Reminiscences of an Astronomer
WE spent most of the winter of 1870— 71 in Berlin, patiently waiting for the end of the Franco-Prussian war, in order that I might rummage among the old manuscripts of the Paris observatory. Delaunay was then director of that institution, having succeeded Leverrier when the emperor removed the latter from his position. I had for some time kept up an occasional correspondence with Delaunay, and while in England, the autumn before, had forwarded a message to him, through the Prussian lines, by the good offices of the London Legation and Mr. Washburn. He was therefore quite prepared for our arrival. We took the first train which was likely to go through to Paris. The evacuation of a country by a hostile army is rather a slow process, so that the German troops were met everywhere on the road, even in France. They had left Paris just before we arrived ; but the French national army was not there, the Communists having taken possession of the city as fast as the Germans withdrew. As we passed out of the station, the first object to strike our eyes was a flaming poster addressed to “ Citoyens,” and containing one of the manifestoes which the Communist government was continually issuing.
Of course we made an early call on Mr. Washburn. His career in Paris was one of the triumphs of diplomacy ; he had cared for the interests of German subjects in Paris in such a way as to earn the warm recognition both of the emperor and of Bismarck, and at the same time had kept on such good terms with the French as to be not less esteemed by them. He was surprised that we had chosen such a time to visit Paris ; but I told him the situation, the necessity of my early return home, and my desire to make a careful search in the records of the Paris observatory for observations made two centuries ago. He advised us to take up our quarters as near to the observatory as convenient, in order that we might not have to pass through the portions of the city which were likely to be the scenes of disturbance.
We were received at the observatory with a warmth of welcome that might be expected to accompany the greeting of the first foreign visitor, after a siege of six months. Yet a tinge of sadness in the meeting was unavoidable. Delaunay immediately began lamenting the condition of his poor ruined country, de spoiled of two of its provinces by a foreign foe, condemned to pay an enormous subsidy in addition, and now the scene of an internal conflict the end of which no one could foresee.
The old records I wished to consult were placed at my disposal, with full liberty not only to copy, but to publish anything of value I could find in them. The mine proved rich beyond the most sanguine expectation. After a little prospecting, I found that the very observations I wanted had been made in great numbers by the Paris astronomers, both at the observatory and at other points in the city. Some explanation of the work I was engaged in may not be devoid of interest, but it necessitates talking a little astronomy.
Millions of stars, visible with large telescopes, are scattered over the heavens; tens of thousands are bright enough to be seen with small instruments, and several thousand are visible to any ordinary eye. The moon performs a monthly course around the heavens, at a distance from us which is very small compared with that of the stars ; consequently, she often passes over a star, and of course hides it from view during the time required for the passage. The great majority of stars are so small that their light is obscured by the effulgence of the moon as the latter approaches them. But quite frequently the star passed over is so bright that the exact moment when the moon reaches it can be observed with the utmost precision. The star then disappears from view in an instant, as if its light were suddenly and absolutely extinguished. This is called an occultation. If the moment at which the disappearance takes place is observed, we know that at that instant the apparent angle between the centre of the moon and the star is equal to the moon’s semi-diameter. By the aid of a number of such observations, the path of the moon in the heavens, and the time at which she arrives at each point of the path, can be determined. From the tables of the moon’s motion, assuming them to be correct, the time of each occultation, as seen from any known station, can be predicted. If the predicted and the observed moments agree, the tables are correct. If they do not, the discrepancy will enable us to determine the error in the moon’s predicted position. In order that the determination may be of sufficient scientific precision, the time of the occultation must be known within one or two seconds ; otherwise, we shall be in doubt how much of the discrepancy may be due to the error of the observation, and how much to the error of the tables.
Occultations of some bright stars, such as Aldebaran and Antares, can be observed by the naked eye ; and yet more easily can those of the planets be seen. It is therefore a curious historic fact that there is no certain record of an actual observation of this sort having been made until after the commencement of the seventeenth century. Even then the observations were of little or no use, because astronomers could not determine their time with sufficient precision. It was not till after the middle of the century, when the telescope had been made part of astronomical instruments for finding the altitude of a heavenly body, and after the pendulum clock had been invented by Huyghens, that the time of an occultation could be fixed with the required exactness. Thus it happens that from 1640 to 1670 somewhat coarse observations of the kind are available, and after the latter epoch those made by the French astronomers become quite comparable with the modern ones in precision.
And how, the reader may ask, did it happen that these observations were not published by the astronomers who made them ? Why should they have lain unused and forgotten for two hundred years ? The answer to these questions is made plain enough by an examination of the records. The astronomers had no idea of the possible usefulness and value of what they were recording. So far as we can infer from their work, they made the observations merely because an occultation was an interesting thing to see ; and they were men of sufficient scientific experience and training to have acquired the excellent habit of noting the time at which a phenomenon was observed. But they were generally satisfied with simply putting down the clock time. How they could have expected their successors to make any use of such a record, or whether they had any expectations on the subject, we cannot say with confidence. It will be readily understood that no clocks of the present time (much less those of two hundred years ago) run with such precision that the moment read from the clock is exact within one or two seconds. The modern astronomer does not pretend to keep his clock correct within less than a minute ; he determines by observation how far it is wrong, on each date of observation, and adds so much to the time given by the clock, or subtracts it, as the case may be, in order to get the correct moment of true time. In the case of the French astronomers, the clock would frequently be fifteen minutes or more in error, for the reason that they used apparent time, instead of mean time as we do. Thus when, as was often the case, the only record found was that, at a certain hour, minute, and second, by a certain clock, une étoile se cache par la lune, a number of very difficult problems were presented to the astronomer who was to make use of the observations two centuries afterward. First of all, he must find out what the error of the clock was at the designated hour, minute, and second ; and for this purpose he must reduce the observations made by the observer in order to determine the error. But it was very clear that the observer did not expect any successor to take this trouble, and therefore did not supply him with any facilities for so doing. He did not even describe the particular instrument with which the observations were made, but only wrote down certain figures and symbols, of a more or less hieroglyphic character. It needed much comparison and examination to find out what sort of an instrument was used, how the observations were made, and how they should be utilized for the required purpose.
Generally the star which the moon hid was mentioned, but not in all cases. If it was not, the identification of the star was a puzzling problem. The only way to proceed was to calculate the apparent position which the centre of the moon must have held to an observer at the Paris observatory, at the particular hour and minute of the observation. A star map was then taken ; the points of a pair of dividers were separated by the length of the moon’s radius, as it would appear on the scale of the map ; one point of the dividers was put into the position of the moon’s centre on the map, and with the other a circle was drawn. This circle represented the outline of the moon, as it appeared to the observer at the Paris observatory, at the hour and minute in question, on a certain day in the seventeenth century. The star should be found very near the circumference of the circle, and in nearly all cases a star was there.
Of course all this could not be done on the spot. What had to be done was to find the observations, study their relations and the method of making them, and copy everything that seemed necessary for working them up. This took some six weeks, but the material I carried away proved the greatest find I ever made. Three or four years were spent in making all the calculations I have described. Then it was found that seventy-five years were added, at a single step, to the period during which the history of the moon’s motion could be written. Previously, this history was supposed to commence with the observations of Bradley, at Greenwich, about 1750 ; now it was extended back to 1675, and with a less degree of accuracy thirty years farther still. Hansen’s tables were found to deviate from the truth, in 1675 and subsequent years, to a surprising extent; but the cause of the deviation is not entirely unraveled even now.
During the time I was doing this work, Paris was under the reign of the Commune and besieged by the national forces. The studies had to be made within hearing of the besieging guns ; and I could sometimes go to a window and see flashes of artillery from one of the fortifications to the south. Nearly every day I took a walk through the town, occasionally as far as the Arc de Triomphe. The story of the Commune has been so often written that I cannot hope to add anything to it, so far as the main course of events is concerned. Looking back on a sojourn at so interesting a period, one cannot but feel that a golden opportunity to make observations of historic value was lost. The fact is, however, that I was prevented from making such observations not only by my complete absorption in my work, but by the consideration that, being in what might be described as a semi-official capacity, I did not want to get into any difficulty that would have compromised the position of an official visitor. I should not deem what we saw worthy of special mention, were it not that it materially modifies the impressions commonly given by writers on the history of the Commune. What an historian says may be quite true, so far as it goes, and yet may be so far from the whole truth as to give the reader an incorrect impression of the actual course of events. The violence and disease which prevail in the most civilized country in the world may be described in such terms as to give the impression of a barbarous community. The murder of the Archbishop of Paris and of the hostages show how desperate were the men who had seized power, yet the acts of these men constitute but a small part of the history of Paris during that critical period.
What one writes at the time is free from the suspicion that may attach to statements not recorded till many years after the events to which they relate. The following extract from a letter which I wrote to a friend, the day after my arrival, may therefore not be devoid of interest : —
DEAR CHARLIE, — Here we are, on this slumbering volcano. Perhaps you will hear of the burst-up long before you get this. We have seen historic objects which fall not to the lot of every generation, the barricades of the Paris streets. As we were walking out this morning, the pavement along one side of the street was torn up for some distance, and used to build a temporary fort. Said fort would be quite strong against musketry or the bayonet ; but with heavy shot against it, I should think it would be far worse than nothing, for the flying stones would kill more than the balls.
The streets are placarded at every turn with all sorts of inflammatory appeals, and general orders of the Comité Central or of the Commune. One of the first things I saw last night was a large placard beginning “ Citoyens ! ” Among the orders is one forbidding any one from placarding any orders of the Versailles government, under the severest penalties ; and another threatening with instant dismissal any official who shall recognize any order issuing from the said government.
I must do all hands the justice to say that they are all very well behaved. There is nothing like a mob anywhere, so far as I can find. I consulted my map this morning, right alongside the barricade and in full view of the builders, without being molested, and wife and I walked through the insurrectionary districts without being troubled or seeing the slightest symptoms of disturbance. The stores are all open, and every one seems to be buying and selling as usual. In all the cafés I have seen, the habitués seem to be drinking their wine just as coolly as if they had nothing unusual on their minds.
From this date to that of our departure I saw nothing suggestive of violence within the limited range of my daily walks, which were mostly within the region including the Arc de Triomphe, the Hôtel de Ville, and the observatory ; the latter being about half a mile south of the Luxembourg. The nearest approach to a mob that I ever noticed was a drill of young recruits of the National Guard, or a crowd in the court of the Louvre being harangued by an orator. With due allowance for the excitability of the French nature, the crowd was comparatively as peaceable as that which we may see surrounding a gospel wagon in one of our own cities. A drill-ground for the recruits happened to be selected opposite our first lodgings, beside the gates of the Luxembourg. This was so disagreeable that we were glad to accept an invitation from Delaunay to be his guests at the observatory, during the remainder of our stay. We had not been there long before the spacious yard of the observatory was also used as a drill-ground ; and yet later, two or three men were given billets de logement upon the observatory ; but I should not have known of the latter occurrence, had not Delaunay told me. I believe he bought the men off, much as one pays an organ-grinder to move on. In one of our walks we entered the barricade around the Hôtel de Ville, and were beginning to make a close examination of a mitrailleuse, when a soldier (beg his pardon, un citoyen membre de la Garde Nationale) warned us away from the weapon. The densest crowd of Communists was along the Rue de Rivoli and in the region of the Colonne Vendôme, where some of the principal barricades were being erected. But even here, not only were the stores open as usual, but the military were doing their work in the midst of piles of trinkets ex posed for sale on the pavement by the shopwomen. The order to destroy the Column was issued before we left, but not executed until later. I have no reason to suppose that the shopwomen were any more concerned while the Column was being undermined than they were before. To complete the picture, not a policeman did we see in Paris ; in fact, I was told that one of the first acts of the Commune had been to drive the police away, so that not one dared to show himself.
An interesting feature of the sad spectacle was the stream of proclamations poured forth by the Communist authorities. They comprised not only decrees, but sensational stories of victories over the Versailles troops, denunciations of the Versailles government, and even elaborate legal arguments, including a not intemperate discussion of the ethical question whether citizens who were not adherents of the Commune should be entitled to the right of suffrage. The conclusion was that they should not. The lack of hummor on the part of the authorities was shown by their commencing one of a rapid succession of battle stories with the words, " Citoyens ! Vous avez soif de la vérité ! ”
The most amusing decree I noticed ran thus : —
“ Article I. All conscription is abolished.
“ Article II. No troops shall hereafter be allowed in Paris, except the National Guard.
“ Article III. Every citizen is a member of the National Guard.”
We were in daily expectation and hope of the capture of the city, little knowing by what scenes it would be accompanied. It did not seem to my unmilitary eye that two or three batteries of artillery could have any trouble in demolishing all the defenses, since a wall of paving-stones, four or five feet high, could hardly resist solid shot, or prove anything but a source of destruction to those behind it if attacked by artillery. But the capture was not so easy a matter as I had supposed.
We took leave of our friend and host on May 5, three weeks before the final catastrophe, of which he wrote me a graphic description. As the barricades were stormed by MacMahon, the Communist line of retreat was through the region of the observatory. The walls of the building and of the yard were so massive that the place was occupied as a fort by the retreating forces, so that the situation of the few non-combatants who remained was extremely critical. They were exposed to the fire of their friends, the national troops, from without, while enraged men were threatening their lives within. So hot was the fusillade that, going into the great dome after the battle, the astronomer could imagine all the constellations of the sky depicted by the bullet-holes. When retreat became inevitable, the Communists tried to set the building on fire, but did not succeed. Then, in their desperation, arrangements were made for blowing it up ; but the most violent man among them was killed by a providential bullet, as he was on the point of doing his work. The remainder fled, the place was speedily occupied by the national troops, and the observatory with its precious contents was saved.
The Academy of Sciences had met regularly through the entire Prussian siege. The legal quorum being three, this did not imply a large attendance. At the time of my visit a score of members were in the city. Among them were Elie de Beaumont, the geologist: MilneEdwards, the zoölogist ; and Chevreul, the chemist. I was surprised to learn that the latter was in his eighty - fifth year ; he seemed a man of seventy or less, mentally and physically. Yet we little thought that he would be the longest-lived man of equal eminence that our age has known. When he died, in 1889, he was nearly one hundred and three years old. Born in 1786, he had lived through the whole French Revolution, and was seven years old at the time of the Terror. His scientific activity, from beginning to end, extended over some eighty years. When I saw him, he was still very indignant at a bombardment of the Jardin des Plantes by the German besiegers. He had made a formal statement of this outrage to the Academy of Sciences, in order that posterity might know what kind of men were besieging Paris. I suggested that the shells might have fallen in the place by accident ; but he maintained that it was not the case, and that the bombardment was intentional.
“ But,” said I, “ the Germans are a scientific nation ; what object could they have had in injuring an establishment so purely scientific as yours ? ”
He replied that some explosives had been stored in one corner of the place, and he supposed that the Germans had found it out. I did not pursue the question further.
The most execrated man in the scientific circle at this time was Leverrier. He had left Paris before the Prussian siege began, and had not returned. Delaunay assured me that this was a wise precaution on his part; for had he ventured into the city he would have been mobbed, or the Communists would have killed him as soon as he was caught. Just why the mob should have been so incensed against one whose life was spent in the sereuest fields of astronomical science was not fully explained. The fact that he had been a senator, and was politically obnoxious, was looked on as an all sufficient indictment. Even members of the Academy could not suppress their detestation of him. He was charged with the most despicable meanness, not to say turpitude ; and altogether, one taking the statements with no grains of salt would have thought him a character that no self-respecting man could associate with.
Four years later I was again in Paris, and attended a meeting of the Academy of Sciences. In the course of the session a rustle of attention spread over the room, as all eyes were turned upon a member who was entering rather late. Looking toward the door, I saw a man of sixty, a decided blond, with light chestnut hair turning gray, a slender form, a shaven face, rather pale and thin, but very attractive, and extremely intelligent features. As he passed to his seat hands were stretched out on all sides to greet him, and not until he sat down did the bustle caused by his entrance subside. He was evidently a notable.
“ Who is that ? ” I said to my neighbor.
Delaunay was one of the most kindly and attractive men I ever met. We spent our evenings walking in the grounds of the observatory, discussing French science in all its aspects. His investigation of the moon’s motion is one of the most extraordinary pieces of mathematical work ever turned out by a single person. It fills two quarto volumes, and the reader who attempts to go through any part of the calculations will wonder how one man could do the work in a lifetime. His habit was to commence early in the morning, and work with but little interruption until noon. He never worked in the evening, and generally retired at nine. I felt some qualms of conscience at the frequency with which I kept him up till nearly ten. I found it hopeless to expect that he would ever visit America, because he assured me that he did not dare to venture on the ocean. The only voyage he had ever made was across the Channel, to receive the gold medal of the Royal Astronomical Society for his work. Two of his relatives, his father, and, I believe, his brother, had been drowned, and this fact gave him a horror of the water. He seemed to feel somewhat like the clients of the astrologists, who, having been told how they were to die, took every precaution to prevent it. I remember, as a boy, reading a history of astrology, in which a great many cases of this sort were described ; the peculiarity being that the very measures which the victim took to avoid the decree of fate became the engines that executed it. The death of Delaunay was not exactly a case of this kind, yet it could not but bring it to mind. He was at Cherbourg in the autumn of 1872. As he was walking on the beach with a relative, a couple of boatmen invited them to take a sail. Through what inducement Delaunay was led to forget his fears will never be known. All we know is that he and his friend entered the boat, that it was struck by a sudden squall when at some distance from the land, and that the whole party were drowned.
There was no opposition to the reappointment of Leverrier to his old place. In fact, at the time of my visit, Delaunay said that President Thiers was on terms of intimate friendship with the former director, and he thought it not at all unlikely that the latter would succeed in being restored. He kept the position with general approval till his death in 1877.
The only occasion on which I met Leverrier was after the incident I have mentioned, in the Academy of Sciences. I had been told that he was incensed against me on account of an unfortunate remark I had made in speaking of his work which led to the discovery of Neptune. I had heard this in Germany as well as in France, yet the matter was so insignificant that I could hardly conceive of a man of philosophic mind taking any notice of it. I determined to meet him, as I had met Hansen, with entire unconsciousness of offense. So I called on him at the observatory, and was received with courtesy, but no particular warmth. I suggested to him that now, as he had nearly completed his work on the tables of the planets, the question of the moon’s motion would be the next object worthy of his attention. He replied that it was too large a subject for him to take up.
To Leverrier belongs the credit of having been the real organizer of the Paris observatory. His work there was not dissimilar to that of Airy at Greenwich ; but he had a much more difficult task before him, and was less fitted to grapple with it. When founded by Louis XIV. the establishment was simply a place where astronomers of the Academy of Sciences could go to make their observations. There was no titular director, every man working on his own account and in his own way. Cassini, an Italian by birth, was the best known of the astronomers, and, in consequence, posterity has very generally supposed he was the director. That he failed to secure that honor was not from any want of astuteness. It is related that the monarch once visited the observatory to see a newly discovered comet through the telescope. He inquired in what direction the comet was going to move. This was a question it was impossible to answer at the moment, because both observations and computations would be necessary before the orbit could be worked out. But Cassini reflected that the king would not look at the comet again, and would very soon forget what was told him ; so he described its future path in the heavens quite at random, with entire confidence that any deviation of the actual motion from his prediction would never be noted by his royal patron.
One of the results of this lack of organization has been that the Paris observatory does not hold an historic rank correspondent to the magnificence of the establishment. The go-as-you-please system works no better in a national observatory than it would in a business institution. Up to the end of the last century, the observations made there were too irregular to be of any special importance. To remedy this state of things, Arago was appointed director early in the present century; but he was more eminent in experimental physics than in astronomy, and had no great astronomical problem to solve. The result was that while he did much to promote the reputation of the observatory in the direction of physical investigation, he did not organize any well-planned system of regular astronomical work.
When Leverrier succeeded Arago, in 1853, he had an extremely difficult problem before him. By a custom extending through two centuries, each astronomer was to a large extent the master of his own work. Leverrier undertook to change all this in a twinkling, and, if reports are true, without much regard to the feelings of the astronomers. Those who refused to fall into line either resigned or were driven away, and their places were filled with men willing to work under the direction of their chief. Unfortunately, the new director was not an adept either in practical astronomy or in the use of instruments. His methods were far from being up to the times, and the work of the Paris observatory, under his direction, so far as observations of precision go, falls markedly behind that of Greenwich and Pulkova.
But in recent times the institution has been marked by an energy and a progressiveness that go far to atone for its former deficiencies. The successors of Leverrier have known where to draw the line between routine, on the one side, and initiative on the part of the assistants, on the other. Probably no other observatory in the world has so many able and well-trained young men, who work partly on their own account, and partly in a regular routine. In the direction of physical astronomy the observatory is especially active, and it may be expected in the future to justify its historic reputation.