Achievements Of The 19th Century:
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( Originally Published Early 1900's )
Progress in astronomy has been so great during the Nineteenth Century that a mere catalogue of the discoveries, and those to whom they are due, would fill vastly more than the space that we have at our disposal, and the work of the astronomer being such that scores of men aid in the making of various discoveries there would be endless controversies as to whom the credit should be awarded. Hence, in this chapter we shall give merely a few of the more important discoveries typical of the direction in which the star-gazers have been working and tell of some things that have been learned about the solar and sidereal heavens during the Century.
Astronomy was further advanced than any other science at the beginning of the Century and during the period there have been no discoveries of great principles rivaling Newton's researches in gravitation. It was known long before the beginning of the Century that the world is not the all-important center of the universe, that it is only one of several planets revolving around the sun, and that that sun was itself only one of millions of other similar-suns that we call stars, and each of which probably has planets revolving around it, some of them larger and others smaller than the earth. Man had come to a realization that his earth was to the universe less than a grain of sand was to the earth. We know of the existence of hundreds of millions of stars like our sun and the nearest star of these is more than 200,000 times as far away as our distance from the sun. These last facts, learned within the last hundred years, illustrate the tendency of astronomical research during the Century. The knowledge of the laws governing heavenly bodies having been discovered, the astronomer of the Nineteenth Century has directed his efforts to finding out what the stars are, and their nature. Their chemistry and physics have been investigated by aid of the spectroscope and the large telescopes have enormously increased the number of stars visible while the application of photography to astronomy has rendered telescopes even more powerful in detecting the presence of celestial bodies and recording their motions.
This attention to detail has been necessary because of the vastness of the subject. Before the days of great telescopes the naked eye of a few astronomers covered the heavens after a fashion. But the new telescopes magnify so greatly that if every man in the world were an astronomer with his eyes glued to a different telescope, and each looked at a different part of the heavens, their whole range would not be covered. And so the astronomer of to-day regards star-gazing as a small part of his duties. There are scores of men who spend a good deal of time at the telescope, but for these there are hundreds of astronomers who never look into a telescope at all except for amusement. The post of observer is not the most important at an observatory, and one active observer will keep ten men busy doing difficult sums in mathematics that lead to their conclusions. Hence the astronomer is familiar with but few stars aside from those he makes his specialty. He knows half a dozen or so and if he wishes to know any-thing about the others he can find their location in the star catalogue; yet not one astronomer in a hundred has observed one thousandth of the stars that are known to exist.
The discovery of the planet Neptune, which is accounted the greatest astronomical discovery of the Century, affords an excellent illustration of astronomical methods. Five planets, or stars revolving around our sun with fixed orbits, Jupiter, Saturn, Mercury, Venus, and Mars had been known to observers of the heavens from the earlier ages, but no addition had been made until William Herschel, then organist at the Octagon Chapel at Bath, had, with the aid of a home-made telescope, discovered on March 13, 1781, the orb which received the name of Uranus. It had been overlooked by earlier astronomers, who had mistaken it for a fixed star. After the discovery of the new planet astronomers looked over the investigations of ancient observers, and, knowing that it would take Uranus eighty-one years to travel around the sun, and the direction of its path they were able to figure where it should have been at various periods in the history of the universe. It was also found that Uranus was not traveling in the course that it should have taken. By 1845 it was the "intolerable quantity of two minutes of arc" out of the way. The only way in such a deflection could be accounted for was by the supposition that it was due to the attraction of gravitation from some planet other than those whose effects had already been made known. From this it followed that there must be another great planet in the solar system besides those of which astronomers then knew.
The search for the new planet might be made by telescope, but that would be like hunting for a needle in a haystack. So two young astronomers set themselves to the effort to figure its location by the aid of mathematics. Only two had the patience and thought it worth while to pursue the calculations. They were Leverrier, of France, and Adams, of England. After each had spent about two years in independent mathematical calculations, both succeeded in finding the track of a hypothetical planet, as well as circumstances of its motion, which would account for the irregularities in Uranus' orbit. At about the same time they announced where the planet was to be found and the locations agreed within a half of a degree. All that was necessary was for astronomers to point their telescopes to the spot indicated. This they did on the night of September 23, 1846, and Neptune was added to the list of planets in the solar system.
The discovery of Leverrier and Adams is chiefly interesting as a proof of the correctness of theoretical astronomy. Sooner or later Neptune would have been discovered by the means of the star charts that are made. Such observations, together with greater telescopes, have brought about the discovery of the asteroids or small planets. All of these discoveries were made during the Nineteenth Century. Piazzi, the Sicilian astronomer, found the first member of the group on the very first night of the present Century. Pallas was discovered by Olbers during the next year, Harding found Juno in 1804, and the fourth, Vesta, the only one brilliant enough ever to be seen with the naked eye, was observed in 1807. Encke discovered the fifth asteriod, Astraea, in 1845 ; three more were discovered in 1847 and since then every year has added to the list, until at the close of 1898 there were 429, six having been added during the year 1898. The new discoveries have been made possible by the use of photography.
Thus the knowledge of the existence of asteroids, and hence their study, has been a development of the Century. Of those we know, Medusa, 198,000,000 miles from the sun, is the nearest to that body, and Thule, which is 400,000,000 miles distant, is the furthest away. Professor Barnard at Lick Observatory measured the diameters of Ceres, Pallas, and Vesta, micrometrically, and found that Ceres is the largest, with a diameter of 488 miles. Pallas is 304 miles in diameter and Vesta 248 miles, while with the exception of Juno, none of the others are greater than 100 miles in diameter, while some are not more than 10 or 12 miles. These Asteroids are located in the space between Mars and Jupiter, and, being in a bunch, the general belief is that they are a part of single planet which either failed to unite in accordance with the nebular hypothesis, or else that they are fragments of an exploded planet.
The first calculated return of a comet was that of Encke's, on May 24, 1822, which was another triumph of theoretical astronomy. Discovered by M. Pons, November 26, 1818, its orbit, motions and perturbations were determined by Encke, who declared that it should return every three years and fifteen weeks, and it has done so in accordance with the calculation. We now know of the existence of 680 comets, although there must be vastly more perhaps hundreds of thousands for, although it is seldom that one is not in sight, yet many too small for our telescopes to detect must be near us all the time. Much has been learned about the nature of comets and astronomers declare nowadays that so far from threatening the earth with danger they do not exert the slightest influence. The most imposing feature of the comet is its tail, and this has been found to be seldom less than 5,000,000 miles in length, in the case of those visible to the naked eye, while several are known to have been 100,000,000 miles in length, a length 120 times as great as the diameter of the sun which latter body is 109 times the diameter of the earth. Cometary matter is so rare that Babinet in 1857 announced that a comet's tail traversed by the earth might be unnoticed. It is certainly so diaphanous that even the stars may be seen through the visible portions of a comet. Babinet also estimated that the chance of a collision between the head of a comet and the earth is so slight that it can occur only on an average of not more than once in about 15,000,000 years. The close connection of comets with the periodical showers of meteors, usually observed in August and November, was first demonstrated by H. A. Newton, of Yale, in 1864, and is now universally admitted. Astronomers suppose the meteors to be the result of the gradual disintegration of the comets. When the earth was passing the track of Biela's comet (discovered by Biela, an Austrian, in 1826) on November 27, 1872, she encountered a wonderful meteoric shower, and it was then declared that Biela's comet was shedding over us the pulverized products of its disintegration.
Comte, the French philosopher and mathematician, who was perhaps the wisest man of his day, declared that it was impossible for us ever to know anything as to the materials of which the stars were composed, because they were so far from us. The distance is great, it is true. Light traveling from the sun at the rate of 186,000 miles a second, requires 499 seconds or about 8 1/3 minutes to reach us, while light coming from the nearest star traveling at the same rate of speed requires seven years to reach us, and from the most distant known from 2,000 to 3,000 years. Thus the problem of learning any-thing about the materials of which the stars are composed might well have seemed impossible even to a man like Comte. But the problem has been solved and we are now able to tell of what certain stars are composed or at least if we cannot tell all the ingredients of which they are composed, we can tell a great many of them. We have, for example, the same certainty of existence of iron in the sun as we have of its existence in the poker and tongs on the hearth.
A remarkable train of discoveries leading to the construction of the spectroscope is responsible for the revelation of the nature of the substances that enter into the composition of the heavenly bodies. That process is called spectrum analysis. Two centuries ago Sir Isaac Newton made his celebrated analysis of light by means of a prism. Every one who is at all observant must have noticed that if a strong light, especially sunlight, falls upon the triangular prisms used to ornament gas fixtures, spots of light containing all the colors of the rainbow will be cast upon nearby objects. Students of science investigated to learn the cause of this action. It was soon ascertained that the shape of the prism and its position to the direction of light would change materially the size of the spot. The reasons for this have been fully investigated, and the stock 0f knowledge relating to the subject is very extensive, but we will not go into it any further than is necessary for the purpose of this article. If a properly constructed prism is fixed in a darkened room so that a small ray of sunlight, coming from through a hole in the door, strikes it at the proper angle, a band of colored light, called a solar spectrum, will be thrown upon a screen placed at a suitable distance. The color of light at one end of this band will be red and at the other violet, while the intermediate colors will be orange, yellow, green, blue, and indigo. The band of light is like a slice cut from a rainbow. Close inspection with a suitable magnifying glass by Wollaston in 1802, revealed the existence, however, of quite a number of dark bands of different widths and located at different distances from each other. Fraunhofer was the first person to study these lines, and he gave a detailed description of them in 1814. Instead of looking through the prism with a naked eye he used a telescope, placing the prism and the telescope a distance of twenty-four feet from the slit, the virtual image of which was thus considerably magnified. Fraunhofer gave a detailed description of these lines and showed the exact position of the more prominent ones, which are therefore known by his name. These lines are always seen in the 'same position when the light that passes through the prism is that of the sun, but if a candle or a gas-jet is used such will not be the case. Some of the light and lines will disappear and other lines, that were not in position before, will come into view.
Experiments carried on by others since Fraunhofer developed the fact that if in the flame of a gas-jet different substances be burned, the lines shown in the spectrum cast by the prism will be changed, and that certain materials will produce certain lines, while others will produce entirely different ones. It has also been found that the actual number of dark lines in the sun spectrum is vastly greater than was at first supposed. As many as 3,000 have been counted with properly constructed instruments. Inasmuch as different substances burned in the flame develop different lines in the spectrum, it was at once inferred that an instrument so constructed as to properly note their number and position would afford ready means for determining the composition of combustible substances. Kirchoff and Bunsen devised such an instrument in 1859, and it is known as the spectroscope. There are many types of this instrument, but the essential parts are one or more prisms, a slit through which the light examined is allowed to enter; a tube having at the other end a lens to render parallel the rays from the slit; a telescope through which the spectrum is viewed, and usually some apparatus by which the different lines may be identified. It is so arranged that two spectrums can be compared one that of a substance whose spectrum is familiar, and the other that of the substance whose spectrum is to be examined.
At first spectrum analysis was used to compare the spectrums of various earthly substances to detect the presence of this or that element and it afforded an absolute detection of any adulteration, while by its aid, as has been told in the article on Chemistry, many new elements of great rarity have been discovered. One of these that affords a striking instance of our knowledge of the composition of celestial bodies is helium. During an eclipse in 1868 Professor Lockyer discovered the presence of a substance in the sun, and being at that time unknown on earth, it was named the sun element helium. For twenty-five years we knew it only as an element in heavenly bodies, but in 1895 Professor Ramsay obtained a gas .from a rare mineral named Clevite, which is found in Norway, and the spectrum of this gas proved it identical with that of the helium of the stars.
Numerous experiments have shown us just what materials must be burned to produce many of the dark lines in the solar spectrum, and in this way we have ascertained that the gaseous covering contains, among other things, iron, magnesium, calcium, chromium copper, zinc, nickel barium, sodium, and other elements, in all thirty-six, known on the earth. Knowing the composition of the surface of the sun we gain an idea of that of the interior mass. The stratum of gases outside of the sun is known as the chromosphere, and is brilliantly scarlet, because of the predominating presence of hydrogen gas. Like a sea of flame it covers the photosphere or shining surface of the sun to a depth of 5,000 to 10,000 miles, and though in-tensely hot, there is no real burning such as combustion as we know it.
By spectroscopic and other examinations of the sun and its rays, it has been found that the sun's light is a mass of heated carbon and we receive 600,000 times as much light from the sun as from the full moon. The sun is surrounded by an extensive and rare atmosphere, the photosphere, the invisible source of solar light; the chromosphere, chiefly of hydrogen gas, and the corona, a vast shell of unknown vapor many thousands of miles in thickness. The diameter of the sun has been found to be 852,000 miles, or 109 times that of the earth, and, as it is a perfect sphere, its surface is 11,900 times as great as that of the earth, while its volume exceeds the earth 1,305,725 times, and its mass is 332,260 times that of the earth. Its density is about one-quarter that of the earth, or rather more than that of water. The heat received from the sun is thirty calories, from which it is computed that the amount of heat reaching this world from the sun in a year would be sufficient to melt a shell of ice 165 feet thick all over the earth's surface. Lord Kelvin calculated that the quantity of fuel required for each square yard of the solar surface would be no less than 13,500 tons of coal an houró. equivalent to the work of a steam-engine of 63,000 horse power. The temperature of the sun is estimated at from 16,000 to 18,000 degrees Fahrenheit. The earth receives only about one part in each 2,200,000,000th part of the total radiation of heat from the sun. The distance of the sun from the earth (by Copernicus) was supposed to he 4,800,000 miles, but spectrum analysis, aided by the known velocity of light, has made astronomers agree that 92,890,000 miles is within 150,000 miles of the correct distance.
The spectra of the stars have been studied as well as those of the sun, and examinations have been made of many hundreds. It has been found from these investigations that while they ,are not all of precisely the same composition, yet in every case they are of the same general character as our own sun and thus the supposition of earlier days that they were suns, the centers of solar system resembling ours, has in this Century become a certainty. Some are hotter, some smaller, and some more luminous than others, they differing as do various species of animals. So far off are many of the stars that it is supposed by some astronomers that they were dead and gone perhaps mil-lions of years ago, and are still visible because the light from them that began traveling millions of years ago has not yet reached us. The nearest of these stars is more than 200,000 times as far from us as is the distance from the sun, and the Lick and Yerkes telescope could make visible at least 100,000,000 stars. Though the stars are called "fixed," merely to distinguish them from the planets, yet they are really in motion at a rate faster than that of a cannon ball. Sirius, the most brilliant of the stars, gives us one seven-thousand-millionth of the light that is given by our own sun, yet Sirius is really radiating forty times as much light as is the sun.
The spectroscope has revealed that the many bright patches seen in the heavens on clear starlight nights, and which were formerly supposed to be the wake of very distant stars, are simply gaseous clouds or nebulae. This was demonstrated by Huggins in 1866, and to him many other discoveries are also due, he having been a leader in spectroscopic astronomy. Some 8,000 nebulae have been noted and many of them photographed, the first work in this latter direction having been done by Henry Draper, of New York, in 1880. Huggins' spectroscopic researches also made possible the noting of movements of stars along the line of vision, and their actual velocity can be deter-mined in many cases. By means of displacements of spectral lines Huggins discovered that stars are receding and approaching us. Arcturus, the one traveling toward us most rapidly, is coming at the rate of fifty-five miles a second, and Sirius, which is becoming more distant, moves away at a rate of twenty-six miles per second. Yet in spite of this motion of the stars, it is so slight that the eye, unaided by the spectroscope, could not detect any change in position during a life-time.
It is interesting to recall at the present time that the largest telescope in its day was constructed for the observatory of Madrid, and was placed in the Spanish capital in 1802. Today the United States has the largest telescope in the world. Important as the size of the telescope is, it must be-remembered that it has been shown there are other things of more importance. It is also true that much depends upon the man at the little end of the telescope as well as upon the size of the lens. Yet great telescopes have undoubtedly been of enormous service to astronomy. They have made possible the detection of thousands of more stars than could be seen with the smaller instruments, and it is due to this and photography that we know of the existence of three hundred times as many stars as we did a Century ago. The principle of the telescope has not changed during the Century, and telescopes are still of two kinds, reflecting and refracting. Of the former type the largest is that erected in 1828-45 by the Earl of Rosse at Parsontown in Ireland, which is 6 feet in diameter and 54 feet long. The largest refracting telescope is that of the Yerkes observatory, which was completed in 1895, and has a 40 inch lens, with a length of 70 feet. The refracting telescope is the favorite in observatories and the Lick telescope at Mt. Hamilton, Cal., was the largest until the Yerkes telescope was mounted. It has a 36 inch refractor, while the largest in the Old World is that at Pulkowa in Russia, with a 30 inch lens, built in 1884. The growth in size of refractors is well shown by the fact that the telescope at Pulkowa, built in 1840, and which was the largest in its day, had a refractor of 14.9 inches.
The work of making the lens of these great instruments requires years of toil, for a single error might destroy $40,000 worth of raw material in a 40 inch glass. The rough disks are smoothed down with revolving con-cave tools, then the surfaces are smoothed and polished with rouge, by hand, no mechanical process being found that will answer the purpose. The lens is tested by looking through it at a star. Handling it for this test is no small task as the lens and its ring weigh about 1,000 pounds. The Yerkes lens is called a 40 inch glass, but its exact diameter is 41 3/8 inches; the crown is about 36 inches thick at the middle and I 1-4 inches thick at the outer edges. Alvin G. Clark, who made the Yerkes glass, said that it was not his limit. He declared before his death that he could grind a perfect lens 45 inches in diameter. The difficulty in the way of making a larger lens is the weight and flexibility of the glass, as a glass larger than 45 inches would surely bend of its own weight while there are those who believe that the 45-inch glass is impossible. The work of mounting a big telescope is no small engineering feat. The tubes of the Yerkes telescope weigh 75 tons, and the instrument must move with precision, keeping time with the stars. Electric motors are employed for the manipulation of the telescope.
While the greatest astronomical discoveries have not been made with big telescopes, yet these gigantic instruments have done much important service. Bond found Hyperion in 1848 with the Harvard telescope, Lassell's telescope in 1846 discovered the satellite of Neptune, while the resolvability of many nebulae and their spiral structure was discovered by the great telescopes which have been of great assistance also in the detection of asteroids. E. E. Barnard's discovery of Jupiter's fifth satellite, Burnham's discovery of many double stars and Asaph- Hall's detection of the two satellites of Mars were made possible by giant lenses.
One of the most important developments of astronomy during the Century has been in the application of photography to the science. The eye of the astronomer gazing at the heavens becomes tired and the longer he looks the less he is able to see. Not so with the camera, which continually records the stars it sees, and is able to, detect many that would be unnoticed by astronomers. On this account telescopes are provided with camera attachments and many important discoveries, such as Keley's securing of actual proof, in 1896, of the meteoric constitution of Saturn's rings. Myriads of stars beyond the ken of the most powerful telescope have been revealed by the aid of the astro-photography. One of its most important uses has been in the work of making a photographic chart of the heavens. Millions of stars will be depicted on this chart, and the undertaking is now well on its road to completion. Millions of those detected by the camera were unknown before its use in astronomy. The preservation of these photographic records will be also of importance in showing the movements of the stars to astronomers of future ages. Thus they will be able to acquire new facts in regard to the mighty voyage through space which is being made by our sun and all his system.