Astronomy

Astronomy (Gr. astron, 'a heavenly body; nomos, 'a law') teaches whatever is known of the heavenly bodies.

The history of astronomy dates from a very early period. It is the most ancient of all the sciences. The Chinese, Hindus, Chaldeans, Egyptians, and even the Greeks, are known to have investigated the heavens very 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; and each of these nations applied it to astrological purposes.

The Chinese, Chaldeans, Hindus, and Egyptians each claim the honour of having been the first students of astronomy, and each have had advocates to support their claim. The Tirvalore tables (said by the Hindus to belong to an epoch 3102 years B.C.) are, so far as their date is concerned, altogether unreliable. A conjunction of the sun, moon, and planets, adduced to fix it, could not possibly have taken place at the time specified. Indeed, many contend that these tables are founded on science communicated to the people of India either by the Greeks or Arabians in much later times. Others maintain that they are original Hindu productions. The matter remains still somewhat uncertain.

The Chinese have astronomical annals claiming to go back 2857 years B.C. In these there is little record of anything but of the appearance of comets and solar eclipses; and regarding the latter phenomena, they tell nothing, save the fact and date of their occurrence. Professional astronomers were compelled to predict every eclipse under pain of death. The popular idea was, that an eclipse was a monster having evil designs on the sun, and it was customary to make a great noise, by shouting, beating of gongs, &c. in order to frighten it away from its solar prey. The many eclipses which the Chinese report have been recalculated, but not more than one anterior to the time of Ptolemy could be verified. At an early period, however, the Chinese appear to have been acquainted with the luni-solar cycle of nineteen years (introduced into Greece by Meton, and since known as the Metonic Cycle), and they had also divided the year into 365$\backslash frac\{1\}\{4\}$ days. Solstitial observances are said to have been made by a gnomon in the 11th century B.C. To the burning of all scientific books by one of their princes (Tsin-Chi-Hong-Ti), 221 B.C., the Chinese attribute the loss of many theories, or methods previously in use. The precession of the equinoxes was not known to the Chinese until

400 A.D., but long prior to that they were familiar with the motion of the planets. They even record an observation of a conjunction of five planets, made between 2514 and 2436 B.C.

The mass of evidence seems in favour of the plains of Chaldea being the primal seat of observational astronomy. The risings and settings of the heavenly bodies and eclipses were subjects of observation and notation by their priests at a very remote period. Simplicius and Porphyry mention that Callisthenes, who had accompanied Alexander the Great to Babylon, on its conquest by that monarch, discovered there a catalogue of eclipses, recorded on tablets of baked clay, the earliest dating from 2234 B.C. These, by Alexander's orders, were sent to Aristotle. They are nearly all now lost, six only being preserved by Ptolemy, the earliest dated 720 B.C., and forming the first reliable observation we possess. The Chaldeans were genuine astronomers. They used different kinds of dials, invented the zodiac, gnomon, and clepsydra, divided the day into the hours we now use, and discovered the Saros, or cycle of about 18 years and 10 days, during which the moon makes 223 synodical revolutions, and eclipses recur in the same order for several cycles. They also were so exact in the orientation of their chief buildings, as to show no mean accuracy of observation. Their proficiency in arithmetic was probably due to their astronomical work.

The Egyptians, it is supposed, were the first instructors of the Greeks in astronomy. They do not, however, appear to have observed much for themselves. The meaning of what data they have left behind them can be guessed at only in a few instances. No mention is made by Ptolemy of the idea ascribed to them, that the planets Mercury and Venus moved round the sun; the probability therefore is, Ptolemy not being likely to overlook such a novel theory, that they entertained no such notion at the time of his visit, but that it is an after-thought of more recent ages. From the accuracy with which the great pyramid faces the cardinal points, it was probably used for astronomical purposes. It is designed on principles requiring much astronomical knowledge. But other pyramids do not show the same correctness of plan; and it is probable that the priests of Herodotus's time had lost the knowledge of their remote ancestors, for the information he received from them about the sun having risen thrice in the west, would seem to show the inaccuracy of their observation and weakness of their theorising power.

Up to this time, astronomy is little else than tradition. The Greeks have the honour of elevating it into a reliable history, and to the dignity of a science. Thales (640 B.C.), the founder of the Ionic school, laid the foundation of Greek astronomy. He it was who first propagated the theory of the earth's sphericity. The sphere is divided into five zones. He predicted the year of a great solar eclipse, but this it is now supposed he must have casually succeeded in doing—the Greeks at this time having no observations of their own to guide them—by means of the Chaldean Saros (above mentioned), which gives a regular recurrence of eclipses. He made the Greeks, who, prior to his time, were content to navigate their vessels by the Great Bear—a rough approximation to the north—acquainted with the Lesser Bear, a much better guide for the mariner. Among other things, he held that the stars were composed of fire, and that the earth was the centre of the universe. 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, on somewhat slender authority, the belief in the idea of the plurality of worlds. Anaxagoras, who transferred the Ionic school from Miletus to Athens, is said to have offered a conjecture that, like the earth, the moon had habitations, hills, and valleys.

Pythagoras (500 B.C.), who was the next astronomer of eminence, was very far in advance of his predecessors. He promulgated, on grounds fanciful enough, the theory—the truth of which, however, has been since established—that the sun is the centre of the planetary world, and that the earth circulates round it. Pythagoras also first taught that the morning and evening star were in reality one and the same planet. But his views met with little or no support from his successors until the time of Copernicus. Between Pythagoras and the advent of the Alexandrian school, nearly a couple of centuries later, the most prominent names in astronomical annals are those of Meton (432 B.C.), who introduced the luni-solar cycle, as already intimated, erected the first sun-dial at Athens, and in conjunction with Euctemon, observed a solstice there in the year 424 B.C.; Callippus (330 B.C.), who improved the Metonic cycle; Eudoxus of Cnidus (370 B.C.), who brought into Greece the year of 365$\backslash frac\{1\}\{4\}$ days, and wrote some works on astronomy; and Nicetas of Syracuse, who is reported to have taught the diurnal motion of the earth on its axis.

The Alexandrian school, fostered by the Ptolemies, originated a connected series of observations relative to the constitution of the universe. The positions of the fixed stars were determined, the paths of the planets carefully traced, and the solar and lunar inequalities more accurately ascertained. Angular distances were measured with instruments suitable to the purpose, and calculated by trigonometrical methods, and ultimately the school of Alexandria presented to the world the first system of theoretical astronomy that had ever comprehended an entire plan of the celestial motions. The system we know to be false, and inferior to the Pythagorean notions; but it had the merit of being founded upon a long and patient observation of phenomena.

The most interesting circumstances connected with the early history of the Alexandrian school are the attempts made to determine the distance of the earth from the sun, and the magnitude of the terrestrial globe. Aristarchus of Samos—the pioneer of the Copernican system, as Humboldt calls him—was the author of an ingenious plan to ascertain the proportion of the moon's distance to that of the sun.

Among other eminent members of this school were Timocharis and Aristyllus, who made the observations which, together with observations of his own, enabled Hipparchus (q.v.) to discover the precession of the equinoxes; and Eratosthenes (q.v.), who determined pretty accurately the obliquity of the ecliptic and the latitude of Alexandria (he also measured an arc of the meridian between Syene and Alexandria, thus determining roughly the size of the earth); and Autolycus, whose books on astronomy are the earliest extant in the Greek language.

We now reach the greatest name in ancient astronomical science—that of Hipparchus of Bithynia (190–120 B.C.), and here may be said to begin the real written history of scientific astronomy; for not until his era were there facts correct enough and sufficient in number upon which to build a system. Hipparchus was at once a theorist, a mathematician, and an observer. He catalogued no less than 1081 stars. This is the first reliable catalogue we have. He discovered, as we have already mentioned, the precession of the equinoxes; he determined with greater exactitude than his predecessors had done, the mean motion, as well as the inequality of the motion of the sun; and also the length of the year. He also determined the mean motion of the moon, her eccentricity, the equation of her centre, and the inclination of her orbit; and he suspected the inequality afterwards discovered by Ptolemy (the evection). He invented processes analogous to plane and spherical trigonometry, and was the first to use right ascensions and declinations, which he afterwards abandoned in favour of latitudes and longitudes.

For more than two centuries and a half after the demise of this indefatigable astronomer, we meet with no name of note. Ptolemy (130–150 A.D.) is the next who rises above the mass of mediocrities. Besides being a practical astronomer, he was accomplished as a musician, a geographer, and mathematician. His most important discovery in astronomy was the evection of the moon. He also was the first to point out atmospheric refraction. He extended and improved many of the theories of Hipparchus, and was the founder of the system known by his name, which was universally accepted as the true theory of the universe, until the researches of Copernicus exploded it. The Ptolemaic system, expounded in the Great Collection, or, as it was called by the Arabs, the Almagest—from which source most of our knowledge of Greek astronomy is derived—placed the earth immovable in the centre of the universe, making the entire heavens revolve round it in the course of twenty-four hours. See PTOLEMAIC SYSTEM.

With Ptolemy closes the originality of the Greek school. His successors were men of no mark, confining themselves for the most part to astrology, or to comments on earlier writers. It is to the Arabs that we owe the next advances in astronomy. They commenced making observations 762 A.D., in the reign of the Caliph Almansor, who gave great encouragement to science, as did also his successors, the 'good Haroun Al-Raschid' and Al-Mamun, both of whom were themselves diligent students of astronomy. Under the latter a small arc of the meridian was measured in Mesopotamia. For four centuries the Arabs prosecuted the study of the science with assiduity, but they are chiefly meritorious as observers. They had little capacity for speculation, and throughout held the Greek theories in superstitious reverence. They, however, determined with much more accuracy than the Greeks had done the precession of the equinoxes, the obliquity of the ecliptic, and the solar eccentricity; and the length of the tropical year was ascertained within a few seconds of the truth. The most illustrious of the Arabian school were Albategnius or Al Batani (880 A.D.), who discovered the motion of the solar apogee (see ANOMALISTIC YEAR), and who was also the first to make use of sines and versed sines instead of chords; he corrected the Greek observations, and was altogether the most distinguished observer between Hipparchus and the Copernican era; Ibn-Yunis (1000 A.D.), an excellent mathematician, who made observations of great importance in determining the disturbances and eccentricities of Jupiter and Saturn; and Abul Wefu, who first employed tangents, cotangents, and secants, and possibly discovered the lunar variation.

In the northern part of Persia, an observatory was erected by a descendant of the renowned warrior Genghis Khan, where some tables were constructed by Nasir-Eddin. The famous Omar Khayyám (q.v.) proposed a reformation of the calendar, which, if adopted, would have been more accurate than the Gregorian reform. And at

Samarqand, Ulugh Beg, a grandson of Timur, made in 1433 A.D. many observations, and the most correct catalogue of stars which, up to his time, had been published.

In the 13th century, astronomy was again introduced into Western Europe, the first translation from the Almagest being made under the Emperor Frederick II. of Germany, about 1230; and in 1252 an impulse was given to the science by the formation of astronomical tables under the auspices of Alfonso X. of Castile. An Englishman, named Holywood (Sacrobosco), in 1220 wrote a book of great repute in its day on the spheres, chiefly abridged from Ptolemy; and among others who did much to promote a taste for astronomy were Purbach (1460), Regiomontanus (John Muller), who died in 1476, and Waltherus, a pupil of the latter, who made numerous observations of merit.

We now come to the illustrious name of Copernicus (1473–1543), to whom was reserved the honour and the danger of exploding the Ptolemaic idea, and of promulgating a correct though imperfect theory of the universe. His system is in some part a revival and systematic application of the opinions said to have been held by Pythagoras. It makes the sun the immovable centre of the universe, around which all the planets revolve in concentric orbits, Mercury and Venus within the earth's orbit, and all the other planets without it. In the Copernican theory there were many of the old notions which have since been exploded. See COPERNICUS.

Among the contemporaries of Copernicus were Reinhold, who constructed the Prutenic tables; Recorde, who was the first to write on astronomy in English; and Nonius, a Portuguese, who invented a method for dividing the circle. The study of astronomy was also much aided about this time by the liberality of the Landgrave of Hesse-Cassel, William IV.

Decidedly the most industrious observer and eminent practical astronomer from the time of the Arabs to the latter half of the 16th century was Tycho Brahè (1546–1601). Some discredit attaches to him on account of his repudiation of the Copernican system, but it should not be forgotten that in the time of Tycho that system was not supported by the conclusive evidence we now possess. Tycho's system, which made the sun move round the earth, and all the other planets round the sun, they moving with it round the earth, explained all natural phenomena then observed equally well, while it must have appeared more probable than the crude and, at that era, undemonstrable theories of Copernicus. Tycho Brahè compiled a catalogue of 777 fixed stars, more perfect than any that had previously appeared. 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 the inclination of the lunar orbit, and rejected the trepidation of the precession, which had hitherto injuriously affected all tables. He also made some interesting cometary investigations.

To his researches are mainly due the discovery by Kepler (1571–1630) of those famous laws which have rendered his name immortal (see KEPLER). To Kepler is due the credit of divesting the Copernican system of its absurdities. He is also said to have had some notion of the law of gravitation.

Galileo Galilei (1564–1642) first applied the telescope (which he made from a general description of the instrument of Hans Lipperhey of Holland, who was the first inventor of the telescope) to the investigation of the heavens. He was rewarded by the discovery of the inequalities on the moon's surface. The important discoveries of the four satellites of Jupiter, the ring of Saturn—not then distinctly recognised as a circle—the spots on the sun, and the crescent form of Venus, followed in quick succession. For propagating the Copernican doctrine of the world, Galileo incurred the displeasure of the priests, and was compelled by the Inquisition to retract his opinions. See GALILEO.

The next great epoch in the history of astronomy brings us to England and Newton (1642–1727). In the interval, practical astronomy had profited largely by the logarithms of Napier; the mathematical researches of Descartes; the work of Horrox, who ascribed the motion of the lunar apsides to the disturbing influence of the sun, so far forestalling Newton, and observed the first recorded transit of Venus; the application of the telescope to the quadrant by Gascoigne, an Englishman, and afterwards by Auzout and Picard; by Römer's discovery of the progressive motion, and measurement of the velocity, of light; by the invention of Vernier; and the application of the pendulum to clocks by Huygens, who also brought into use the spiral spring, and made some valuable observations on the ring and satellites of Saturn; as well as by the investigations of Norwood, Hooke, Hevelius, Gilbert, Leibnitz, and Dominicus Cassini, to the last of whom especially the scientific world owes much. Among a variety of other valuable observations and discoveries may be mentioned his thorough investigation of the zodiacal light, his determination of the rotations of Jupiter and Mars, and of the motions of Jupiter's satellites from their eclipses, his discovery of the dual character of Saturn's ring, and also of four of his satellites. Newton's fame rests upon his discovery of the law of gravitation, upon which the common belief is he was led to speculate by the fall of an apple. Newton announced his discovery in the Principia in 1687, which was briefly that every particle of matter is attracted by, or gravitates to, every other particle of matter, with a force inversely proportional to the squares of their distances. The first gleam of this grand conclusion is said to have so overpowered Newton that he had to suspend his calculations, and call in a friend to finish the few arithmetical computations that were incomplete. This discovery is perhaps the grandest effort of human genius of which we have any record. Newton also made the important discovery of the revolution of comets round the sun in conic sections, proved the earth's form to be an oblate spheroid, gave a theory of the moon and tides, invented fluxions, and wrote upon Optics.

While the foundations of physical astronomy were thus broadly laid by Newton, Flamsteed—the first astronomer royal at Greenwich, to whom, until recently, scant justice has been done—and Halley were greatly improving and extending the practical department of the science. To the former we are indebted for numerous observations on the fixed stars, on planets, satellites, and comets, and for a catalogue of 2884 stars. His Historia Cælestis, published in 1725, 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 of 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 only direct and conclusive proof we have of the earth's annual motion. To him also we are indebted for our knowledge of the nutation of the earth's axis. He was, besides, an unwearied observer, and left behind him at his death upwards of 60,000 observations. Altogether, Bradley's is deservedly one of the most honoured names in modern astronomy. Dr Maskelyne, who was appointed to the observatory after Bradley, originated the Nautical Almanac.

These three Greenwich observers (Flamsteed, Bradley, and Maskelyne) span with their labours (from 1676 to 1811) a period during which both practical and theoretical astronomy were greatly developed. The discovery of gravitation gave men power to reduce the wanderings of the moon and planets to order, while the accuracy of calculation demanded more correct observation and better instruments. Dollond, Bird, Harrison, and Graham, famous instrument-makers, provided the last, while the three above mentioned, together with Römer, Bianchini, Lacaille, Cassini, and others, made observations far more correct than any before. On these observations much was founded of important theory, and new problems in celestial mechanics were thus presented. In solving these, Euler, who generally investigated the planetary motions in his Theoria Motuum, and published solar and lunar tables; Clairaut, who improved the theory of the earth's figure, and investigated the motion of the lunar apogee; D'Alembert, who assisted in investigating the planetary theory, precession, nutation, and the earth's figure; Lalande, who treated the orbit of Halley's comet, and published his planetary tables; Lagrange, who discussed the lunar libration (applying first the principle of virtual velocities), the theory of Jupiter's satellites, and the attraction of spheroids, were all eminent workers. Greatest in this field, however, was the Marquis de Laplace. With his investigations on the solar system, Jupiter's satellites, Saturn's ring, the theory of tides, and above all his great work, the Mécanique Céleste, he brings us into the 19th century, and to something like the full development of theoretical astronomy.

How complete this had now become will be best seen by the manner of discovery of the planet Neptune. The motions of Uranus, the outermost then-known planet, had been carefully watched since its discovery by Sir W. Herschel, and an orbit was speedily assigned to it. For about fourteen years the planet kept to this path, and then began to gain on its predicted place, continuing to do so for about twenty-seven years, when it ceased to advance and soon began to fall behind, continuing steadily to do so. It was seen by Leverrier, a young French astronomer, and Adams, then a student at Cambridge, that these movements could be explained by the action of a planet exterior to Uranus, and they both independently tried to solve the problem thus presented, and indicate the disturbing planet's place. This problem could be solved so as to indicate any one of an infinite number of planets, each of which would produce the observed disturbance of Uranus. It was treated differently by the two investigators. Both assigned certain probable values to the distance and periodic time of the unknown body, which made their work possible. Each wrought out his solution, and found the elements of the unknown body's orbit. Adams sent word to Professor Challis of Cambridge, and Leverrier later advised Dr Galle of Berlin where to look for it. Dr Galle first saw it, on September 23, 1846, within a degree of Leverrier's calculated place, and three degrees of Adams's. It is true the planet was found to have a different orbit from that assigned by the calculators. Their planets were in fact not identical, nor were they the planet Neptune. But they must ever have credit for the sagacity and ability with which, aiming at so indefinite a target, they so nearly struck the centre.

But partly parallel to this advance of theoretical astronomy there had been an enormous development of physical astronomy, so that it practically became a new science. Sir W. Herschel discovered double binary stars (see STARS), catalogued vast numbers of Nebulae (q.v.), and by new methods framed daring theories of the constitution of the universe and the stars. Earlier than Laplace he thought of the nebular hypothesis, since confirmed in many ways, and by his discoveries gave an impulse to the new work of determining the physical state of the heavenly bodies. His son, Sir John Herschel, did for the southern heavens what his father had for the north. Their giant reflecting telescopes, with the refractors of Fraunhofer, Merz, and Mahler of Munich, and the larger ones since constructed by Cooke, Grubb, Alvan Clark, and continental makers, enabled this work to go on. W. and O. Struve at Dorpat and Pulkowa, with a host of other observers, largely amateurs, carried on the observation of double stars. Beer and Mädler, with Schmidt of Athens, mapped with great accuracy the surface of the moon. And the surface of the planets has been scrutinised, with some results in the case of Mars and Jupiter, by a multitude of telescopes, which have been so cheapened and improved as to have attracted to their use, especially in America, numbers of amateur workers. By the invention of the Spectroscope (q.v.), the investigation of the chemical constitution and physical state of the sun, stars, and nebulae, was rendered possible. Fraunhofer, Balfour Stewart, and Kirchhoff, all deserve mention in connection with this great discovery. By it the heavenly bodies have been shown to consist of similar matter to the earth; the constitution of many stars, their physical state and temperature, the causes of the variability of some, and the fresh outbursts of others, with their motions in the line of sight, have all been investigated with success. The sun, however, has been the chief field of triumph for spectroscopic astronomy. Its physical constitution, vast atmosphere, and enormous gaseous eruptions, have been observed, and the problems they raise so far solved. Young and Langley in America, Janssen in France, Secchi in Italy, Zöllner in Germany, and Huggins and Lockyer in England, are leading names in solar research. It is in this field of physics that astonishing advances are now made even daily. Through it astronomy has largely absorbed into itself all the other sciences, and become so extensive that its history must henceforth be theirs. It has in turn assisted them all. Through the physical changes of the sun and planets, light has been thrown on the meteorology of the earth, on geology and chemistry, on electrical and magnetic science.

Photography has also played a prominent part in astronomy. Daily photographs of the sun's surface are now made in more than one observatory. The form and number of the spots on his surface (discovered to have a periodical increase and diminution in about eleven years, by Schwabe of Dessau) are thus continually recorded. The planets, stars, and even nebulae, have also been pictured by the camera, and vast fields opened for the future extension of the science. Dr Draper of New York, the brothers Henry of Paris, Captain Abney and Mr Common in England, have done good service in this field.

Since the beginning of the 19th century, there have been added to our solar system upwards of 400 Planetoids (q.v.) or asteroids. Ceres, the first seen of these, was discovered by Piazzi at Palermo on January 1, 1801. Their number has been increased almost monthly by the work of observers such as Peters (of Clinton, New York) and Palisa. On the nights of August 11 and 17, 1877, Professor Asaph Hall, at the United States Naval Observatory, discovered two satellites of the planet Mars; and in 1892 Barnard discovered at the Lick Observatory a fifth satellite of Jupiter. Astro- nomers now, too, look with interest for the results of the work of those lately charged with the use of the gigantic and perfect instruments of the Lick Observatory, California, which in effective power seem likely to surpass all their predecessors.

Branches of Astronomy.—Astronomy has three main divisions: practical astronomy, which deals with the observation of phenomena; theoretical astronomy, which treats of the real motions of the heavenly bodies; and physical astronomy, which regards their physical state, chemical constitution, and the configuration of their surface.

In practical astronomy, the errors of instruments and observations, the construction of observatories, and the division of the celestial sphere by the circles and points to which the positions of the stars are referred, have all to be considered.

Theoretical astronomy is the application to the explanation of the discoveries of practical astronomy of the law of gravitation.

Physical astronomy applies the sciences of terrestrial nature, by proper instruments, to the heavenly bodies, and has come to be really the great welding science of the universe.

The science is also otherwise divided into sidereal astronomy, treating of the stars and nebulae; solar physics, the study of the sun's physical state; selenography, the mapping of the moon; planetary astronomy, regarding the planets; meteoric astronomy, and other divisions, taking their name from the instruments used, or the subject investigated. Some of these have their practical, theoretical, and physical sides, as will be evident from their titles; but as yet they are scarcely all agreed on as separate departments.

According to the plan of this work, the detailed treatment of the extensive subject of astronomy falls to be given in the separate articles on the most important departments of investigation and instruments. The principal articles will be found under the following heads:

Readers may consult for further information Sir J. Herschel's Outlines of Astronomy; Lardner and Dunkin's Handbook of Astronomy; Newcomb's Popular Astronomy; Grant's History of Physical Astronomy; A. M. Clarke's History of Astronomy during the 19th Century (ed. 1887); Ball's Story of the Heavens; Whewell's History of the Inductive Sciences; Chambers's Handbook of Astronomy (1890); Bonney's Story of the Planet (1893); and Lockyer's Dawn of Astronomy (1894).