The recent progrss of astronomy; especially in the United States

Preface

This little volume is designed to exhibit in a popular form the most important astronomical discoveries of the past ten years. It does not profess even to enumerate all the discoveries of this period. Such an enumeration would have swelled the volume much beyond its present size. Those topics have been selected in which it was supposed the public generally would feel the deepest interest. The book was designed, not for astronomers, but for the pubhc ; and I have accordingly aimed to exhibit results in common language, as free as possible from technical terms. It is hoped that the book may be read with profit by those who have never made Astronomy a subject of special study ; and that it may be found an appropriate supplement to the popular treatises on this science, most of which are very deficient in their notice of recent discoveries. It is hoped that the fourth chapter, which exhibits the progress of Astronomy in the United States, may be found to have a special interest for American readers. They may here see how far we are deserving of the compliment recently bestowed by

IV PREFACE.

the Astronomer Royal of Great Britain ; " The Americans of the United States^ although late in the field of astronomical enterprise, have now taken up that science with their characteristic energy, and have already shoivn their ability to instruct their former masters^

I have endeavored to award equal and exact justice to all American astronomers ; and if any individual should feel that his labore in this department have not been fairly represented, he is requested to furnish me in writing a minute account of the same, and I will endeavor to make amends in a second edition of the M'ork.

Chapter I. Recent Additions To Our Knowledge Of The Planetary System

SECTION" I.

Page

THE DISCOVERT OF THE PLANET NEPTUNE 9

Irregularities iii the Motion of Uranus, .... 10

Labors of Mr. Adams, 16

Labors of M. Le Terrier, It

Discovery of the Planet by Dr. Galle, .... 24

Search for old Observations, 28

True Orbit of the Planet, 33

Name given to the Planet, 35

Is Neptune surrounded by a Ring ? 40

Discovery of a Satellite, . . . . . . .43

Will Neptune account for the Anomalies of Uranus ? , 45

Is Neptune the Planet predicted by Le Verrier ? . . .48 Why was not the Planet sooner discovered ? . . . 56

SECTION IL

THE DISCOVERY OF SEVERAL ASTEROIDS, .

Six new Asteroids,

Are they all Fragments of a single Body ?

60 63 66

SECTION in.

DISCOVERY OF AN EIGHTH SATELLITE OF SATURN, .

SECTION IV.

NEW OBSERVATIONS OF THE SATELLITES OF URANUS,

VI COKTENTS.

CHAPTER 11.

RECENT ADDITIONS TO OUR KNOWLEDGE OF COMETS.

Chapter III. Additions To Our Knowledge Of Fixed Stars And Nebula

DETERMINATION OF THE P.VRALL.\X OF FIXED STARS, . .116

Xabors of Bessel, 118

Parall^ of, 61 Cygni and other Stars, ..... 120

CONTENTS.

Vll

SECTION 11.

Page OBSERVATIONS OF NEW AND VARIABLE STARS, . . . 122

The star Eta Argus, . . . . . . . .123

Hind's Star of 1848, 124

Kepler's Star of 1604, . .125

SECTION III.

DISTRIBUTION OF THE STARS IN SPACE, 128

Herschel's View of the Milky "Way, . . . . .128

Abandoned by its Author in 1817, 132

Struve's Researches on the Milky "Way, . . . .133

Sir J. Herschel's Observations in the Southern Hemisphere, 13*7

SECTION IV.

MOTION OF THE SUN AND FIXED STARS, 144

Point of Space toward which the Sun is moving, . . 145 Madler's Speculations respecting the Central Sun, . , 14*7

SECTION V.

RESOLUTION OF REMARKABLE NEBULA, . . . . .151

Lord Rosse's Telescope, ...*... 151

Various Nebulae resolved, 153

Nebulae resolved by the Cambridge Telescope, . . 156

Section I. History Of American Observatories

Transit of Venus in 1769, Yale College Observatory, Williams College Observatory, Western Reserve College Observatory, Philadelphia High School Observatory, West Point Observatory, .

National Observatory at Washington, . Georgetown Observatory, D. C,

Vlll CUN TENTS.

Page

Cincinnati Observatory, , .182

Cambridge Observatory, 184

Sharon Observatory, near Philadelphia, . . . .194

Tuscaloosa (Alabama) Observatory, 196

Mr. Rutherford's Observatory, N. Y., 196

Dartmouth CoUege Observatory, 198

Amherst College Observatory, ...... 200

Brooklyn Observatory, 200

Section II. Astronomical Expedition To Chili, 203

SECTION" IIL

ASTEONOMICAL RESULTS OF TUBLIC SURVEYS, . . , 201

SECTION IV. / A"l^ DETERMINATION OF LONGITUDE BY THE ELECTRIC TELEGRAPH, 212

^^ Experiments between New York and Washington, . .213

Experiments between New York and Cambridge, . . 218

Dr. Locke's Electro-magnetic Clock, 223

Apphcation to Astronomical Observations, . . . 225

Notice of similar Inventions, 230

Different Modes of Registering, 234

Velocity of the Electric Wave, 235

Section I. The Discovery Of The Planet Neptune

The discovery of the planet Neptune, took place under circumstances most extraordinary. The existence of the planet was predicted, its path in the heavens was assigned, its mass was calculated from considerations purely theoretical. The astronomer was told where to direct his telescope, and he would see a planet hitherto unobserved. The telescope was pointed, and there the planet was found. In the whole history of astronomy we can find few things equally wonderful. This discovery resulted from the study of the motions of the planet Uranus.

Uranus was first discovered to be a planet in 1781, but it had been repeatedly observed before by different astronomers, and mistaken for a fixed star. Nineteen observations of this description are on record, one of them dating as far back as 1690. In 1821, M. Bouvard, of Paris, published a set of tables for computing the place of this planet. The materials for the construction of these tables consisted of forty years' regular observations at Greenwich and Paris since 1781, and the nineteen accidental observations, reaching back almost a century further. Upon comparing these observations, Bouvard found unexpected difficulties. He was unable to combine all the observations in one elliptic orbit. When he attempted to unite the ancient with the modern observations, the former might be tolerably well represented, but the latter exhibited discordances too great to be ascribed to errors of observation. Not being able to explain this discrepancy in any satisfactory manner, he rejected the ancient observations, and founded his tables upon the observations since 1781. "It being necessary," says he, " to decide between the ancient and the modern observations, I have held to the modern ones as being the most likely to be accurate, and I leave it to time to show whether the difficulty of reconciling the two sets of observations depends upon the inaccuracy of the ancient ones, or on some foreign and unknown injiuence to which the planet is subjected."

These tables represent very well the observations of the forty years from which they w^ere derived ; but soon after 1821, new discrepancies began to appear, which have lately increased with great rapidity. In 1832, the discordance between the observed and computed place of the planet, amounted to near half a minute of space, and now the error exceeds two minutes. In order to exhibit more palpably the nature of these discrepancies, I have represented them upon the figure on the next page. If the straight line, A B, be taken to represent the path of Uranus, as computed from the elements of Bouvard, the broken line will represent the observed orbit. The deviation of these two lines indicates the discrepancy between theory and observation for the dates at the top of the page, the amount of the discrepancy being given on the left margin. If it w^as doubtful whether this difference in the case of the ancient observations was not due to the carelessness of the observers, no such supposition is admissible in the case of the observations since 1821. The discrepancy between Bouvard's orbit and the observations is now enormous, and is increasing with alarming rapidity.

What can be the cause of these discrepancies ? Do they indicate errors in the computations of M.

j-)/-s(-)^0 O OOOO OOOQOOQ t^ t-

A

A

-80"

B

J y

A >

Bouvard ? In order to decide this question, the illustrious Bessel, about the year 1840, subjected all the observations to a new calculation ; and although he detected one or two errors of Bouvard, they did not materially influence the results. He satisfied himself that the ancient and modern observations could not be reconciled by any modification of the elements, and that the differences could not be attributed to inaccuracy of instruments, or to methods of observation. Are these anomalies due to the attraction of some unknown disturbing body ? This idea was seriously entertained more than twelve years ago by Bouvard, Hansen, Hussey, Bessel, and some others. In a public lecture delivered in the vear 1840, Bessel stated, " I have arrived at the full conviction that we have in Uranus a case to which Laplace's assertion, that the law of gravitation explains all the motions observed in our solar system, is inapplicable. We have here to do with discordances whose explanation can only be found in a new physical discovery. Farther attempts to explain them must be based upon the endeavor to discover an orbit and a mass for some unknown planet, of such a nature, that the resulting perturbations of Uranus may reconcile the present want of harmony in the observations."

Mr. Hussey, in 1834, proposed to compute an approximate place of the supposed disturbing bod}^ and then commence searching for it with his large reflector. Mr. Airy, now Astronomer Royal of Great Britain, at that time professor in Cambridge, pronounced the problem hopeless. His words were : " If it were certain that there was any extraneous action upon Uranus, I doubt much the possibility of determining the place of the planet which produced it. / am sure it could not be done till the nature of the irregularity ivas well determined from several successive revolutions ;" that is, till after the lapse of several centuries.

This deliberate opinion from one who, by common consent, stood at the head of British mathematicians and astronomers, would have deterred any but the most daring mathematician from attacking the problem. Again in 1837, Mr. Airy repeats the same idea : " If these errors are the effect of any unseen body, it will he nearly impossible ever to find out its place."

In the year 1842, the Royal Society of Sciences of Gottingen, proposed as a prize question, the full discussion of the theorv of the motions of Uranus, with special reference to the cause of the large and increasing error of Bouvard's tables. During the same year, 1842, Bessel was engaged in researches relative to this problem ; but his labors were soon interrupted by sickness and subsequent death ; and from this time we find but two mathematicians, Mr. Adams of Cambridge University, in England, and M. Le Verrier, of Paris, who busied themselves with the problem.

It should be remembered, that in accordance with the Newtonian law of gravitation, every body in the solar system attracts every other ; that the attraction of each body is proportioned to its quantity of matter ; and that in the same body the power of attraction varies inversely as the square of the distance. In order, therefore, to compute the exact place of a planet in its orbit about the sun, it is necessary not merely to regard the attraction of the central body, but also to allow for the influence

of all the other bodies of the solar

system. For instance, if the sun alone had acted, Jupiter w^ould revolve around the central luminary in a perfect ellipse, which we may represent by the annexed black curve ; but the real orbit vibrates around this curve as in the dotted line, a curve which is not reproduced at every revolution, but will pass through an infinite number of variations. To compute the exact orbit which a planet will describe, subject to the attractions of all the members of the solar system, is one of the grandest problems in astronomy.

Hitherto, mathematicians had only aspired to compute the disturbing influence of one body upon another, when the magnitude and position of both bodies were known. But in the case of Uranus it was necessary to solve the inverse problem which Professor Airy had pronounced hopeless, viz., from the observed disturbances of one body, to compute the place of the disturbing body.

After taking his degree of Bachelor of Arts in January, 1843, with the honor of Senior Wrangler, Mr. Adams ventured to attack this problem, and obtained an approximate solution by supposing the disturbing body to move in a circle at twice the distance of Uranus from the sun. His results were so far satisfactory, as to encourage him to attempt a more complete solution. Accordingly, in February, 1844, having obtained through Professor Airy a complete copy of the Greenwich observations of Uranus, he renewed his computations, which he continued during that and the subsequent years. In September, 1845, he had obtained the approximate orbit of the disturbing planet, which he showed to Professor Chalhs, the director of the observatory at Cambridge; and near the close of the next month, he communicated his results to the Astronomer Royal, together with a comparison of his theory with the observations. The discrepancies were quite small, except for the single observation of 1690. Professor Airy, in acknowledging the receipt of this letter, pronounced the results extremely satisfactory, and inquired of Mr. Adams whether his theory would explain the error of the tables in regard to the distance of Uranus from the sun, which error he had shown to be very great. To this inquiry Mr. Adams returned no answer for nearly a year ; probably because he was not able to answer the question entirely to his own satisfaction.

Meanwhile, this grand problem was undertaken by another mathematician who was entirely ignorant of the progress which Mr. Adams had made ; for none of his results had yet been published. In the summer of 1845, M. Arago, of Paris, requested M. Le Verrier, a young mathematician who had already distinguished himself by his improved tables of Mercury, to attempt the solution of this problem. This he accordingly did, and his success astonished all Europe. He commenced his investigations by inquiring whether the observations of Uranus could be reconciled with the supposition, that this body is subject to no other attraction than that of the sun and the known planets, acting according to the Newtonian law of gravitation. He carefully computed the effects due to the action of Jupiter and Saturn, neglecting no quantities until he had proved that their influence was insensible. He thus discovered some important terms which had been neglected by Laplace. He then compared his theory with observation, and proved conclusively that the observations of Uranus could not be reconciled with the law of gravitation, except by admitting some extraneous action. These results were communicated to the Academy of Sciences, Nov. 10, 1845 ; and such was the reputation secured by this and his preceding memoirs, that in January, 1846, he was elected to fill the vacancy which had occurred in the Institute in the section of Astronomy, by the death of Cassini. This memoir was but preliminary to his grand investigation ; and it should be remarked, that Mr. Adams had already deposited with the Astronomer Royal at Greenwich, a paper containing the elements of the supposed disturbing planet, and agreeing closely with the results which Le Verrier subsequently obtained.

Le Verrier next proceeds to inquire after the cause of the discovered irregularities. Is it possible that at the immense distance of Uranus from the sun, the force of attraction does not vary inversely as the square of the distance ? The law of gravitation is too firmly established to permit such a supposition, until every other resource has failed. Are these irregularities due to the resistance of a rare ether diffused everywhere through space ? No other planet has aflforded any indication of such a resistance. Can they be ascribed to a great satellite accompanying the planet? Such a cause would produce inequalities having a very short period ; while the observed anomalies of Uranus are precisely the reverse. Moreover, it would be necessary to assign it such dimensions that it could not fail to have been visible in our telescopes. Has a comet impinged upon Uranus, and changed the form of its orbit? Such a cause might render it impossible to represent the entire series of observations by a single elliptic orbit ; but the observations hefor-e the supposed collision, would all be consistent with each other, and the observations after collision would also be consistent with each other. Yet the observations of Uranus from 1781 to 1821, as maybe seen from the diagram, page 12, accord neither with the earher observations nor with the more recent ones.

There seems to remain no other probable supposition than that of an undiscovered planet. But if these disturbances are due to such a body, we can not suppose it situated within the orbit of Saturn. This would disturb the orbit of Saturn more than that of Uranus, while we know that its influence on Saturn is inappreciable, for Saturn's motion is well represented by the tables. Can this body be situated between Saturn and Uranus ? We must then place it much nearer Uranus than Saturn, for the reason already assigned, in which case its mass must be supposed to be small, or it would produce too great an effect upon Uranus. Under these circumstances, its action would only be appreciable when in the immediate neighborhood of Uranus, which supposition does not accord well with the observations. The disturbing body must then be situated beyond Uranus, and at a considerable distance from it, for reasons already given. Now the distance of each of the more remote planets from the sun, is about double that of the preceding one. It is natural then to conjecture that the disturbing planet may be at a distance from the sun double that of Uranus, and it must move nearly in the ecliptic, because the observed inequalities of Uranus are chiefly in the direction of the ecliptic. Le Verrier then propounds the following specific problem : —

" Are the irregularities in the motion of Uranus due to the action of a planet situated in the ecliptic, at a distance from the sun double that of Uranus ? If so, what is its present place, its mass, and the elements of its orbit'' This problem he proceeds to resolve.

If we could determine for each day the precise effect produced by the unknown body, we could deduce from it the direction in which Uranus is drawn; that is, we should know the direction of the disturbing body. But the problem is far from being thus simple. The amount of the disturbance can not be deduced directly from the observations, unless we know the exact orbit which Uranus would describe, provided it were free from this disturbing action ; and this orbit in turn can not be computed unless we know the amount of the disturbance. Le Verrier therefore computes for every nine degrees of the entire circumference, the effect which would be produced by supposing a planet situated in different parts of the ecliptic. He finds that when he locates the supposed disturbing planet in one part of the ecliptic, the discrepancies between the observed and computed effects are enormous. By varying the place of the planet, the discrepancies become smaller, until at a certain point they nearly disappear. Hence he concludes that there is but one point of the ecliptic where the planet can be placed, so as to satisfy the observations of Uranus. Having thus determined its approximate place, he proceeds to compute more rigorously its effects ; and on the first of June, 1846, he announces as the result of his investigations, that the longitude of the disturbing planet for the beginning of 1847, must be about 325°.

The result thus obtained by Le Verrier, differed but one degree from that communicated by Mr. Adams to Professor Airy, more than seven months previous. Upon receiving this intelligence. Professor Airy expressed himself satisfied with regard to the general accuracy of both computations, and immediately wrote to Le Verrier, inquiring as he had done before of Mr. Adams, whether his theory explained the eiTor of the tables in respect to the distance of Uranus from the sun. Le Verrier answered that it did this perfectly. Professor Airy was now so well convinced of the existence of a planet yet undiscovered, that he was anxious to have a systematic search for it forthwith undertaken. The Observatory of Cambridge is provided with one of the finest telescopes of Europe, presented by the late Duke of Northumberland. Professor Airy urged upon the director, Professor Challis, to undertake the desired search, and recommended the examination of a belt of the heavens ten degrees in breadth, and extending thirty degrees in the direction of the ecliptic. This belt was to be swept over at least three times. If any star in the second sweep had a different position from that observed in the first, it might be presumed that it was the planet. If two sweeps failed of detecting the planet, it might be caught in the third.

Professor Challis commenced his search July 29th, and continued it each favorable evening, recording the exact position of every star down to the eleventh magnitude. Meanwhile Le Verrier was proceeding with his computations, and on the 31st of August, he announced to the Academy the elements he had obtained for the supposed planet. He assigned its exact place in the heavens, and estimated that it should appear as a star of the eighth magnitude, \^nth an apparent diameter of about three seconds ; and consequently that the planet ought to be visible in good telescopes, and with a perceptible disc. The fixed stars are situated at such immense distances from us, that to the most powerful telescopes they appear only as points, although with a brilliancy augmented in proportion to the size of the telescope; while all the planets exhibit a measurable disc. Le Verrier was of opinion that the new planet might be found by its possession of a visible disc, and therefore without any very great labor.

Soon after this communication was made to the Academy, Le Verrier wrote to Dr. Galle, of the Berlin Observatory, (where is found one of the largest telescopes of Europe,) requesting him to undertake a search for his computed planet, and assigning its supposed place in the heavens. The Berlin Academy had just published a chart of this part of the heavens, indicating the exact place of every star down to the tenth magnitude. On the evening of the very day upon which this letter was received, (September 23,) Galle found near the place computed by Le Verrier, a star of the eighth magnitude, not contained on the Berlin chart. Its place was carefully measured ; and the observations beinw repeated on the succeeding evening, showed a motion of more than a minute of space. The new star was found in longitude 325° 52' ; the place of the planet computed by Le Verrier was 324° 58' ; so that this body was within one degree of the computed point. Its diameter measured nearly three seconds. A coincidence so exact, left no doubt that this was really the body whose effects had been detected in the motions of Uranus. Mr. Galle accordingly writes to Le Verrier, " The planet whose position you marked out actually exists.^' The news of the discovery spread rapidly over Europe. The planet was observed at Gottingen on the 27th of September, at Altona and Hamburgh on the 28th, and at London on the 30th.

We must now return to Professor Challis, whom we left exploring a large zone of the heavens, and recording the exact position of every star down to the eleventh magnitude. These observations were continued from the 29th of July to the 29th of September, during which time he had made more than three thousand observations of stars. On the 29th of September, Professor Challis saw for the first time Le Verrier's memoir communicated to the Academy August 31st. Struck with the confidence which Le Verrier manifested in his own conclusions. Professor Challis immediately changed his mode of observation, and endeavored to distinguish the planet from the fixed stars by means of its disc. On the same evening he swept over the zone marked out by Le Verrier, paying particular attention to the physical appearance of the brighter stars. Out of three hundred stars whose positions were recorded that night, he selected one which appeared to have a disc, and which proved to be the planet. On the first of October, he heard of the discovery at Berlin ; and now on comparing his numerous observations, he finds that he has twice observed the planet before, viz., on August fourth and twelfth ; but he lost the opportunity of being first to announce the discovery by deferring too long the discussion of his observations.

The news of this capital discovery was brought to this country by the steamer of Oct. 4th, and every telescope was immediately turned upon the planet. It was observed at Cambridge by Mr. Bond, Oct. 21st; it was seen at Washington Oct. 23d, and was regularly observed there till Jan. 27th, when it approached too near the sun to be longer followed.

Le Verrier, although quite a young man, thus established at once an enviable reputation. He was literally overwhelmed with honors received fi'om

THE TLANET NEl'TUNE, 27

the sovereigns and academies of Europe. He was created an officer of the Legion of Honor, by the King of France, and a special chair of Celestial Mechanics was established for him at the Faculty of Sciences. From the King of Denmark, he received the title of Commander of the Royal Order of Dannebroga ; and the Royal Society of London conferred on him the Copley medal. The Academy of St. Petersburg resolved to offer him the first vacancy in their body ; and the Royal Society of Gottingen elected him to the rank of Foreign Associate.

Thus were the predictions of Adams and Le Verrier, with regard to the direction of the planet at the present time, wonderfully fulfilled ; but is this body pursuing the orbit which these mathematicians had prescribed for it ? Since its first discovery, the planet has advanced but six or seven degrees in its orbit. We have less than four years' observations to determine an orbit which it requires more than a century to complete. The computation has been made ; but a result derived from so short a period must be received with some distrust, on account of the unavoidable imperfection of all observations. The best observations are liable to small errors ; and a slight error in the measurement of a minute portion of the orbit, would lead to a much larger error in the computed length of the remainder of the path. We must have observations for a long series of years, to furnish the orbit with all desirable precision. Under these circumstances it becomes a question of the highest interest, whether this body may not have been observed by astronomers of former years, and mistaken for a fixed star. If we could obtain one good observation, made some time in the last century, it would enable us at once to determine the orbit with nearly the same precision as that of Jupiter itself. It will then be presumed that astronomers have not neglected to explore the records of the past, to discover if possible some chance observation of the new planet.

Mr. Hind, of London, adopting the predicted elements of Le Verrier, examined Lalande's and other observations for this purpose, and satisfied himself that the new planet was not there to be found. An American astronomer, Mr. Sears C Walker, was more fortunate. Mr. Walker proceeded in the following manner. He first computed the orbit which best represented all the observations which had been made at the Washington Observatory, as well as those which had been re-

THE PLANET NEPTUNE. S9

ceived from Europe. He then computed the planet's probable place for a long series of preceding years, and sought among the records of astronomers for observations of stars in the neighborhood of the computed path. Bradley, Mayer, and Lacaille have left us an immense collection of observations, yet they seldom recorded stars so small as the body in question. Among the observations of Piazzi, no one was found which could be identified with the planet. The Madras observations were generally confined to the stars of Piazzi's catalogue. The Paramatta Catalogue seldom extends north of the thirty-third parallel of south declination; and Bessel, in preparing his zones of 75,000 stars, did not sweep far enough south to comprehend the planet. The only remaining chance of finding an observation of the planet was among the observations of Lalande. The Histoire Celeste Fran9aise embraces 50,000 stars, and Mr. Walker soon found that Lalande had swept over the supposed path of the planet, on the 8th and 10th of May, 1795. He accordingly computed more carefully the place of the planet for this period, making small variations in the elements of the orbit, so as to include the entire region within which the planet could possibly have been comprised. He then selected from the

30 HIStdiftY OF ASTRONOMY.

Histoire Celeste, all the stars within a quarter of a degree of the computed path. These stars were nine in number ; of which six had, however, been subsequently observed by Bessel, and of course were to be set down as fixed stars. But three stars remained which required special examination ; and of these, one was too small to be mistaken for the planet, and a second was thought to be too far from the computed place. The remaining star was distant only two minutes from the computed place of the planet ; it was of the same magnitude, and was not to be found in Bessel's observations, although this part of the heavens must have been included in the field of his telescope. This discovery was made on the 2d of February, 1847 ; and on the first clear subsequent evening, February 4th, the great equatorial of the Washington Observatory was pointed to the heavens, and this star was missing. Where Lalande, in 1795, saw a star of the ninth magnitude, there remained only a blank. The conclusion seemed almost certain, that Mr. Walker had here obtained the object of his search. He accordingly computed the path upon this supposition, and found that a single elliptic orbit would represent, with almost mathematical precision, the observation of 1795, and all the observations of 1846.