corrections as the subsequent advance of astronomical science demands. The amended results thus obtained are ready for comparison, at their proper value, with the additional and, it is hoped, better results to be obtained from the observations of next December. Still others have used, and are now using, their utmost skill in constructing instruments of hitherto unequalled excellence for the great occasion. Besides great improvements in the instruments known in 1769, they have devised others, perhaps more valuable, and of a character then not dreamed of. Others, again, have devoted months to the nicest and most intricate calculations of the movements of the earth and the planets, in order to know in full time beforehand what special stations on the surface of the earth will, that day and at the required hours, afford the most eligible positions from which to make the desired observations.

Finally, governments have been appealed to, to aid in preparing the means and in bearing the expense; and they have responded. Every civilized nation is acting in the matter. Russia leads off with, as we are assured, twenty‐seven stations, mostly on her own territory, all duly provided with instruments and observers. France, England, and Germany will have ten or a dozen each. Austria will have her quota. Belgium, Holland, Denmark, and Italy will establish stations and send observers and instruments. Even distracted Spain is at least talking of it. From the Western World, the United States will send eight corps. Nor will Brazil, Peru, and Chili prove laggard. The whole civilized world seems to move in this undertaking with a singular unanimity, doing what only governments can do. Many of the stations must be in bleak and inhospitable lands beyond the confines of civilization. They will be furnished with all that is needful, and thousands of miles of telegraphic wires will be stretched to put them in connection with the observatories of Europe. Other stations will be on distant islands in mid‐ocean. Thither national vessels will bear the observers and their instruments. It were well for the world if governments would manifest such generous rivalry in doing good when other and more important interests than those of astronomy are in question.

What, then, is that little black spot which they are so anxious to examine as it passes across the sun next December? How comes it to be of such importance that all these mighty efforts are made to have it fully and correctly observed? To what great results, scientific or other, will a correct knowledge of everything about it lead the world?

That little black spot is the planet Venus, then passing directly between the earth and the sun, and producing an homœopathic solar eclipse, just as, under similar circumstances, the moon might produce an annular or a total solar eclipse. As ordinarily seen in her character of morning or evening star, Venus shines more brightly and joyously in the heavens than any other star. But on this occasion the whole of her illuminated half is turned towards the sun. Towards the earth she shows only her dark, unillumined half, which even looks darker by contrast with the bright face of the sun, on which it is projected. This passage across the sun is called the transit of Venus. If the observations are successfully made, they will give us the means of ascertaining with sufficient precision what as yet is not so known—the actual distance of the earth from the sun.

This knowledge is all‐important in a scientific point of view. From it we can deduce the distance of every other planet of the solar system. With it we can carry our survey beyond that system into the stellar world. The distance of our earth from the sun—the orbital radius of the earth, is, for the astronomer, his unit of measure—his yard‐stick, as it has been termed—when he would estimate or measure stellar distances or velocities. Any error in it is multiplied millions of times in such surveys. Any uncertainty or reasonable apprehension of error about it casts a cloud of embarrassment over almost every portion of the newly acquired domain of astronomy. No wonder, then, that no effort is spared to secure as soon as possible, and in the easiest and most certain way we know of, an accurate solution of the question. This, more than anything else, is the spring of the whole movement.

The earth, as all know, revolves, as do the other planets, round the sun, not precisely in a circle, but in an oval or ellipse not differing much from a circle. The length of our year, or time of one complete revolution of the earth around the sun, is 365 days, 5 hours, 48 minutes, 49.657 seconds.

Inside the earth, and next to us, among the planets, comes Venus, revolving around the sun in her elliptical orbit in 224 days, 16 hours, 48 minutes, and 42 seconds.

Were both orbits on the same level, in the same plane, Venus and the earth would come to be in the same direction or line from the sun as often as Venus, moving on her inner and shorter course, and more rapidly, would overtake the more sluggish earth. Such conjunctions would happen once in every 584 days nearly; and every such conjunction would show a transit, and Venus could be seen between the earth and the sun. But the orbits, though both around the same sun, are not on the same level. That of Venus is somewhat tilted up or inclined, so that one‐half of it lies above the level of the earth’s orbit, and the other half sinks correspondingly below. The line where the orbits cross or intersect each other is the nodal diameter, the only one common to both orbits. Venus overtakes the earth regularly, but ordinarily elsewhere than on or in the immediate vicinity of this nodal line. The planet then, in her apparent journeying from one side of the sun to the other, generally seems to pass near that luminary, either to the north or the south of it. But whenever, as sometimes happens, Venus overtakes the planet on the line of the nodes, either as she is descending on her orbit on one side, or ascending on the other, then the planet is seen to pass across the sun, and there is a transit. It is not necessary that Venus should be precisely on the line uniting the earth’s centre to the sun’s centre. The apparent size of the sun, 32’ in diameter, and the size of the earth, and the smallness of the angle of inclination between the orbits, all combine to give a little latitude in the matter. The earth arrives punctually every year at one end of this line in June, and at the other in December. The astronomical question is, When will Venus be there also at the same time? To answer requires a calculation which appalls. First, there is the planetary velocity proper of Venus, varying according as in the various parts of her elliptical orbit she is nearer to or further from the sun. Then there are the influences of planetary attraction—the earth and the other planets acting on Venus, accelerating or retarding her movements, and tending sometimes to draw her to one side of her orbit. Then there is or may be question of that nodal diameter shifting its position, and trying, as it were, to swing round the circle of the earth’s orbit. When all these calculations have been made, the diurnal movement of the earth must be taken into account, and the geography of her surface must be duly studied, to determine finally when the transit will take place, across what portion of the sun’s face the planet will be seen to travel, and from what portion of the earth’s surface that transit can be seen, and where in that portion stations for observing it can be placed with the greatest probability of success.

It is a fearful sight even to look over a seemingly endless series of pages all bristling with serried columns of figures, broken every now and then by mysterious formulas of higher calculus, like a group of officers commanding a brigade. Mathematicians and astronomers may delight in them; we shall be satisfied to take the results.

The transits of Venus go in pairs eight years apart. There can be only one pair to a century; some centuries will have none. The pairs occur alternately in June, as Venus descends from the upper to the lower half of her orbit, and in December, as she ascends again from it. Thus there were transits in December, 1631, and December, 1639. A second pair occurred in June, 1761, and June, 1769. A third pair is near at hand, in December, 1874, and December, 1882. The next century will have none. The fourth pair will appear in June, 2004, and June, 2012.

So much on the character of that dark little round spot, the passage of which across the sun hundreds of astronomers, with all manner of telescopes, spectroscopes, and photographic instruments, will watch, examine, measure, and record, as they see it sweeping on in its course on the 8th of next December. What will be the special purpose animating observers as they view the transits of 2004 and 2012—if, despite the prophetic and apocalyptic Dr. Cumming, the world lasts till then—no one can now tell. Astronomy by that time may be advanced as far beyond the present state of the science as the present state surpasses the state of two centuries ago. It is probable that new and, to that generation, most interesting questions may have then arisen, which they will strive to solve by their observations of the transits—questions now perhaps undreamed of. But at present our astronomical world is deeply impressed with the advantage and necessity of definitely ascertaining the distance of the earth from the sun. This is the paramount, though by no means the only, purpose of all this expenditure of time and skill and money in preparing for, in making the observations, and afterwards in laboriously working out the results.

How, by merely looking never so attentively at an object whose distance you do not know, as it stands in a line with, and perhaps far in front of, another, likewise of unknown distance, you can tell how far off that second object is, may seem as difficult as the king’s requirement of the prophet first to tell him the dream he had forgotten, and then to explain its meaning. It might seem almost an impossibility; but a few words will explain how the difficulty is turned by availing ourselves of other data.

When two planets, as is the case with the earth and Venus, both revolve in elliptical orbits around the sun, in virtue of the law of gravitation, then their respective times of orbital revolution are to each other as the cubes of their respective mean distances from the sun.

This is one of the laws of Kepler. It was announced by him as the wonderful result of seventeen long years of calculations. He took the data given by the observations of Tycho Brahe and of others, and those made by himself. He tried, by every imaginable form of arithmetical supposition, to combine them together somehow, and under the form of some mathematical law. This was his last result, perhaps the most surprising result of hard plodding, long‐continued labor in the field of science. All honor to his memory. There are few discoveries in the mathematics of astronomy to be compared to this and the other laws of Kepler. He established them as experimental facts. The mathematical reason of them he did not learn.

Since his day, gravity has been discovered to be the bond which binds the solar system together, and its laws have been studied out. The differential and integral calculus, also discovered and perfected since his day, has enabled the scholar to grapple with intricate questions of higher mathematics, which, without its aid, would have remained insoluble. Availing themselves of the laws of gravity and of the aid of the calculus, astronomers have been able to give us a mathematical demonstration of Kepler’s laws, which, from being mere isolated facts or numerical coincidences, have passed into the realm of scientific truths.

Now, we know the length of our own year—365.2422414 days; we know also the length of the year of Venus—224.70048625 days. If we divide the former by the latter, square the quotient, and then extract the cube root of this quotient, we shall obtain the number which indicates the proportion between the two mean distances. Applying this, we learn that if the distance of the earth from the sun be taken as 100,000,000 miles, the mean distance of Venus will be 72,333,240 miles. And consequently, when they are in the same direction from the sun, and supposing both to be at their mean distances from that luminary, the distance between them must be, according to the same proportion, 27,666,760 miles. It is obviously enough to know the actual value of either of those three distances to learn very easily the other two. The observations of the transit are intended to ascertain the last and smaller one. How this is done, and what difficulties are to be surmounted in doing it, we shall see further on. Just now we will remark that supposing the observer to have ascertained to the very furlong this distance, during the transit, between the planets, he must still do much before he can apply his proportion. That holds good only for the mean distances. There are only two points in the orbit or ellipse of each planet around the sun which are at the mean distance from that focus. Were those points for both planets to be found on the lines of the nodes, the matter would be easy. But it is not so. In June, the earth is approaching her greatest distance; in December, she is nearing her smallest distance from the sun. A similar embarrassment exists for the orbit of Venus. But the astronomer can bravely grapple with this double difficulty. He has learned the eccentricity and consequent shape of each ellipse, and he can calculate how far, proportionately, the actual distance of either planet, at any given point of its orbit, exceeds or falls short of the true mean distance. Such calculations have to be made for the earth and for Venus as they will stand on the 8th of next December. When this is done, the astronomer is at liberty to make use of the actual distance learned by observation, and to apply the Keplerian formula.

But perhaps the question suggests itself, why take all this trouble of a circuitous route? Why not measure the distance of the sun directly, if such things can be done at all? If it is possible to measure the distance of Venus by observations, surely the sun, which has an apparent diameter thirty times as great, and which can be seen every day, and from any accessible point of the earth’s surface, gives a far ampler field for such observations. If we have instruments so delicate as to disclose to us the presence in the sun of iron, copper, zinc, aluminium, sodium, manganese, magnesium, calcium, hydrogen, and other substances, surely it will be possible to determine that comparatively gross fact—its distance from the earth. And, in truth, what becomes of the lesson we learned in our school‐ days, that the sun was just ninety‐five millions of miles away from us?

And yet, strange as it may seem to those unacquainted with the subject, it has been found impossible to decide, by direct observations, the actual distance; and the distance usually accepted was not derived from such observations. As for our lately acquired knowledge of some of the constituent substances of the sun, that is derived from the spectroscope, which as yet throws no light on the question of distance.

How do we ascertain the distance of bodies from us? Practice enables us to judge, and judge correctly, of the distance and size of things immediately around us almost without any consciousness of how we do it. But if we analyze the process, it will be found that we do it chiefly by using both eyes at the same time. They are separated by an interval of two and a half to three inches. As we look at an object near to us, the rays from each visible point of it must separate, in order to enter both eyes. The images thus formed on the retina of each eye differ sensibly, and we instinctively take cognizance of that difference. Speaking mathematically, the interval is a base line, at each end of which a delicate organism takes the angle of the object viewed, and our conclusion is based on our perception of the difference between them. Ordinarily, we estimate distances by the cross‐sight thus obtained. When, however, the body is so far off that the lines of light from it to the eyes become so nearly parallel that the eyes fail to perceive the minute difference between the representations formed on the retina, then we must recur to the results of past experience, and judge, as best we may, of the distance from other data than that given us at the moment by our eyesight. Thus a sailor at sea judges of the distance of a vessel on the horizon from the faintness with which he sees her; for he knows that the intervening atmosphere absorbs some of the light, so that distant objects are dim. He judges from the fact that a vessel of the form and rig of the one he is looking at is usually of a given size, and a certain distance is required to cause the entire vessel to look so small, and certain portions, the size of which he is familiar with, to become indistinguishable. He is guided, also, by the amount to which, on account of the earth’s curvatures, the vessel seems to be sunk below the horizon. These are data from experience. It is wonderful with what accuracy they enable him to judge. A landsman by the seaman’s side, and without such aid, could give only the most random guesses as to the distance of the vessel.

That we really do make this use of both eyes in judging of the distance of bodies near us will be evident if we bandage one eye and try to determine their distances, only using the other. It will require caution to avoid mistakes. We knew an aged painter, who had lost the sight of one eye, but still continued to play, at least, with his brush. He had to use the finger of his left hand to ascertain by touch whether the tip of his brush, loaded with the proper color, was sufficiently near the canvas or not. If he relied on his eye alone, it often happened that when he thought it near, not the eighth of an inch away, it failed in reality by an inch and a half to reach the canvas. He would ply the brush, and, noticing that the color was not delivered, would smile sadly at what he called his effort to paint the air. So long as he had retained the use of both eyes, this mishap, of course, had never occurred to him.

When a surveyor desires to ascertain the distance of a visible object which he cannot approach, he must avail himself of the same principle of nature. He measures off on the ground where he is a suitable baseline, and takes the angle of the object from each end of it, not vaguely by his unaided eyesight alone, but with a well‐graduated instrument. It is, as it were, putting his eyes that far apart, and taking the angles accurately. From the length of the measured base‐line and the size of the two angles he can easily calculate the distance of the object. In taking such measurements, the surveyor must make his base sufficiently large in proportion to the distance sought. If the base be disproportionately small, the angles at the extremities will not serve. Their sum will be so near 180° that the possible errors which are ever present in observations will more than swallow up the difference left for the third angle, and the distance is not obtained. In our excellent Coast Survey, which, in exactness of working, is not surpassed anywhere in the world, the bases carefully measured may be five or seven miles long, and angles under 30° are avoided when possible.

From such measuring of distant objects on the surface of the earth, the passage was easy to an attempt to measure the distance of heavenly bodies. How far is the moon from us? It was soon found that a base of ten miles or of a hundred miles was entirely too short to give satisfactory angles. The moon was too distant. A far larger base was required. Suppose two places to be selected on the same meridian of longitude, and therefore agreeing in time, and situated sixty degrees of latitude apart. The distance between them will be equal to a radius of the earth. At each station, and at the same hours, the angles are taken which the moon makes with the zenith, or, better still, with some star near it, coming to the meridian at the same time. In such a case, the angles are, satisfactory. The base is large enough. The result of such observations, and of others which we need not dwell on, is that, when nearest to us, the centre of the moon is distant from the centre of the earth 222,430 miles; when at her greatest distance, 252,390 miles. These numbers are based on the fact that the equatorial radius or semi‐diameter of the earth is 3962.57 miles. This value, however, may in reality be a quarter of a mile too short. The mean distance of the moon is roughly stated at 60 semi‐diameters of the earth.

When observers essayed to apply to the sun the same procedure which had proved so successful in regard to the moon, they encountered disastrous failures, partly because the base, even the largest practicable one, was found to be comparatively very small; partly because, when the sun shines, no star is visible near by from which to measure an angle; and also because the atmosphere is so disturbed by the rays of solar heat that, when seen through a large telescope, the sun’s edge is quite tremulous. Hence a very large element of uncertainty is introduced when angles are taken with the zenith. No astronomer would look with confidence on the result obtained under such circumstances. Two hundred years ago, their instruments were much less perfect than those we now have; yet, even with our best instruments, to‐day, too much uncertainty remains. That mode of ascertaining the sun’s distance has been abandoned.

Ancient astronomers, long before the invention of telescopes, and before the discovery of the Copernican system, devised an ingenious method of getting some light on the distance of the sun. It is attributed to Aristarchus of Samos. They reflected that, when the moon appeared precisely half full, this arose from the fact that the sun and the earth were at right angles to her; the sun illumining the half turned to him, and the plane of division between the illumined and unillumined portions extended stretching directly to the earth. They conceived the three bodies to stand at the angles of a right‐angled triangle, of which the distance of the moon from the earth was the base, and the distance of the sun was the hypothenuse. Hence they had only to measure the angle at the earth, which they could do, and then take into account their estimate of the moon’s distance, to arrive at the result sought. The plan is ingenious, and taught them that the sun was at least twenty times further off than the moon. But their estimate of the moon’s distance was altogether wide of the mark. They had no means of correctly estimating it. Moreover, even keen eyesight is a bad judge of whether the moon is precisely half full or not. The error of half a dozen hours would give a large mistake. Even with instruments such as we have, it cannot be precisely determined by direct observations; for the surface of the moon, as developed in a powerful telescope, is so uneven, jagged, and volcanic that the division between light and shade is a line too uneven and broken to be determined except by guessing at its mean course.

Another method has been also used in these later centuries. Kepler’s law applies to all the planets. The planet next outside the earth is Mars, whose mean distance from the sun is about one‐third greater than that of the earth. It periodically happens that Mars is in opposition—that is, is precisely on the other side of the earth from the sun. In that case, he makes his nearest approach to our planet. Cannot his distance from the earth be then observed and determined, so that he will give us the means of calculating by Kepler’s formula the distance of the sun? It was tried, and with some success. The base‐line was found large enough; the observations were made at night, when the atmosphere is comparatively quiescent, and when fixed stars may be seen in the vicinity of the planet, to aid in taking the requisite angles. Yet, as in the case of Venus, there are, as we have stated, subsidiary calculations to be made on account of the eccentricity of his orbit and his varying velocity. In the case of Mars, these variations were too full of anomalies to allow confidence in the calculations. When afterwards these anomalies were understood to proceed from interplanetary attraction, they were so complicated that their numerical value almost escaped calculation. The whole subject has been gone over in our own day under the light of more perfect observations, and with the aid of the highest calculus. We doubt, however, if even now the results are sufficiently established to warrant a calculation as to the sun’s distance to which reasonable exception may not be taken.

Anyhow, this method cannot compare, either in facility of calculation or in accuracy of result, with the method of determining the solar distance by observations for the transit of Venus.

Of the theory and mode of such observations we will now say a few words.

In 1677, while Halley, the great English astronomer, was at St. Helena, for the purpose of observing and cataloguing stars south of the equator, he observed a transit of Mercury across the face of the sun, and, from his efforts to measure its positions and movements, was led to believe that a transit of Venus could be so accurately observed and measured as to yield a precise and definite determination of the sun’s distance. From the knowledge he had of the movements of Venus, he knew that there had been a transit of Venus in 1631, as Kepler had predicted, although no eye in Europe had seen it; and another in 1639, which had been observed, but, of course, not for this purpose, which in 1639 was yet unthought of. The next transit would be in 1761. He could not hope to live to see it. But he did the next best thing. He studied out all the conditions of the question, published his plans, and made all the preliminary calculations required, so as to aid in securing, as far as possible, good observations and good results when the time came.

As the year 1761 was approaching, the scientific world was astir, pretty much as it is now. Halley’s computations were again gone over, and such corrections and improvements were introduced as the advance of astronomy since his day warranted and required. Governments gave their aid and supplied means liberally. One hundred and twenty positions had been carefully chosen, and the best results were confidently expected. The grand problem was about to receive a final and definite solution. The error in the ultimate result would certainly not exceed one‐fifth of one per cent.

The astronomers were doomed to a sad disappointment. Wars then waging prevented some of the most important positions from being occupied by the observers. It was bitter for a well‐appointed party to sail for months and months over two oceans, only to see a hostile flag floating over the port they were about to enter. Sadly they sailed away, and could only see the transit from the rolling deck of their ship. Cloudy weather rendered other positions valueless. And even where everything seemed to promise success, an unforeseen phenomenon interfered to mar their work. The astronomer might have his best telescope duly mounted, and directed to the proper point of the heavens, and carefully adjusted; his eye might be glued to the instrument, as he watched on one side of his field of vision a portion of the circular edge of the sun’s disk, and on the other the round, black spot gradually approaching. As they drew near, his hand was raised to give the signal; his assistant stood ready to mark the very second when the two edges, coming nearer and nearer, would at last just touch. They hoped to seize the time of that first contact so accurately as to escape even the one second of error or doubt which Halley thought unavoidable. Vain hope! Before the contact, while Venus was still distant about two‐thirds of her own diameter from the edge of the sun, a dark streak or band seemed to interpose between them like a black cushion or wedge. As they pressed against it, the curved outlines of their edges seemed to be pressed back or flattened, as if by the resistance of the cushion, and to lose their normal shape. There was a pause in the onward movement, a quivering, a struggle, and then, by an irregular, convulsive jump, like that of two drops of water coalescing into one, Venus was seen to have already entered some way on the disk of the sun. The discomfited and astonished observer was forced to record that his uncertainty as to the precise time of the contact was not of one second only, but of at least twelve or fifteen seconds. Was it the defect of the instrument, or the fault of his own eye, over‐strained by long use, by the brilliant light, or by his intense anxiety? Or was there some unknown atmospheric cause at work producing this band? Anyhow, he might hope that other observers would be more fortunate than he had been. Again he was in error. Everywhere the same unexpected and puzzling phenomenon appeared. There was trouble in the astronomical world. The fault was generally thrown on the instruments. But whatever the cause of the mishap, there was some room for consolation. They would soon have another opportunity, and might make another trial. In 1769, only eight years off, there would be another transit, and by that time some means would certainly be devised for escaping the evil.

In 1769, the stations were as numerous, the governmental aid fully as great, the instruments, they said, more perfect, and the observers, we may be sure, as earnest and as careful as before. Perhaps they were more skilful because of their previous experience. But again all in vain. The same evil reappeared. The resulting uncertainty was even greater. It was held to reach fully twenty seconds. When they undertook to calculate, from such observations, the distance of the sun, some made it not more than 87,890,780 miles, while, according to others, it reached 108,984,560 miles, the majority finding intermediate values. On the whole, it did not appear that there was much improvement on the estimate made by Cassini a century and a half before, that it was not less than 85,000,000 miles. Again and again were the records of the observations studied, scrutinized, and weighed, and the calculations based on them repeated and criticised. Finally, in 1824, Encke, after several years of special study of them, summed all up, and gave, as the best result attainable, 95,274,000 miles. The scientific world, hopeless of anything better, seemed for a time to acquiesce. Some even upheld the estimate of Encke as “so successfully determined as to leave no sensible doubt of its accuracy.”

But, despite this, its accuracy has since been impugned, and on very strong grounds. It was known that light travels from the sun to the earth in about 8 minutes 13 seconds. Experiments carefully and ingeniously made by Arago, Foucault, and Fizeau show that light travels with a velocity of nearly 186,000 miles a second. This would give the distance of about 91,400,000 miles.

The irregularities of the moon and of Mars have been studied out and calculated on the theory of interplanetary attraction modifying the attraction of the sun. Though the results vary somewhat, yet they all tend in the same direction. Leverrier found 91,759,000 miles; Hansen, the Dane, found 91,659,000 miles; Airey, the Astronomer‐Royal of England, whose earlier opinion of Encke’s estimate we quoted above, has changed his opinion, and now proposes 91,400,000 miles.

A fact in practical optics, calculated to affect some observations rather seriously, has been discovered within the last few years. It is this: When a white body is viewed on a dark ground, its size is exaggerated by some illusion of our vision; and, on the contrary, a dark body seen on a bright ground appears smaller than it would were the ground of a dark color, differing from that of the body only as much as is required to render them distinguishable. Now, in the transit, a dark body is seen on an intensely bright ground. It becomes necessary, therefore, to bring in a correction which will compensate for the error arising from this optical illusion. This has been done by Stone, who studied out the whole matter, arrived at certain modes of correction, applied them to Encke’s calculation, and maintains that the true result of the observations of 1761 and 1769 should be 91,730,000 miles.

Thus all seem to agree that the sun’s distance must be less than 92,000,000 miles, and that Encke’s estimate was too great by 3 or 4 per cent.

This is the stage at which our astronomers now take up the question, and aim to obtain a yet more definite and precise result. Will they succeed? They are full of confidence now; what they will say after their observations we may know a year hence.

Some of our readers may like to know what is the course followed in making the observations and in calculating the results. We will give a slight account of the chief points, sufficiently detailed to enable one with an ordinary knowledge of trigonometry to understand how the conclusion is reached.

The astronomers will follow two methods, known as those of Halley and of Delisle. They each require two suitable stations, so far apart on the surface of the earth as to give a satisfactory base‐line. In fact, the further apart, the better, all things else being equal. For Halley’s method, the two stations lie as nearly north and south as may be. For Delisle’s, they lie east and west.

Let us suppose two such stations to be chosen on or nearly on the same meridian of longitude, and 6,000 miles apart. From each of these stations the planet is seen to traverse the disk of the sun, like a dark spot moving steadily across an illuminated circular dial‐plate. The lines as seen from stations so far apart are sensibly different. What the observers first seek to know is the apparent distance between these lines, the angle they form, when seen from the earth. Were both visible at once from the same station, through the same telescope, it would not be difficult for a skilful observer to measure the angle directly. But at each station only one line is seen, if, indeed, we may properly give that name to the course of the dark spot that passes on and leaves no trace behind. Each observer must determine correctly the position of his line on the face of the sun, in order that it may be afterwards compared with the other line similarly determined at the other, and the apparent distance between them is then determined by calculation.

How to determine the true position of such a line is the delicate and difficult task. One mode is to take the measurements in two directions on the face of the sun, northward and eastward, from the position of the planet to the edge of the solar disk. This must be done for a number of positions which the planet occupies successively as it moves onward. But such measurements are very hard to be obtained with the desired precision. The edge of the sun, viewed in a large telescope, appears always tremulous, on account of the action of solar heat on our own terrestrial atmosphere. The better and larger the telescope, and the brighter the day, the greater and the more embarrassing does this tremulousness appear. Such measurements are difficult, and are open to too much uncertainty.

There is another mode, which, if successfully used, is far more accurate. The lines or paths which the planet, viewed from the observatories, is seen to follow are chords across a circle—largest when they pass through the sun’s centre and become diameters, smaller as their course is more distant from the sun’s centre. Being both due to the motion of the same body moving at what we may hold to be a uniform velocity, their lengths must be proportional to the times required for tracing them. Being chords, a knowledge of their relative lengths determines with accuracy their position on the circular disk of the sun, and consequently their distance apart. Hence the importance of catching, with the utmost exactness, the beginning and the ending of the transit. The first exterior contact is noted when the circular edge of Venus just touches the circular edge of the sun; then the first interior contact when the entire little, dark circle of Venus is just fully on the sun. Midway between the two, the centre of Venus was just on the edge of the sun. Similarly, the second interior contact and the second exterior contact, if accurately and successfully observed, will show the instant of time when the centre of Venus passed off from the sun’s surface. It was, as we saw, in making these delicate observations, that the observers of 1761 and 1769 failed, to a great extent, on account of the mysterious appearance of the black band, of which we gave an account. Will this embarrassing phenomenon again make its appearance next December? If it be due, as some think, to an aberration of sphericity in the lenses of the instruments, it may not be seen. For our telescopes are far more perfect than those of 1769. If it is due, as others maintain, to an interference of light in the observation, a more delicate manipulation of the instrument may, it is hoped, avoid it. If it is due to some optical illusion in our own eye, it will, of course, appear again, and must be grappled with. The observers now being trained at Greenwich, in preparation for the grand day, have a facsimile of the sun and Venus, which are made to move in such manner as to give as exact a representation of the transit as is possible; and they practise observations on this artificial transit. It is said that even in this fac‐ simile the black band has shown itself, and that one important lesson now being learned is how to judge of the instant of contact, despite of this obstacle.