The "Dumb-Bell" Nebula.Photographed by Keeler, July 31, 1899.Exposure, three hours.
The "Dumb-Bell" Nebula.Photographed by Keeler, July 31, 1899.Exposure, three hours.
It is never advisable to push philosophical speculation very far when supported by too slendera basis of fact. But if we are to regard the visible universe as made up on the whole of a single system of bodies, we may well ask one or two questions to be answered by speculative theory. We have said the stars are not uniformly distributed in space. Their concentration in the Milky Way, forming a narrow band dividing the sky into two very nearly equal parts, must be due to their being actually massed in a thin disk or ring of space within which our solar system is also situated. This thin disk projected upon the sky would then appear as the narrow star-band of the Milky Way. Now, suppose this disk has an axis perpendicular to itself, and let us imagine a rotation of the whole sidereal system about that axis. Then the fact that the visible nebulæ are congregated far from the Milky Way means that they are actually near the imaginary axis.
Possibly the diminished velocity of motion near the axis may have something to do with the presence of the nebulæ there. Possibly the nebulæ themselves have axes perpendicular to the plane of the Milky Way. If so, we shouldsee the spiral nebulæ near the Milky Way edgewise, and those far from it without foreshortening. Thus, the paucity of nebulæ near the Milky Way may be due in part to the increased difficulty of seeing them when looked at edgewise. Indeed, there is no limit to the possibilities of hypothetical reasoning about the nebular structure of our universe; unfortunately, the whole question must be placed for the present among those intensely interesting cosmic problems awaiting elucidation, let us hope, in this new century.
Nothing can be more erroneous than to suppose that the stellar multitude has continued unchanged throughout all generations of men. "Eternal fires" poets have called the stars; yet they burn like any little conflagration on the earth; now flashing with energy, brilliant, incandescent, and again sinking into the dull glow of smouldering half-burned ashes. It is even probable that space contains many darkened orbs, stars that may have risen in constellations to adorn the skies of prehistoric time—now cold, unseen, unknown. So far from dealing with an unvarying universe, it is safe to say that sidereal astronomy can advance only by the discovery of change. Observational science watches with untiring industry, and night hides few celestial events from the ardent scrutiny of astronomers. Old theories are tested and newer ones often perfected by the detection of some slight and previouslyunsuspected alteration upon the face of the sky. The interpretation of such changes is the most difficult task of science; it has taxed the acutest intellects among men throughout all time.
If, then, changes can be seen among the stars, what are we to think of the most important change of all, the blazing into life of a new stellar system? Fifteen times since men began to write their records of the skies has the birth of a star been seen. Surely we may use this term when we speak of the sudden appearance of a brilliant luminary where nothing visible existed before. But we shall see further on that scientific considerations make it highly probable that the phenomenon in question does not really involve the creation of new matter. It is old material becoming suddenly luminous for some hidden reason. In fact, whenever a new object of great brilliancy has been discovered, it has been found to lose its light again quite soon, ending either in total extinction or at least in comparative darkness. It is for this reason that the name "temporary star" has been applied to cases of this kind.
The first authenticated instance dates from theyear 134B.C., when a new star appeared in the constellation Scorpio. It was this star that led Hipparchus to construct his stellar catalogue, the first ever made. It occurred to him, of course, that there could be but one way to make sure in the future that any given object discovered in the sky was new; it was necessary to make a complete list of everything visible in his day. Later astronomers need then only compare Hipparchus's catalogue with the heavens from time to time in order to find out whether anything unknown had appeared. This work of Hipparchus became the foundation of sidereal study, and led to most important discoveries of various kinds.
But no records remain concerning his new star except the bare fact of its appearance in Scorpio. Hipparchus's published works are all lost. We do not even know the exact place of his birth, and as for those two dates of entry and exit that history attaches to great names—we have them not. Yet he was easily the first astronomer of antiquity, one of the first of all time; and we know of him only from the writings of Ptolemy, who lived three hundred years after him.
More than five centuries elapsed before another temporary star was entered in the records of astronomy. This happened in the year 389A.D., when a star appeared in Aquila; and of this one also we know nothing further. But about twelve centuries later, in November, 1572, a new and brilliant object was found in the constellation Cassiopeia. It is known as Tycho's star, since it was the means of winning for astronomy a man who will always take high rank in her annals, Tycho Brahe, of Denmark. When he first saw this star, it was already very bright, equalling even Venus at her best; and he continued a careful series of observations for sixteen months, when it faded finally from his view. The position of the new star was measured with reference to other stars in the constellation Cassiopeia, and the results of Tycho's observations were finally published by him in the year 1573. It appears that much urging on the part of friends was necessary to induce him to consent to this publication, not because of a modest reluctance to rush into print, but for the reason that he considered it undignified for a nobleman of Denmark to be the author of a book!
An important question in cosmic astronomy is opened by Tycho's star. Did it really disappear from the heavens when he saw it no more, or had its lustre simply been reduced below the visual power of the unaided eye? Unfortunately, Tycho's observations of the star's position in the constellation were necessarily crude. He possessed no instruments of precision such as we now have at our disposal, and so his work gives us only a rather rough approximation of the true place of the star. A small circle might be imagined on the sky of a size comparable with the possible errors of Tycho's observations. We could then say with certainty that his star must have been situated somewhere within that little circle, but it is impossible to know exactly where.
It happens that our modern telescopes reveal the existence of several faint stars within the space covered by such a circle. Any one of these would have been too small for Tycho to see, and, therefore, any one of them may be his once brilliant luminary reduced to a state of permanent or temporary semi-darkness. These considerations are, indeed, of great importance in explaining thephenomena of temporary stars. If Tycho had been able to leave us a more exact determination of his star's place in the sky, and even if our most powerful instruments could not show anything in that place to-day, we might nevertheless theorize on the supposition that the object still exists, but has reached a condition almost entirely dark.
Indeed, the latest theory classes temporary stars among those known as variable. For many stars are known to undergo quite decided changes in brilliancy; possibly inconstancy of light is the rule rather than the exception. But while such changes, when they exist, are too small to be perceptible in most cases, there is certainly a large number of observable variables, subject to easily measurable alterations of light. Astronomers prefer to see in the phenomena of temporary stars simple cases of variation in which the increase of light is sudden, and followed by a gradual diminution. Possibly there is then a long period of comparative or even complete darkness, to be followed as before by a sudden blazing up and extinction. No temporary star, however, has been observed to reappear in the same celestial place where once had glowed itssudden outburst. But cases are not wanting where incandescence has been both preceded and followed by a continued existence, visible though not brilliant.
For such cases as these it is necessary to come down to modern records. We cannot be sure that some faint star has been temporarily brilliant, unless we actually see the conflagration itself, or are able to make the identity of the object's precise location in the sky before and after the event perfectly certain by the aid of modern instruments of precision. But no one has ever seen the smouldering fires break out. Temporary stars have always been first noticed only after having been active for hours if not for days. So we must perforce fall back on instrumental identification by determinations of the star's exact position upon the celestial vault.
Some time between May 10th and 12th in the year 1866 the ninth star in the list of known "temporaries" appeared. It possessed very great light-giving power, being surpassed in brilliancy by only about a score of stars in all the heavens. It retained a maximum luminosity onlythree or four days, and in less than two months had diminished to a point somewhere between the ninth and tenth "magnitudes." In other words, from a conspicuous star, visible to the naked eye, it had passed beyond the power of anything less than a good telescope. Fortunately, we had excellent star-catalogues before 1866. These were at once searched, and it was possible to settle quite definitely that a star of about the ninth or tenth magnitude had really existed before 1866 at precisely the same point occupied by the new one. Needless to say, observations were made of the new star itself, and afterward compared with later observations of the faint one that still occupies its place. These render quite certain the identity of the temporary bright star with the faint ones that preceded and followed it.
Such results, on the one hand, offer an excellent vindication of the painstaking labor expended on the construction of star-catalogues, and, on the other, serve to elucidate the mystery of temporary stars. Nothing can be more plausible than to explain by analogy those cases in which no previous or subsequent existence has been observed. It is merely necessary to suppose that, instead of varying from the ninth or tenth magnitude, other temporary objects have begun and ended with the twentieth; for the twentieth magnitude would be beyond the power of our best instruments.
Nor is the star of 1866 an isolated instance. Ten years later, in 1876, a temporary star blazed up to about the second magnitude, and returned to invisibility, so far as the naked eye is concerned, within a month, having retained its greatest brilliancy only one or two days. This star is still visible as a tiny point of light, estimated to be of the fifteenth magnitude. Whether it existed prior to its sudden outburst can never be known, because we do not possess catalogues including the generality of stars as faint as this one must have been. But at all events, the continued existence of the object helps to place the temporary stars in the class of variables.
The next star, already mentioned under "nebula," was first seen in 1885. It was in one respect the most remarkable of all, for it appeared almost in the centre of the great nebula in theconstellation Andromeda. It was never very bright, reaching only the sixth magnitude or thereabouts, was observed during a period of only six months, and at the end of that time had faded beyond the reach of our most powerful glasses. It is a most impressive fact that this event occurred within the nebula. Whatever may be the nature of the explosive catastrophe to which the temporary stars owe their origin, we can now say with certainty that not even those vast elemental luminous clouds men call nebulæ are free from danger.
The last outburst on our records was first noticed February 22, 1901. The star appeared in the constellation Perseus, and soon reached the first magnitude, surpassing almost every other star in the sky. It has been especially remarkable in that it has become surrounded by a nebulous mass in which are several bright condensations or nuclei; and these seem to be in very rapid motion. The star is still under observation (January, 1902).
Among the figures that stand out sharply upon the dim background of old-time science, there is none that excites a keener interest than Galileo. Most people know him only as a distinguished man of learning; one who carried on a vigorous controversy with the Church on matters scientific. It requires some little study, some careful reading between the lines of astronomical history, to gain acquaintance with the man himself. He had a brilliant, incisive wit; was a genuine humorist; knew well and loved the amusing side of things; and could not often forego a sarcastic pleasantry, or deny himself the pleasure of argument. Yet it is more than doubtful if he ever intended impertinence, or gave willingly any cause of quarrel to the Church.
His acute understanding must have seen that there exists no real conflict between science andreligion; for time, in passing, has made common knowledge of this truth, as it has of many things once hidden. When we consider events that occurred three centuries ago, it is easy to replace excited argument with cool judgment; to remember that those were days of violence and cruelty; that public ignorance was of a density difficult to imagine to-day; and that it was universally considered the duty of the Church to assume an authoritative attitude upon many questions with which she is not now required to concern herself in the least. Charlatans, unbalanced theorists, purveyors of scientific marvels, were all liable to be passed upon definitely by the Church, not in a spirit of impertinent interference, but simply as part of her regular duties.
If the Church's judgment in such matters was sometimes erroneous; if her interference now and again was cruel, the cause must be sought in the manners and customs of the time, when persecution rioted in company with ignorance, and violence was the law. Perhaps even to-day it would not be amiss to have a modern scientific board pass authoritatively upon novel discoveries and inventions, so as to protect the public against impostors as the Church tried to do of old.
Galileo was born at Pisa in 1564, and his long life lasted until 1642, the very year of Newton's birth. His most important scientific discoveries may be summed up in a few words; he was the first to use a telescope for examining the heavenly bodies; he discovered mountains on the moon; the satellites of Jupiter; the peculiar appearance of Saturn which Huygens afterward explained as a ring surrounding the ball of the planet; and, finally, he found black spots on the sun's disk. These discoveries, together with his remarkable researches in mechanical science, constitute Galileo's claim to immortality as an investigator. But, as we have said, it is not our intention to consider his work as a series of scientific discoveries. We shall take a more interesting point of view, and deal with him rather as a human being who had contracted the habit of making scientific researches.
What must have been his feelings when he first found with his "new" telescope the satellites of Jupiter? They were seen on the night of January 7, 1610. He had already viewed the planet through his earlier and less powerful glass, and was aware that it possessed a round disk like the moon, only smaller. Now he saw also three objects that he took to be little stars near the planet. But on the following night, as he says, "drawn by what fate I know not," the tube was again turned upon the planet. The three small stars had changed their positions, and were now all situated to the west of Jupiter, whereas on the previous night two had been on the eastern side. He could not explain this phenomenon, but he recognized that there was something peculiar at work. Long afterward, in one of his later works, translated into quaint old English by Salusbury, he declared that "one sole experiment sufficeth to batter to the ground a thousand probable Arguments." This was already the guiding principle of his scientific activity, a principle of incomparable importance, and generally credited to Bacon. Needless to say, Jupiter was now examined every night.
The 9th was cloudy, but on the 10th he againsaw his little stars, their number now reduced to two. He guessed that the third was behind the planet's disk. The position of the two visible ones was altogether different from either of the previous observations. On the 11th he became sure that what he saw was really a series of satellites accompanying Jupiter on his journey through space, and at the same time revolving around him. On the 12th, at 3A.M., he actually saw one of the small objects emerge from behind the planet; and on the 13th he finally saw four satellites. Two hundred and eighty-two years were destined to pass away before any human eye should see a fifth. It was Barnard in 1892 who followed Galileo.
To understand the effect of this discovery upon Galileo requires a person who has himself watched the stars, not, as a dilettante, seeking recreation or amusement, but with that deep reverence that comes only to him who feels—nay, knows—that in the moment of observation just passed he too has added his mite to the great fund of human knowledge. Galileo's mummied forefinger still points toward the stars from itslittle pedestal of wood in theMuseoat Florence, a sign to all men that he is unforgotten. But Galileo knew on that 11th of January, 1610, that the memory of him would never fade; that the very music of the spheres would thenceforward be attuned to a truer note, if any would but hearken to the Jovian harmony. For he recognized at once that the visible revolution of these moons around Jupiter, while that planet was himself visibly travelling through space, must deal its death-blow to the old Ptolemaic system of the universe. Here was a great planet, the centre of a system of satellites, and yet not the centre of the universe. Surely, then, the earth, too, might be a mere planet like Jupiter, and not the supposed motionless centre of all things.
The satellite discovery was published in 1610 in a little book called "Sidereus Nuncius," usually translated "The Sidereal Messenger." It seems to us, however, that the word "messenger" is not strong enough; surely in Papal Italy anunciuswas more than a mere messenger. He was clothed with the very highest authority, and wethink it probable that Galileo's choice of this word in the title of his book means that he claimed for himself similar authority in science. At all events, the book made him at once a great reputation and numerous enemies.
But it was not until 1616 that the Holy Office (Inquisition) issued an edict ordering Galileo to abandon his opinion that the earth moved, and at the same time placed Copernicus'sDe Revolutionibusand two other books advocating that doctrine on the "Index Librorum Prohibitorum," or list of books forbidden by the Church. These volumes remained in subsequent editions of the "Index" down to 1821, but they no longer appear in the edition in force to-day.
Galileo's most characteristic work is entitled the "Dialogue on the Two Chief Systems of the World." It was not published until 1632, although the idea of the book was conceived many years earlier. In it he gave full play to his extraordinary powers as a true humorist, afine lameamong controversialists, and a genuine man of science, valuing naked truth above all other things. As may be imagined, it was nosmall matter to obtain the authorities' consent to this publication. Galileo was already known to hold heretical opinions, and it was suspected that he had not laid them aside when commanded to do so by the edict of 1616. But perhaps Galileo's introduction to the "Dialogue" secured the censor'simprimatur; it is even suspected that the Roman authorities helped in the preparation of this introduction. Fortunately, we have a delightful contemporary translation into English, by Thomas Salusbury, printed at London by Leybourne in 1661. We have already quoted from this translation, and now add from the same work part of Galileo's masterly preface to the "Dialogue":
"Judicious reader, there was published some years since inRomea salutiferous Edict, that, for the obviating of the dangerous Scandals of the Present Age, imposed a reasonable Silence upon the Pythagorean (Copernican) opinion of the Mobility of the Earth. There want not such as unadvisedly affirm, that the Decree was not the production of a sober Scrutiny, but of an ill-formed passion; and one may hear somemutter that Consultors altogether ignorant of Astronomical observations ought not to clipp the wings of speculative wits with rash prohibitions."
Galileo first states his own views, and then pretends that he will oppose them. He goes on to say that he believes in the earth's immobility, and takes "the contrary only for a mathematicalCapriccio," as he calls it; something to be considered, because possessing an academical interest, but on no account having a real existence. Of course any one (even a censor) ought to be able to see that it is the Capriccio, and not its opposite, that Galileo really advocates. Three persons appear in the "Dialogue": Salviati, who believes in the Copernican system; Simplicio, of suggestive name, who thinks the earth cannot move; and, finally, Sagredus, a neutral gentleman of humorous propensities, who usually begins by opposing Salviati, but ends by being convinced. He then helps to punish poor Simplicio, who is one of those persons apparently incapable of comprehending a reasonable argument. Here is an interesting specimen of the "Dialogue"taken from Salusbury's translation: Salviati refers to the argument, then well known, that the earth cannot rotate on its axis, "because of the impossibility of its moving long without wearinesse." Sagredus replies: "There are some kinds of animals which refresh themselves after wearinesse by rowling on the earth; and that therefore there is no need to fear that the Terrestrial Globe should tire, nay, it may be reasonably affirmed that it enjoyeth a perpetual and most tranquil repose, keeping itself in an eternal rowling." Salviati's comment on this sally is, "You are too tart and satyrical, Sagredus."
There is no doubt that the "Dialogue" finished the Ptolemaic theory, and made that of Copernicus the only possible one. At all events, it brought about the well-known attack upon Galileo from the authorities of the Holy Office. We shall not recount the often-told tale of his recantation. He was convicted (very rightly) of being a Copernican, and was forced to abjure that doctrine. Galileo's life may be summed up as one of those through which the world has been made richer. A clean-cutting analytic wit, neverbecoming dull: heated again and again in the fierce blaze of controversy, it was allowed to cool only that it might acquire a finer temper, to pierce with fatal certainty the smallest imperfections in the armor of his adversaries.
The discovery of a new and important planet usually receives more immediate popular attention and applause than any other astronomical event. Philosophers are fond of referring to our solar system as a mere atom among the countless universes that seem to be suspended within the profound depths of space. They are wont to point out that this solar system, small and insignificant as a whole in comparison with many of the stellar worlds, is, nevertheless, made up of a large number of constituent planets; and these in turn are often accompanied with still smaller satellites, or moons. Thus does Nature provide worlds within worlds, and it is not surprising that public attention should be at once attracted by any new member of our sun's own special family of planets. The ancients were acquainted with only five of the bodies now counted as planets, viz.: Mercury, Venus, Mars, Jupiter,and Saturn. The dates of their discovery are lost in antiquity. To these Uranus was added in 1781 by a brilliant effort of the elder Herschel. We are told that intense popular excitement followed the announcement of Herschel's first observation: he was knighted and otherwise honored by the English King, and was enabled to lay a secure foundation for the future distinguished astronomical reputation of his family.
Herschel's discovery quickened the restless activity of astronomers. Persistent efforts were made to sift the heavens more and more closely, with the strengthened hope of adding still further to our planetary knowledge. An association of twenty-four enthusiastic German astronomers was formed for the express purpose of hunting planets. But it fell to the lot of an Italian, Piazzi, of Palermo, to find the first of that series of small bodies now known as the asteroids or minor planets. He made the discovery at the very beginning of our century, January 1, 1801.
But news travelled slowly in those days, and it was not until nearly April that the German observers heard from Piazzi. In the meantime, he hadhimself been prevented by illness from continuing his observations. Unfortunately, the planet had by this time moved so near the sun, on account of its own motions and those of the earth, that it could no longer be observed. The bright light of the sun made observations of the new body impossible; and it was feared that, owing to lack of knowledge of the planet's orbit, astronomers would be unable to trace it. So there seemed, indeed, to be danger of an almost irreparable loss to science. But in scientific, as in other human emergencies, someone always appears at the proper moment. A very young mathematician at Göttingen, named Gauss, attacked the problem, and was able to devise a method of predicting the future course of the planet on the sky, using only the few observations made by Piazzi himself. Up to that time no one had attempted to compute a planetary orbit, unless he had at his disposal a series of observations extending throughout the whole period of the planet's revolution around the sun. But the Piazzi planet offered a new problem in astronomy. It had become imperatively necessary to obtain an orbit from a few observations made at nearly the same date. Gauss's work was signally triumphant, for the planet was actually found in the position predicted by him, as soon as a change in the relative places of the planet and earth permitted suitable observations to be made.
But after all, Piazzi's planet belongs to a class of quite small bodies, and is by no means as interesting as Herschel's discovery, Uranus. Yet even this must be relegated to second rank among planetary discoveries. On September 23, 1846, the telescope of the Berlin Observatory was directed to a certain point on the sky for a very special reason. Galle, the astronomer of Berlin, had received a letter from Leverrier, of Paris, telling him that if he would look in a certain direction he would detect a new and large planet.
Leverrier's information was based upon a mathematical calculation. Seated in his study, with no instruments but pen and paper, he had slowly figured out the history of a world as yet unseen. Tiny discrepancies existed in the observed motions of Herschel's planet Uranus. No manhad explained their cause. To Leverrier's acute understanding they slowly shaped themselves into the possible effects of attraction emanating from some unknown planet exterior to Uranus. Was it conceivable that these slight tremulous imperfections in the motion of a planet could be explained in this way? Leverrier was able to say confidently, "Yes." But we may rest assured that Galle had but small hopes that upon his eye first, of all the myriad eyes of men, would fall a ray of the new planet's light. Careful and methodical, he would neglect no chance of advancing his beloved science. He would look.
Only one who has himself often seen the morning's sunrise put an end to a night's observation of the stars can hope to appreciate what Galle's feelings must have been when he saw the planet. To his trained eye it was certainly recognizable at once. And then the good news was sent on to Paris. We can imagine Leverrier, the cool calculator, saying to himself: "Of course he found it. It was a mathematical certainty." Nevertheless, his satisfaction must have been of the keenest. No triumphs give a pleasurehigher than those of the intellect. Let no one imagine that men who make researches in the domain of pure science are under-paid. They find their reward in pleasure that is beyond any price.
The Leverrier planet was found to be the last of the so-called major planets, so far as we can say in the present state of science. It received the name Neptune. Observers have found no other member of the solar system comparable in size with such bodies as Uranus and Neptune. More than one eager mathematician has tried to repeat Leverrier's achievement, but the supposed planet was not found. It has been said that figures never lie; yet such is the case only when the computations are correctly made. People are prone to give to the work of careless or incompetent mathematicians the same degree of credence that is really due only to masters of the craft. It requires the test of time to affix to any man's work the stamp of true genius.
While, then, we have found no more large planets, quite a group of companions to Piazzi's little one have been discovered. They are allsmall, probably never exceeding about 400 miles in diameter. All travel around the sun in orbits that lie wholly within that of Jupiter and are exterior to that of Mars. The introduction of astronomical photography has given a tremendous impetus to the discovery of these minor planets, as they are called. It is quite interesting to examine the photographic process by which such discoveries are made possible and even easy. The matter will not be difficult to understand if we remember that all the planets are continually changing their places among the other stars. For the planets travel around the sun at a comparatively small distance. The great majority of the stars, on the contrary, are separated from the sun by an almost immeasurable space. As a result, they do not seem to move at all among themselves, and so we call them fixed stars: they may, indeed, be in motion, but their great distance prevents our detecting it in a short period of time.
Now, stellar photographs are made in much the same way as ordinary portraits. Only, instead of using a simple camera, the astronomer exposes his photographic plate at the eye-end ofa telescope. The sensitive surface of the plate is substituted for the human eye. We then find on the picture a little dot corresponding to every star within the photographed region of the sky. But, as everyone knows, the turning of the earth on its axis makes the whole heavens, including the sun, moon, and stars, rise and set every day. So the stars, when we photograph them, are sure to be either climbing up in the eastern sky or else slowly creeping down in the western. And that makes astronomical photography very different from ordinary portrait work.
The stars correspond to the sitter, but they don't sit still. For this reason it is necessary to connect the telescope with a mechanical contrivance which makes it turn round like the hour-hand of an ordinary clock. The arrangement is so adjusted that the telescope, once aimed at the proper object in the sky, will move so as to remain pointed exactly the same during the whole time of the photographic exposure. Thus, while the light of any star is acting on the plate, such action will be continuous at a single point. Consequently, the finished picture will show thestar as a little dot; while without this arrangement, the star would trail out into a line instead of a dot. Now we have seen that the planets are all moving slowly among the fixed stars. So if we make a star photograph in a part of the sky where a planet happens to be, the planet will make a short line on the plate; whereas, if the planet remained quite unmoved relatively to the stars it would give a dot like the star dots. The presence of a line, therefore, at once indicates a planet.
This method of planet-hunting has proved most useful. More than 400 small planets similar to Piazzi's have been found, though never another one like Uranus and Neptune. As we have said, all these little bodies lie between Mars and Jupiter. They evidently belong to a group or family, and many astronomers have been led to believe that they are but fragments of a former large planet.
In August, 1898, however, one was found by Witt, of Berlin, which will probably occupy a very prominent place in the annals of astronomy. For this planet goes well within the orbit ofMars, and this will bring it at times very close to the earth. In fact, when the motions of the new planet and the earth combine to bring them to their positions of greatest proximity, the new planet will approach us closer than any other celestial body except our own moon. Witt named his new planet Eros. Its size, though small, may prove to be sufficient to bring it within the possibilities of naked-eye observation at the time of closest approach to the earth.
To astronomers the great importance of this new planet is due to the following circumstance: For certain reasons too technical to be stated here in detail, the distance from the earth to any planet can be determined with a degree of precision which is greatest for planets that are near us. Thus in time we shall learn the distance of Eros more accurately than we know any other celestial distance. From this, by a process of calculation, the solar distance from the earth is determinable. But the distance from earth to sun is the fundamental astronomical unit of measure; so that Witt's discovery, through its effect on the unit of measure, will doubtless influenceevery part of the science of astronomy. Here we have once more a striking instance of the reward sure to overtake the diligent worker in science—a whole generation of men will doubtless pass away before we shall have exhausted the scientific advantages to be drawn from Witt's remarkable observation of 1898.
Long before clocks and watches had been invented, people began to measure time with sun-dials. Nowadays, when almost everyone has a watch in his pocket, and can have a clock, too, on the mantel-piece of every room in the house, the sun-dial has ceased to be needed in ordinary life. But it is still just as interesting as ever to anyone who would like to have the means of getting time direct from the sun, the great hour-hand or timekeeper of the sky. Any person who is handy with tools can make a sun-dial quite easily, by following the directions given below.
In the first place, you must know that the sun-dial gives the time by means of the sun's shadow. If you stick a walking-cane up in the sand on a bright, sunshiny day, the cane has along shadow that looks like a dark line on the ground. Now if you watch this shadow carefully, you will see that it does not stay in the same place all day. Slowly but surely, as the sun climbs up in the sky, the shadow creeps around the cane. You can see quite easily that if the cane were fastened in a board floor, and if we could mark on the floor the places where the shadow was at different hours of the day, we could make the shadow tell us the time just like the hour-hand of a clock. A sun-dial is just such an arrangement as this, and I will show you how to mark the shadow places exactly, so as to tell the right time without any trouble whenever the sun shines.
If you were to watch very carefully such an arrangement as a cane standing in a board floor, you would not find the creeping shadow in just the same place at the same time every day. If you marked the place of the shadow at exactly ten o'clock by your watch some morning, and then went back another day at ten, you would not find the shadow on the old mark. It would not get very far from it in a day or two, but in amonth or so it would be quite a distance away. Now, of course, a sun-dial would be of no use if it did not tell the time correctly every day; and in fact, it is not easy to make a dial when the shadow is cast by a stick standing straight up. But we can get over this difficulty very well by letting the shadow be cast by a stick that leans over toward the floor just the right amount, as I will explain in a moment. Of course, we should not really use the floor for our sun-dial. It is much better to mark out the hour-lines, as they are called, on a smooth piece of ordinary white board, and then, after the dial is finished, it can be screwed down to a piazza floor or railing, or it can be fastened on a window-sill. It ought to be put in a place where the sun can get at it most of the time, because, of course, you cannot use the sun-dial when the sun is not shining on it. If the dial is set on a window-sill (of a city house, for instance) you must choose a south window if you can, so as to get the sun nearly all day. If you have to take an east window, you can use the dial in the morning only, and in a west window only in the afternoon. Sometimesit is best not to try to fasten the dial to its support with screws, but just to mark its place, and then set it out whenever you want to use it. For if the dial is made of wood, and not painted, it might be injured by rain or snow in bad weather if left out on a window-sill or piazza.
Fig. 1.
Fig. 1.
It is not quite easy to fasten a little stick to a board so that it will lean over just right. So it is better not to use a stick or a cane in the way I have described, but instead to use a piece of board cut to just the right shape.
Fig. 1 shows what a sun-dial should look like.The lines to show the shadow's place at the different hours of the day will be marked on the board ABCD, and this will be put flat on the window-sill or piazza floor. The three-cornered piece of boardabcis fastened to the bottom-board ABCD by screws going through ABCD from underneath. The edgeabof the three-cornered boardabcthen takes the place of the leaning stick or cane, and the time is marked by the shadow cast by the edgeab. Of course, it is important that this edge should be straight and perfectly flat and even. If you are handy with tools, you can make it quite easily, but if not, you can mark the right shape on a piece of paper very carefully, and take it to a carpenter, who can cut the board according to the pattern you have marked on the paper.
Fig. 2.
Fig. 2.
Now I must tell you how to draw the shape of the three-cornered boardabc. Fig. 2 shows how it is done. The sideacshould always be just five inches long. The sidebcis drawn at right angles toac, which you can do with an ordinary carpenter's square. The length ofbcdepends on the place for which the dial is made. The following table gives the length ofbcfor various places in the United States, and, after you have marked out the length ofbc, it is only necessary to complete the three-cornered piece by drawing the sideabfromatob.
Table Showing the Length of the Sidebc.
Place.b cPlace.b cInches.Inches.Albany411-16New York43-8Baltimore41-16Omaha43-8Boston41-2Philadelphia43-16Buffalo411-16Pittsburg43-8Charleston31-4Portland, Me413-16Chicago41-2Richmond315-16Cincinnati41-16Rochester411-16Cleveland41-2San Diego31-4Denver43-16San Francisco315-16Detroit41-2Savannah31-8Indianapolis41-16St. Louis315-16Kansas City315-16St. Paul5Louisville315-16Seattle59-16Milwaukee311-16Washington, D. C.41-16New Orleans27-8
If you wish to make a dial for a place not given in the table, it will be near enough to use the distancebcas given for the place nearest to you. But in selecting the nearest place from the table, please remember to take that one of the cities mentioned which is nearest to you in a north-and-southdirection. It does not matter how far away the place is in an east-and-west direction. So, instead of taking the place that is nearest to you on the map in a straight line, take the place to which you could travel by going principally east or west, and very little north or south. The figure drawn is about the right shape for New York. The board used for the three-cornered piece should be about one-half inch thick. But if you are making a window-sill dial, you may prefer to have it smaller than I have described. You can easily have it half as big by making all the sizes and lines in half-inches where the table calls for inches.
After you have marked out the dimensions for the three-cornered piece that is to throw the shadow, you can prepare the dial itself, with the lines that mark the place of the shadow for every hour of the day. This you can do in the manner shown in Fig. 3. Just as in the case of the three-cornered piece, you can draw the dial with a pencil directly on a smooth piece of white board, about three-quarters of an inch thick, or you can mark it out on a paper pattern and transfer it afterwardto the board. Perhaps it will be as well to begin by drawing on paper, as any mistakes can then be corrected before you commence to mark your wood.