Astronomy
the science that treats vided into two branches, mathematical and physical, and these are almost sy- Astronomy may to-day be broadly dinonymous with two terms recently introduced, the old and the new astronomy, as defined by the statement that the old tells us where the heavenly bodies are, the new, what they are. The characteristic feature of the instruments and methods of the new versus the old characteristic feature of the instruments measuring or analyzing the vibrations astronomy is that the new deals with transmitted throughout all space by some special form of radiant energy, means ether.
of the elastic medium called above, mathematical astronomy would Under the two broad divisions stated include the following divisions, which Spherical astronomy, which treats of angles and directions on the celestial include the following divisions, which of the instruments, methods of observasphere; practical astronomy, treating tion, and of calculation employed to get at the facts and data of astronomy; the orbits, tables and ephemerides of the ing the effect of their mutual attractions, and gravitational or mechanical sun, moon, planets, and comets, includastronomy, which treats of the forces (principally gravitation) at work in space and the motions resulting therefrom. This last was formerly called physical astronomy, but the name has space and the motions resulting therebranch, likewise called astronomical within the last few decades, and must physics and astro-physics, attempts to now be reserved for it. This second enly bodies are, the nature and constitution of their interiors, surfaces, answer the question of what the heavatmosphere, their temperatures and radi- ASTRONOMY ations, and the effect of these radiations upon other bodies, and all allied questions arising out of these. Its principal instrument, the spectroscope, has likewise furnished data otherwise unattainable in the field of mathematical astronomy, viz., the determination of the motion to or from us of the heavenly bodies by displacement of the lines of their spectra due to this motion.
COELOSTAT RAYS FROM NOS. -2ND. MIRROR -150 FI FOCUS LENS DADAN VAVAVAYAVAVAVAVAV 4 OBSERVATION HOUSE PHOTOGRAPHIC PLATE -RAYS UPWARD TO PLATE ASTRONOMY cycle of 223 lunations, nearly equal to 18 years, by which they predicted the return of previously observed eclipses and made use of other empirical cycles or periods.
Thales (640 в. c.), the founder of the Ionic school, school, laid the foundation of Greek astronomy. The successors of Thales held opinions which, in many respects, are wonderfully in accordance with modern ideas. Anaximander, it is said, held that the earth moved about its own axis, and that the moon's light was reflected from the sun. To him is also attributed the belief in the plurality of worlds. Pythagoras (500 B. c.) promulgated the true theory that the sun is the center of the planetary world, and that the earth revolves round it. But the views of Pythagoras met with little or no support from his successors until the time of Copernicus. Between Pythagoras and the advent of the Alexandrian school, nearly two centuries later, among the most prominent names in astronomical annals is that of Meton, who introduced the Metonic Cycle, consisting of 125 months of 30 days each, and of 100 of 29 days, making a period of 6,940 days, nearly equal to 19 solar years.
To the Alexandrian school, owing its existence to the Ptolemies, we are indebted for the first systematic observations in astronomy. Hipparchus of Bithynia (160-125 B. C.), was a theorist, a mathematician, and observer. He catalogued no less than 1,081 stars. He discovered the precession of the equinoxes; he determined the mean motion as well as the inequality of the motion of the sun, and the length of the year; also the mean motion of the moon, her eccentricity, the equation of her center and the inclination of her orbit; and he suspected RAYS DOWNWARD the inequality afterward found by Ptol- LENS GRATING DIAGRAM OF COELOSTAT OF MT. WILSON OBSERVATORY. THE TOWER IS 164' HIGH History. The Chinese, Hindus, Chaldeans, Egyptians, and Greeks investigated the heavens long before the Christian era. In China, astronomy was intimately associated with state politics: the Indians, Chaldeans, and Egyptians made it a matter of religion. The Greek historians attribute the earliest knowledge of astronomical science to the Chaldeans and Egyptians. They say that the former discovered the Saros or emy (the evection). After the death of Hipparchus, astronomy languished for nearly three centuries.
Ptolemy (130-150 A. D.), besides being a practical astronomer, was accomplished as a musician, a geographer, and a mathematician. His most important discovery in astronomy was the evection of the moon. He also was the first to point out the effect of refraction.
He was the founder of the false system known by his name, and which was universally accepted as the true theory of the universe until the researches of Copernicus exploded it. The Ptolemaic system placed the earth, immovable, in the center of the universe, making the entire heavens revolve round it in the ASTRONOMY course of 24 hours. The work by which he is best known, however, is the colthe ancient observations in his great the ancient observations in his great gives a complete résumé of the astrogives a complete résumé of the astronomical knowledge of the day. The most important part of it is the seventh and eighth books, which contain the catalogue of stars which bears Ptolemy's name, though it is only a compilation of name, though it is only a compilation the catalogue of Hipparchus with the positions brought up to the time of Ptol- ASTRONOMY moon's motion, the variation, and determined its amount. The revival of astronomy in Europe may be said to have begun with George Purbach, who translated the "Almagest" at Vienna. His pupil, John Muller, translated into Latin the works of Ptolemy and the conics of Appolonius, built an observatory at Nuremberg, and invention. He died in 1476. equipped it with instruments of his own Copernicus (1473-1543) exploded the Ptolemaic idea, and promulgated a cor- SHUTTER CANVAS DOME PROTECTION ✓ SKELETON TUBE OBSERVATION PLATFORM 21 FOOT SPECTROGRAPH PER
THE DOME AND MOUNTING FOR A 60" REFLECTING TELESCOPE emy. These latter are in use to-day, rect theory. It makes the sun the imthough the gaps between them have been filled up in some cases by more modern asterisms.
To the Arabs we owe the next adbeen filled up in some cases by more trious of the Arabian school were Alba- To the Arabs we owe the next adcovered the motion of the solar apogee, and who was the first to make use of sines and versed sines instead of chords; covered the motion of the solar apogee, mathematician, who made observations of great importance in determining the and Ibn-Yunis (1000 A. D.), an excellent ter and Saturn, and who was the first of great importance in determining the wise, at about the same time, bul Wefa discovered the third inequality in the movable center of the universe, around tric orbits, Mercury and Venus within which all the planets revolve in concenthe earth's orbit, and all other planets without it.
Decidedly the most industrious obwithout it. Decidedly the most industrious obfrom the time of the Arabs to the latter from the time of the Arabs to the latter Brahe (1546-1601). He made the first table of refractions, and discovered the variation and annual equation of the moon, the inequalities of the motion of the nodes, and of the inclination of the lunar orbit. He also demonstrated that the region of the comets is far beyond the orbit of the moon, and he determined the positions of 777 stars with ASTRONOMY an accuracy far surpassing anything before done in that line. He left behind him a mass of observations of the sun and planets which he had made to demonstrate the truth of his system of the universe, but which afterward became, in Kepler's hands, the means of ASTRONOMY haps the greatest of these was the invention of logarithms by Lord Napier. In 1603, John Bayer, of Augsburg, published his "Uranometria," or maps of the 48 constellations which had been handed down from Hipparchus through Ptolemy in the "Almagest," and, on THE 40" REFRACTOR TELESCOPE AT YERKES OBSERVATORY its overthrow and the final and permanent establishment of the truth of the Copernican system. Kepler's brilliant discovery of the three laws of planetary motion made his name immortal.
Galileo Galilei was the contemporary of Kepler, and, as his discoveries were of a more popular character, he obtained a more immediate fame and reputation. In the interval between the great discoveries of Kepler and Galileo and those of Newton various astronomers made valuable additions to astronomical knowledge or invented new apparatus for observing the heavenly bodies. Perthese maps, he for the first time assigned to the individual stars the letters that are used to-day. The researches of Descartes gave a new help to mathematical analysis. Horrox observed the transit of Venus in 1639, the first ever seen by man.
The most accurate determinations of the positions of the heavenly bodies made without the help of the telescope were those of Hevelius, a rich citizen of Danzig. The catalogue of stars which bears his name, and by whose numbers in the different constellations the individual stars are still called to-day with ASTRONOMY the distinguishing letter "H," is the greatest of the results of his labors.
Newton's fame rests on his discovery of the law of gravitation, announced in the "Principia" in 1677. Newton also made the important discovery of the revolution of comets around the sun in conic sections, proved the earth's form to be that of an oblate spheroid, gave a theory of the moon and tides, invented fluxions and wrote on optics. While the foundations of gravitational astronomy were thus broadly laid by Newton, Flamsteed, the first astronomer-royal at Greenwich, and Halley were greatly improving and extending the practical department of the science. To Flamsteed we are indebted for numerous observations on the fixed stars, on planets, satellites and comets, and for a catalogue of 2,884 stars. His "Historia Cœlestis" formed a new era in sidereal astronomy. Dr. Halley, who succeeded Flamsteed as astronomer-royal, discovered the accelerated mean motion of the moon, and certain inequalities in Jupiter and Saturn, but he is most famed for his successful investigations into the motions and nature of comets. His successor was Dr. Bradley, who, in the year of Newton's death, made the important discovery of the aberration of light, which furnishes the most conclusive proof we have of the earth's annual motion. While Bradley was at work at Greenwich, at the middle of the 18th century, Lacaille, a celebrated French astronomer, undertook a voyage to the Cape of Good Hope to determine the sun's parallax, by observations of Mars and Venus simultaneously with similar ones in Europe, and to form a catalogue of southern circumpolar stars. In a single year and single-handed he observed the positions of over 10,000 stars and computed the places of 1,942 of them.
The latter half of the 18th century was marked by the brilliant work of Sir William Herschel, who discovered the planet Uranus and its four satellites, and two additional satellites of Saturn; determined the direction of the motion of the solar system in space; resolved the Milky Way into countless myriads of stars, and opened up a boundless field of discovery and research among the nebulæ and double and multiple stars.
Maskelyne perfected the method of reducing observations of lunar distances at sea for the determination of longitudes, and had tables of lunar distances first published in the British "Nautical Almanac." Lalande observed the positions of by far the largest number of ASTRONOMY stars that had been catalogued up to the end of the 18th century. These were afterward reduced and published by Baily in a catalogue which contains over 47,000 star positions. Mayer, besides making a valuable catalogue of zodiacal stars at about the same time as those of Bradley and Lacaille, perfected lunar tables which were for many years the most accurate in existence. The 18th MT. WILSON OBSERVATORY and 19th centuries were astronomically connected by the work of Piazzi in the observatory established at Palermo in 1790, where on the night of Jan. 1, 1801, a new planet, the first of the numerous belt of planetoids between Mars and Jupiter, was discovered.
Modern Astronomy. - Friedrich Wilhelm Bessel contributed more than any other to the solid advancement of the science in the 19th century. Bessel combined in an extraordinary degree the qualities of an able mathematician and a skillful observer. Before mentioning particularly any of the other prominent astronomers of the early part of the 19th century, the celebrated optician Fraunhofer, who contributed so much to their success, deserves special notice. In connection with his experiments on light for the further perfection of his lenses, Fraunhofer was led to the discovery of ASTRONOMY the host of lines of the solar spectrum, of which he counted 600 and mapped 324, and which are to-day known as the "Fraunhofer Lines." Though he did not have the time to carry this discovery to its legitimate conclusion, this being afterward done by Kirchhoff and Bunsen, Fraunhofer's labors may be considered as the beginning of the new astronomy.
Friedrich Georg Wilhelm Struve (1793-1864) rendered his name immortal by the accurate determination, with the 9½ inch Dorpat refractor, of the position-angles, distances, colors and relative brightness of 3,112 double and multiple stars, about 2,200 of which were new discoveries. Friedrich Wilhelm August Argelander (1799-1875) ranks next to Bessel among the great astronomers of the 19th century. A pupil of the latter, he thoroughly imbibed the ideas of exactitude in astronomical observations for which his great master was pre-eminent, and he carried them out in all his subsequent work.
His first work was the observations, made while his observatory at Bonn was being completed, for the formation of the "Uranometria Nova," the accepted standard of stellar magnitudes.
John F. W. Herschel (1792-1871), following in the footsteps of his father, in 1834 began at the Cape of Good Hope a survey of the southern heavens, using an 18-inch reflector of his own construction. With this, in the course of four years, he accumulated a vast store of material, in the way of new double and multiple stars, nebulæ and starclusters, photometric measures of stellar brightness, "soundings" or "stargauges" in the Milky Way, to show the laws of the distribution of the stars in space, all of which form the starting point of our knowledge of the southern heavens. The work of Sir George Bid- Airy (1801-1892) next deserves attention. Appointed in 1835 to the attention. Appointed directorship of the Royal Observatory of Greenwich, he first carried to completion the great work begun at his suggestion two years before the complete reduction, on a uniform system, of all the Greenwich planetary observations from 1750 onward.
The greatest event of the century was the discovery of the planet Neptune.
The work of Lagrange and Laplace in the domain of gravitational astronomy was continued and vastly extended in the 19th century by several eminent mathematicians, astronomers, notably by Leverrier. His life was devoted to the perfection of the theory of the planetary motions. Adams, the equal sharer with ASTRONOMY him in the glory of the discovery of Neptune, has also made very important additions to our knowledge in the same field, and in the United States we have, in the persons of Simon Newcombe and George W. Hill, their worthy successors and collaborators. The amount of work which Newcomb published in the line of fundamental star places, the discussion of old eclipses and occultations, with their bearing on the theory of the moon's motion, the motion of Mercury, etc., was prodigious.
The theory of the moon's motion, or the lunar theory, as it is generally called, has from the beginning attracted the attention of the ablest mathematical investigators. The two who stand_out prominently before all others are Hansen and Delaunay. In various other branches of gravitational astronomy, several names deserve special mention.
Olbers, besides being the discoverer of several comets and of the second and fourth planetoid, is best known for his development of the best method of computing cometary orbits. Encke, a pupil of Gauss, developed the best methods of applying the method of least squares to computation, determined a value of the solar parallax, which stood a long time as giving the accepted value of 95,000,- 000 miles as the distance of the sun, but is best known for the discovery of the remarkably short period of the comet which bears his name.
Hall's detection of the two minute and remarkable satellites of Mars ranks next to that of Neptune as the most brilliant of the century. It was not an accidental picking up of easily visible object in sweeps, but the result of a well planned and careful search at the most favorable time, the opposition of 1877, after the erection of the 26-inch refractor of the United States Naval Observatory. Hall also kept up systematiellite systems of Saturn, Uranus, and cally the observation of the difficult sat- Neptune after taking charge of the 26inch Washington ref Neptune refractor. Next came the discovery of the fifth satellite of Jupiter, by Prof. Edward E. Barnard, of the Lick Observatory, Sept. 9, 1892.
This was followed by the discovery, March 18, 1899, of the ninth satellite of Saturn, by Prof. William H. Pickering, of the Harvard Observatory. The work done at Cordoba, in the Argentine Republic, by Dr. Benjamin Apthorp Gould and his assistants in 1870, must next be mentioned. Dr. Gould began the observation for a uranometry of the southern heavens, to include all stars down to the seventh magnitude.
This great work contains the names, ASTRONOMY ASTRONOMY positions, magnitudes, to the nearest son, Secchi, Crova, Violle, Langley, and 10th, or 7,730 stars situated between -10° and the South Pole, and the mag- nitudes, to the nearest quarter, of more than 1,000 others, mostly companions of these, or situated in clusters, the joint light of which equaled a seventh magnitude star. The New Astronomy. The spectroscope has been the principal instrument of investigation in the new astronomy. After the work of Kirchhoff and Bun- sen, the next important step was the investigation, with the diffraction spectroscope, by Angstrom and Thalen, of the formation of the so-called normal spectrum, in which the distances of the lines are proportional to their wavelengths. The map of the solar spectrum constructed in this way has been the standard for the wave-lengths of the Fraunhofer line until within a very few years. The work of Rowland at the ing directly the spectrum formed from Johns Hopkins University, photograph- ing directly the spectrum formed from vented by him), has so far exceeded the Angstrom maps that the latter may now ena attending the solar eclipses and of comets offered a new field for the spectroscope, and in this a host of names at once claim attention, principal among comets offered a new field for the spec- troscope, and in this a host of names at once claim attention, principal among which are those of Young, Hale, direc- Secchi, Huggins, Lockyer, Janssen, and Langley.
The simultaneous and independent discoveries by Lockyer and Janssen in connection with the Indian solar eclipse of August, 1868 (that the solar prominences, or hydrogen clouds surrounding the sun, can be studied at any time without the help of an eclipse), revoluthe sun, can be studied at any time part of the sun's surroundings.
Photometry, or the measurement of the brightness of the different heavenly bodies, so far as its results are concerned, is properly classed under the new astronomy. It has, however, been employed from the earliest times, without instrumental assistance, in classifying the stars into a scale of magnitudes, and in later days in observation of the employed from the earliest times, withstars.
Solar Investigations. - Sir John Herout instrumental assistance, in classify- ing the stars into a scale of magnitudes, and in later days in observation of the the amount of heat which we ure receive from the sun by noting the increase in the temperature of a given amount of water upon which a given beam of sunlight is allowed to fall for a certain time. Using various forms of receive from the sun by noting the increase in the temperature of a given or the amount of radiant energy which The most remarkable work of all in tions of the so-called "solar-constant," falls upon a square meter of surface at the upper limits of the atmosphere.
The most remarkable work of all in amounts of such radiations, which were entirely beyond the reach of all previous of this instrument minute that of Langley with his bolometer. By means amounts of such radiations, which were entirely beyond the reach of all previous experiments, can be detected and accurately measured. Further Progress. - In summarizing the growth of astronomy during the 19th century we enumerate the researches of double-star discoveries and measures of Struve (Otto), Dawes, Dembowski, and Elkin in stellar parallax; the double-star discoveries and measures of Struve (Otto), Dawes, Dembowski, Burnham, and Stone; the discoveries of comets by Pons, Tuttle, Tempel, Swift, Brooks, Barnard, and many others; the discovery and cataloguing of nebulæ by rington on sun spots and the positions and Dreyer; the elaborate work of Carrington on sun spots and the positions of northern circumpolar stars; the cornac, the Henry brothers, and especornac, the Henry brothers, and especially Peters; Chandler's important work in variable stars and in variation of latitude; the work of Schmidt on vawork in variable stars and in variation discovery of difficult planetary satellites by Lassel and Bond; the spectroscopic rious stars and in selenography; the discovery of difficult planetary satellites researches of Young, Schuster, Draper, Thollon, and Lohse; the determinations of the velocity of light by Fizeau, Fouof the velocity of light by Fizeau, Fouwork upon the parallax of Mars and some of the asteroids; Elkin's thorough eter; Darwin's investigation of the remeasurement of the position of the stars of the Pleiades with the heliomentirely new subject of the bearing of tidal friction upon the development of planetary and satellite systems and Stone's observations at the Cape, replanetary and satellite systems and Catalogue," which ranks next to the work of Gould in furnishing us exact positions of the stars of the southern the transit of Venus should also be noted. heavens. Harkness' work upon the reduction of the American observations of Instruments. The history of the progress of astronomy in the 19th century would be incomplete without a mention of the remarkable opticians and mechanicians whose handiwork has made it possible. We have already mentioned Fraunhofer. Pre-eminent among them all are the names of the late Alvan Clark, of Cambridge, Mass., and his sons, ASTROPHYSICS George B. and Alvan G. Their latest masterpieces are the huge 36-inch objective of the Lick telescope, and that of the Yerkes instrument.
In the matter of the polishing of optical cal surfaces, surfaces, the figuring of lenses, and ruling of gratings, American artisans have excelled all others from their first attempts. Only in the production of instruments of precision and in the making of optical glass do they still yield superiority to European artisans.
Celestial Photography. As early as 1840 Dr. John W. Draper, of New York, obtained a few photographs of the moon about an inch in diameter. In 1845, at Cambridge, Mass., Bond obtained photographic impressions of Vega and Castor, and in 1850 obtained the picture of the moon.
Among the successes of photography as an adjunct to the new astronomy have been photographs of stellar spectra by Pickering at the Harvard University Observatory, and the photographic normal spectrum of the sun recently completed by Rowland at the Johns Hopkins University.
The Mount Wilson (Cal.) Solar Observatory of the Carnegie Institute (1904), designed for spectroscopic study of the sun and stars, rs, has produced marvelous photographs of these bodies that have proved invaluable to astronomers. roduced