In 1615, Galileo was summoned before the Inquisition to reply to the accusation of heresy. ‘He was charged with maintaining the motion of the Earth and the stability of the Sun; with teaching this doctrine to his pupils; with corresponding on the subject with several German mathematicians; and with having published it, and attempted to reconcile it to Scripture in his letters to Mark Velser in 1612.’
These charges having been formally investigated by the Inquisition, Cardinal Bellarmine was authorised to communicate with Galileo, and inform him that unless he renounced the obnoxious doctrines, and promised ‘neither to teach, defend, or publish them in future,’ it was decreed that he should be committed to prison. Galileo appeared next day before the Cardinal, and, without any hesitation, pledged himself that for the future he would adhere to the pronouncement of the Inquisition.
Having, as they imagined, silenced Galileo, the Inquisition resolved to condemn the entire Copernican system as heretical; and in order to effectually accomplish this, besides condemning the writings of Galileo, they inhibited Kepler’s ‘Epitome of the Copernican System,’ and Copernicus’s own work, ‘De Revolutionibus Orbium Celestium.’
Whether it was that Galileo regarded the Inquisition as a body whose decrees were tooabsurd and unreasonable to be heeded, or that he dreaded the consequences which might have followed had he remained obstinate, we know that, notwithstanding the pledges which he gave, he was soon afterwards engaged in controversial discussion on those subjects which he promised not to mention again.
On the accession of his friend Cardinal Barberini to the pontifical throne in 1623, under the title of Urban VIII., Galileo undertook a journey to Rome to offer him his congratulations upon his elevation to the papal chair. He was received by his Holiness with marked attention and kindness, was granted several prolonged audiences, and had conferred upon him several valuable gifts.
Notwithstanding the kindness of Pope Urban and the leniency with which he was treated by the Inquisition, Galileo, having ignored his pledge, published in 1632 a book, in dialogue form, in which three persons were supposed to express their scientific opinions. The first upheld the Copernican theory and the more recent philosophical views; the second person adopted a neutral position, suggested doubts, and made remarks of an amusing nature; the third individual, called Simplicio, was a believer in Ptolemy and Aristotle, and based his arguments upon the philosophy of the ancients.
As soon as this work became publicly known, the enemies of Galileo persuaded the Pope that the third person held up to ridicule was intended as a representation of himself—an individualregardless of scientific truth, and firmly attached to the ideas and opinions associated with the writings of antiquity.
Almost immediately after the publication of the ‘Dialogues’ Galileo was summoned before the Inquisition, and, notwithstanding his feeble health and the infirmities of advanced age, he was, after a long and tedious trial, condemned to abjure by oath on his knees his scientific beliefs.
‘The ceremony of Galileo’s abjuration was one of exciting interest and of awful formality. Clothed in the sackcloth of a repentant criminal, the venerable sage fell upon his knees before the assembled cardinals, and, laying his hand upon the Holy Evangelists, he invoked the Divine aid in abjuring, and detesting, and vowing never again to teach the doctrines of the Earth’s motion and of the Sun’s stability. He pledged himself that he would nevermore, either in words or in writing, propagate such heresies; and he swore that he would fulfil and observe the penances which had been inflicted upon him.’ ‘At the conclusion of this ceremony, in which he recited his abjuration word for word and then signed it, he was conveyed, in conformity with his sentence, to the prison of the Inquisition.’[2]
Galileo’s sarcasm, and the bitterness which he imparted into his controversies, were more the cause of his misfortunes than his scientific beliefs. When he became involved in difficulties he did not possess the moral courage to enable him to abideby the consequences of his acts; nor did he care to become a martyr for the sake of science, his submission to the Inquisition having probably saved him from a fate similar to what befell Bruno. Though it would be impossible to justify Galileo’s want of faith in his dealings with the Inquisition, yet one cannot help sympathising deeply with the aged philosopher, who, in this painful episode of his life, was compelled to go through the form of making a retractation of his beliefs under circumstances of a most humiliating nature.
But the persecution of Galileo did not delay the progress of scientific inquiry nor retard the advancement of the Copernican theory, which, after the discovery by Newton of the law of gravitation, was universally adopted as the true theory of the solar system.
Ferdinand, Duke of Tuscany, having exerted his influence with Pope Urban on behalf of Galileo, he was, after a few days’ incarceration, released from prison, and permission was given him to reside at Siena, where he remained for six months. He was afterwards allowed to return to his villa at Arcetri, and, though regarded as a prisoner of the Inquisition, was permitted to pursue his studies unmolested for the remainder of his days.
Galileo died at Arcetri on January 8, 1642, when in the seventy-eighth year of his age.
Though not the inventor, he was the first to construct a refracting telescope and apply it to astronomical research. With this instrument hemade a number of important discoveries which tended to confirm his belief in the truthfulness of the Copernican theory.
On directing his telescope to the Sun, he discovered movable spots on his disc, and concluded from his observation of them that the orb rotated on his axis in about twenty-eight days. He also ascertained that the Moon’s illumination is due to reflected sunlight, and that her surface is diversified by mountains, valleys, and plains.
On the night of January 7, 1610, Galileo discovered the four moons of Jupiter. This discovery may be regarded as one of his most brilliant achievements with the telescope; and, notwithstanding the improvement in construction and size of modern instruments, no other satellite was discovered until near midnight on September 9, 1892, when Mr. E. E. Barnard, with the splendid telescope of the Lick Observatory, added ‘another gem to the diadem of Jupiter.’
The phases of Venus and Mars, the triple form of Saturn, and the constitution of the Milky Way, which he found to consist of a countless multitude of stars, were additional discoveries for our knowledge of which we are indebted to Galileo and his telescope. Galileo made many other important discoveries in mechanical and physical science. He detected the law of falling bodies in their accelerated motion towards the Earth, determined the parabolic law of projectiles, and demonstratedthat matter, even if invisible, possessed the property of weight.
In these pages a short historical description is given of the progress made in astronomical science from an early period to the time in which Milton lived. The discoveries of Copernicus, Kepler, and Galileo had raised it to a position of lofty eminence, though the law of gravitation, which accounts for the form and permanency of the planetary orbits, still remained undiscovered. Theories formerly obscure or conjectural were either rejected or elucidated with accuracy and precision, and the solar system, having the Sun as its centre, with his attendant family of planets and their satellites revolving in majestic orbits around him, presented an impressive spectacle of order, harmony, and design.
The seventeenth century embraces the most remarkable epoch in the whole history of astronomy. It was during this period that those wonderful discoveries were made which have been the means of raising astronomy to the lofty position which it now occupies among the sciences. The unrivalled genius and patient labours of the illustrious men whose names stand out in such prominence on the written pages of the history of this era have rendered it one of the most interesting and elevating of studies. Though Copernicus lived in the preceding century, yet the names of Tycho Brahé, Kepler, Galileo, and Newton, testify to the greatness of the discoveries that were made during this period, which have surrounded the memories of those men with a lustre of undying fame.
Foremost among astronomers of less conspicuous eminence who made important discoveries in this century we find the name of Huygens.
Christian Huygenswas born at The Hague in 1629. He was the second son of Constantine Huygens, an eminent diplomatist, and secretary to the Prince of Orange. Huygens studied at Leydenand Breda, and became highly distinguished as a geometrician and scientist. He made important investigations relative to the figure of the Earth, and wrote a learned treatise on the cause of gravity; he also determined with greater accuracy investigations made by Galileo regarding the accelerated motion of bodies when subjected to the influence of that force.
Huygens admitted that the planets and their satellites attracted each other with a force varying according to the inverse ratio of the squares of their distances, but rejected the mutual attraction of the molecules of matter, believing that they possessed gravity towards a central point only, to which they were attracted. This supposition was at variance with the Newtonian theory, which, however, was universally regarded as the correct one.
Huygens originated the theory by which it is believed that light is produced by the undulatory vibration of the ether; he also discovered polarization.
Up to this time the method adopted in the construction of clocks was not capable of producing a mechanism which measured time with sufficient accuracy to satisfy the requirements of astronomers. Huygens endeavoured to supply this want, and applied his mechanical ingenuity in constructing a clock that could be relied upon to keep accurate time. Though the pendulum motion was first adopted by Galileo, he was unable to arrange its mechanism so that it should keep up a continuous movement. The oscillation of the pendulum ceasedafter a time, and a fresh impulse had to be applied to set it in motion. Consequently, Galileo’s clock was of no service as a timekeeper.
Huygens overcame this difficulty by so arranging the mechanism of his clock that the balance, instead of being horizontal, was directed perpendicularly, and prolonged downwards to form a pendulum, the oscillations of which regulated the downward motion of the weight. This invention, which was highly applauded, proved to be of great service everywhere, and was especially valuable for astronomical purposes.
Huygens next directed his attention to the construction of telescopes, and displayed much skill in the grinding and polishing of lenses. He made several instruments superior in power and accuracy to any that existed previously, and with one of these made some remarkable discoveries when observing the planet Saturn.
The telescopic appearance of Saturn is one of the most beautiful in the heavens. The planet, surrounded by two brilliant rings, and accompanied by eight attendant moons, surpasses all the other orbs of the firmament as an object of interest and admiration. To the naked eye, Saturn is visible as a star of the first magnitude, and was known to the ancients as the most remote of the planets. Travelling in space at a distance of nearly one thousand millions of miles from the Sun, the planet accomplishes a revolution of its mighty orbit in twenty-nine and a half years.
Galileo was the first astronomer who directed atelescope to Saturn. He observed that the planet presented a triform appearance, and that on each side of the central globe there were two objects, in close contact with it, which caused it to assume an ovoid shape. After further observation, Galileo perceived that the lateral bodies gradually decreased in size, until they became invisible. At the expiration of a certain period of time they reappeared, and were observed to go through a certain cycle of changes. By the application of increased telescopic power it was discovered that the appendages were not of a rounded form, but appeared as two small crescents, having their concave surfaces directed towards the planet and their extremities in contact with it, resembling the manner in which the handles are attached to a cup.
These objects were observed to go through a series of periodic changes. After having become invisible, they reappeared as two luminous straight bands, projecting from each side of the planet; during the next seven or eight years they gradually opened out, and assumed a crescentic form; they afterwards began to contract, and on the expiration of a similar period, during which time they gradually decreased in size, they again became invisible. It was perceived that the appendages completed a cycle of their changes in about fifteen years.
In 1656, Huygens, with a telescope constructed by himself, was enabled to solve the enigma which for so many years baffled the efforts of the ablest astronomers. He announced his discovery in the formof a Latin cryptograph which, when deciphered, read as follows:—
‘Annulo cingitur, tenui plano, nusquam cohaerente, ad eclipticam inclinatio.’
‘The planet is surrounded by a slender flat ring everywhere distinct from its surface, and inclined to the ecliptic.’
Huygens perceived the shadow of the ring thrown on the planet, and was able to account in a satisfactory manner for all the phenomena observed in connection with its variable appearance.
The true form of the ring is circular, but by us it is seen foreshortened; consequently, when the Earth is above or below its plane, it appears of an elliptical shape. When the position of the planet is such that the plane of the ring passes through the Sun, the edge of the ring only is illumined, and then it becomes invisible for a short period. In the same manner, when the plane of the ring passes through the Earth, the illumined edge of the ring is not of sufficient magnitude to appear visible, but as the enlightened side of the plane becomes more inclined towards the Earth, the ring comes again into view. When the plane of the ring passes between the Earth and the Sun, the unillumined side of the ring is turned towards the Earth, and during the time it remains in this position it is invisible.
Huygens discovered the sixth satellite of Saturn (Titan), and also the Great Nebula in Orion.
Johann Hevelius, a celebrated Prussian astronomer, was born at Dantzig in 1611, and died inthat city in 1687. He was a man of wealth, and erected an observatory at his residence, where, for a period of forty years, he carried out a series of astronomical observations.
He constructed a chart of the stars, and in order to complete his work, formed nine new constellations in those spaces in the celestial vault which were previously un-named. They are known by the names Camelopardus, Canes Venatici, Coma Bernices, Lacerta, Leo Minor, Lynx, Monoceros, Sextans, and Vulpecula. He also executed a chart of the Moon’s surface, wrote a description of the lunar spots, and discovered the Libration of the Moon in Longitude.
On May 30, 1661, Hevelius observed a transit of Mercury, a description of which he published, and included with it Horrox’s treatise on the first-recorded transit of Venus. This work, after having passed through several hands, became the property of Hevelius, who was capable of appreciating its merits. The manuscript was sent to him by Huygens, and in acknowledging it he writes: ‘How greatly does my Mercury exult in the joyous prospect that he may shortly fold within his arms Horrox’s long looked-for and beloved Venus! He renders you unfeigned thanks that by your permission this much-desired union is about to be celebrated, and that the writer is able, with your concurrence, to introduce them both together to the public.’
Hevelius made numerous researches on comets,and suggested that the form of their paths might be a parabola.
Giovanni Domenico Cassiniwas born at Perinaldo, near Nice, in 1625. He studied at Genoa and Bologna, and was afterwards appointed to the Chair of Astronomy at the latter University. He was a man of high scientific attainments, and made many important astronomical discoveries.
In 1671 he became Director of the Royal Observatory at Paris, and devoted a long life to trying and difficult observations, which in his later years deprived him of his eyesight.
In 1644 Cassini proved beyond doubt that Jupiter rotated on his axis, and also assigned his period of rotation with considerable accuracy. He published tables of the planet’s satellites, and determined their motions from observations of their eclipses. He ascertained the periods of rotation of Venus and Mars; executed a chart of the lunar surface, and observed an occultation of Jupiter by the Moon.
Cassini discovered the dual nature of Saturn’s ring, having perceived that instead of one there are two concentric rings separated by a dark space. He also discovered four of the planet’s satellites—viz. Japetus, Rhea, Dione, and Tethys. He made a near approximation to the solar parallax by means of researches on the parallax of Mars, and investigated some irregularities of the Moon’s motion. Cassini discovered the belts of Jupiter, and also theZodiacal Light, and established the coincidence of the nodes of the lunar equator and orbit.
Jaques Cassini, son of Giovanni, was born at Paris in 1677. He followed in his father’s footsteps, and wrote several treatises on astronomical subjects. He investigated the period of the rotation of Venus on her axis, and upheld the results arrived at by his father, which were afterwards confirmed by observations made by Schroeter. Cassini made some valuable researches with regard to the proper motion of the stars, and demonstrated that their change of position on the celestial vault was real, and not caused by a displacement of the ecliptic. He attempted to ascertain the apparent diameter of Sirius, and made observations with regard to the visibility of the stars. The Cassini family produced several generations of eminent astronomers, whose discoveries and investigations were of much value in advancing the science of astronomy.
Olaus Roemer, an eminent Danish astronomer, was born at Copenhagen September 25, 1644. When Picard, a French astronomer, visited Denmark in 1671, for the purpose of ascertaining the exact position of ‘Uranienburg,’ the site of Tycho Brahé’s observatory, he made the acquaintance of Roemer, who was engaged in studying mathematics and astronomy under Erasmus Bartolinus. Having perceived that the young man was gifted with no ordinary degree of talent, he secured his services to assist him in his observations, and, on the conclusionof his labours, Picard was so much impressed with the ability displayed by Roemer, that he invited him to accompany him to France. This invitation he accepted, and took up his residence in the French capital, where he continued to prosecute his astronomical studies.
In 1675 Roemer communicated to the Academy of Sciences a paper, in which he announced his discovery of the progressive transmission of light. It was believed that light travelled instantaneously, but Roemer was able to demonstrate the inaccuracy of this conclusion, and determined that light travels through space with a measurable velocity.
By diligently observing the eclipses of Jupiter’s satellites, Roemer perceived that sometimes they occurred before, and sometimes after their predicted times. This irregularity, he discovered, depended upon the position of the Earth with regard to Jupiter. When the Earth, in traversing her orbit, moved round to the opposite side of the Sun, thereby bringing Jupiter into conjunction, an eclipse occurred sixteen minutes twenty-six seconds later than it did when Jupiter was in opposition or nearest to the Earth. As there existed an impression that light travelled instantaneously, it was believed that an eclipse occurred at the moment it was perceived in the telescope. This, however, was not so. Roemer, after a long series of observations, concluded that the discrepancies were due to the fact that light travels with a measurable velocity, and that it requires a greater length oftime, upwards of sixteen minutes, to traverse the additional distance—the diameter of the Earth’s orbit—which intervenes between the Earth and Jupiter, when the planet is in conjunction, as compared with the distance between the Earth and Jupiter, when the latter is in opposition. This discovery of Roemer’s was the means of enabling the velocity of light to be ascertained, which, according to recent calculations, is about 187,000 miles a second. As an acknowledgment of the importance of his communication, Roemer was awarded a seat in the Academy, and apartments were assigned to him at the Royal Observatory, where he carried on his astronomical studies.
In 1681 Roemer returned to Denmark, and was appointed Professor of Mathematics in the University of Copenhagen; he was also entrusted with the care of the city observatory—a duty which his reputation as an astronomer eminently qualified him to undertake. The transit instrument—a mechanism of much importance to astronomers—was invented by Roemer in 1690; it consists of a telescope fixed to a horizontal axis, and adjusted so as to revolve in the plane of the meridian. It is employed in observing the passage of the heavenly bodies across the observer’s meridian. To note accurately by means of the astronomical clock the exact instant of time at which a celestial body crosses the centre of the field of view is the essential part of a transit observation. Small transit instruments are employed for taking the time andfor regulating the observatory clock, but large instruments are used for delicate and exact observations of Right Ascensions and Declinations of stars of different magnitudes. Meridian, and altitude and azimuth circles, are important astronomical appliances, which owe their existence to the inventive skill of this distinguished astronomer.
Roemer resided for many years at the observatory in the city of Copenhagen, where he pursued his astronomical studies until the time of his death, which occurred in 1710. He meritoriously attempted to determine the parallax of the fixed stars; and it is said that the astronomical calculations and observations which he left behind him were so voluminous as to equal in number those made by Tycho Brahé, nearly all of which perished in a great conflagration that destroyed the observatory and a large portion of the city of Copenhagen in 1728.
Among other astronomers of this century whose names deserve recording were Descartes and Gassendi, whose mathematical researches in their application to astronomy were of much value; Fabricius, Torricelli, and Maraldi, who by their observations and investigations added many facts to the general knowledge of the science; and Bayer, to whom belongs the distinction of having constructed the first star-atlas.
In our own country during this period astronomy was cultivated by a few enthusiastic men, who devoted their time and talents to promotingthe advancement of the science. It, however, received no recognition as a subject of study at any of the Universities, and no public observatory existed in Great Britain.
Though it was not until towards the close of the century that the attention of all Europe was directed to England in admiration of the discoveries of the illustrious Newton, yet astronomy had its humble votaries, and chief among those was a young clergyman of the name of Horrox.
Jeremiah Horroxwas born at Toxteth, near Liverpool, in 1619—close on three centuries ago. Little is known of his family. His parents have been described as persons who occupied a humble position in life, but, as they were able to give their son a classical education which fitted him for one of the learned professions, it is probable they were not so obscure as they have been represented to be.
Having received his early education at Toxteth, Horrox afterwards proceeded to Cambridge, and was entered as a student at Emmanuel College on May 18, 1632, when in his fourteenth year.
At the University he devoted himself to the study of classics, especially Latin, which in those days was the language adopted by men of learning, when engaged in writing works of a philosophical and scientific character.
After having remained at Cambridge for three years, Horrox returned to his native county, and was appointed curate of Hoole, a place about eight miles distant from Preston. Hoole isdescribed as a narrow low-lying strip of land consisting largely of moss, and almost converted into an island by the waters of Martin Mere on the south, and the Ribble on the north; and, though doubtless an open and favourable situation for astronomical observation, it could not have been attractive as a place of residence. Yet it was here on November 24, 1639, that Horrox made his famous observation of the first recorded transit of Venus, an occurrence with which his name will be for ever associated.
It was while at Cambridge that Horrox first turned his attention to the study of astronomy. His love of the sublime, and the captivating influence exerted on his mind by the contemplation of the heavenly bodies, induced him to adopt astronomy as a pursuit congenial to his tastes, and capable of exercising his highest mental powers. Having this object in view, he applied himself with much earnestness to the study of mathematics; he had, however, to rely mainly upon his own exertions, for at that time no branch of physical or mathematical science was taught at Cambridge, and consequently he obtained no professional instruction.
It was so also with astronomy, which, as a science, was scarcely known in this country; no regular record of astronomical observations was kept by any individual observer, and no public observatory existed in England or in France.
The disadvantages and obstacles which Horroxhad to encounter may be best described by quoting his own words. He writes: ‘There were many hindrances. The abstruse nature of the study, my inexperience and want of means dispirited me. I was much pained not to have any one to whom I could look for guidance, or indeed for the sympathy of companionship in my endeavours, and I was assailed by the languor and weariness which are inseparable from every great undertaking. What then was to be done? I could not make the pursuit an easy one, much less increase my fortune, and least of all imbue others with a love for astronomy; and yet to complain of philosophy on account of its difficulties would be foolish and unworthy. I determined, therefore, that the tediousness of study should be overcome by industry; my poverty—failing a better method—by patience; and that instead of a master I would use astronomical books. Armed with these weapons I would contend successfully; and, having heard of others acquiring knowledge without greater help, I would blush that any one should be able to do more than I, always remembering that word of Virgil’s—
Totidem nobis animaeque manusque.’
Totidem nobis animaeque manusque.’
Having heard much praise bestowed upon the works of Lansberg, a Flemish astronomer, Horrox thought it would be to his advantage to procure a copy of his writings. This he succeeded in obtaining after some difficulty, and devoted a considerable time to calculating Ephemerides, based upon theLansberg Tables, but after making a number of computations he discovered that they were unreliable and inaccurate.
In the year 1636 Horrox made the acquaintance of William Crabtree, a devoted astronomer, who lived at Broughton, a suburb of Manchester. A close friendship soon existed between the two men, and they carried on an active correspondence about matters relating to the science which they both loved so well.
Crabtree, who was an unbeliever in Lansberg, urged Horrox to discard the Flemish astronomer’s works, and devote his talents to the study of Tycho Brahé and Kepler. This advice led Horrox to make a more rigorous examination of the Lansberg Tables, and after comparing them with the observations made by Crabtree, which coincided with his own, he resolved to renounce them. Acting on the advice of his friend, Horrox directed his attention to the writings of Kepler. The youthful astronomer soon realised their value, and was charmed with the accuracy of observation and inductive reasoning displayed in the elucidation of those general laws which constituted a new era in the history of astronomy.
The Rudolphine Tables, which were the astronomical calculations commenced by Tycho Brahé, and completed by Kepler, were regarded by Horrox as much superior to those of Lansberg; but it occurred to him that they might be improved by changing some of the numbers, and yet retainingthe hypotheses. To this task he applied himself with much earnestness and assiduity, and after close application and laborious study he accomplished the arduous undertaking of bringing those tables to a high state of perfection.
In his investigation of the Lunar theory, Horrox outstripped all his predecessors, and Sir Isaac Newton distinctly affirms he was the first to discover that the Moon’s motion round the Earth is in the form of an ellipse with the centre in the lower focus. Besides having made this discovery, Horrox was able to explain the causes of the inequalities of the Moon’s motion, which render the exact computation of her elements so difficult.
The Annual Equation, an irregularity discovered by Tycho Brahé, which is produced by the increase and decrease of the Sun’s disturbing force as the Earth approaches or recedes from him in her orbit, had its value first assigned by Horrox. This he calculated to be eleven minutes sixteen seconds, which is within four seconds of what it has since been proved to be by the most recent observations.
The Evection, an irregular motion of the Moon discovered by Ptolemy, whereby her mean longitude is increased or diminished, was explained by Horrox as depending upon the libratory motion of the apsides, and the change which takes place in the eccentricity of the lunar orbit.
These discoveries were made by Horrox before he attained the age of twenty years, and if his reputation had alone rested upon them his namewould have been honourably associated with those who have attained to the highest eminence in astronomy.
Another achievement which adds lustre to Horrox’s name consists in his detection of the inequality in the mean motions of Jupiter and Saturn.
He also directed his attention to the study of cometary bodies, and arrived at certain conclusions with regard to the nature of their movements. At first, he believed like Kepler that comets were projected in straight lines from the Sun; this supposition having been upheld on account of the great elongation of their orbits. He next perceived that their velocity increased as they approached the Sun, and decreased as they receded from him. Afterwards he says, ‘They move in an elliptic figure or near it,’ and finally he arrived at the conclusion that ‘comets move in elliptical orbits, being carried round the Sun with a velocity which is probably variable.’ This theory has been verified by numerous observations, and is now generally accepted by astronomers.
Horrox also made a series of observations on the tides. He notified the extent of their rise and fall at different periods, and investigated other phenomena associated with their ebb and flow. After having continued his observations for some time, he wrote to his friend Crabtree, and informed him that he had perceived many interesting details which had not been previously described, and hehoped to be able to arrive at some important conclusions with regard to their nature and cause. Unfortunately, Horrox’s writings on this subject, along with many other important papers, have been lost or destroyed. We are therefore ignorant of the result of his researches, which were the first undertaken by any person for the purpose of scientific inquiry.
From his study of the Lansberg and Rudolphine Tables, Horrox arrived at the conclusion that a transit of Venus would occur on November 24, 1639. This transit was for some unaccountable reason overlooked by Kepler, who predicted one in 1631, and the next not until 1761. The transit of 1631 was not visible in Europe.
We are indebted to Horrox for a description of the transit of 1639—the first that was ever observed of which there is any record; and were it not for the accuracy of his calculations, the occurrence of the phenomenon would have been unperceived, and no history of the conjunction would have been handed down to posterity. As soon as Horrox had assured himself of the time when the transit would take place, he wrote to Crabtree to inform him of the date, and asked him to make observations with his telescope, and especially to examine the diameter of the planet, which he thought had been over-estimated. He also requested him to write to Dr. Foster of Cambridge, and inform him of the expected event, as it was desirable that the transit should be observed from several places in consequence of thepossibility of failure, owing to an overcast sky. His letter is dated October 26, 1639. He says: ‘My reason for now writing is to advise you of a remarkable conjunction of the Sun and Venus on the 24th of November, when there will be a transit. As such a thing has not happened for many years past, and will not occur again in this century, I earnestly entreat you to watch attentively with your telescope in order to observe it as well as you can.
‘Notice particularly the diameter of Venus, which is stated by Kepler to be seven minutes, and by Lansberg to be eleven, but which I believe to be scarcely greater than one minute.’
In describing the method which he adopted for observing the transit, Horrox writes as follows: ‘Having attentively examined Venus with my instrument, I described on a sheet of paper a circle, whose diameter was nearly equal to six inches—the narrowness of the apartment not permitting me conveniently to use a larger size. I divided the circumference of this circle into 360 degrees in the usual manner, and its diameter into thirty equal parts, which gives about as many minutes as are equivalent to the Sun’s apparent diameter. Each of these thirty parts was again divided into four equal portions, making in all one hundred and twenty; and these, if necessary, may be more minutely subdivided. The rest I left to ocular computation, which, in such small sections, is quite as certain as any mechanical division. Suppose,then, each of these thirty parts to be divided into sixty seconds, according to the practice of astronomers. When the time of the observation approached, I retired to my apartment, and, having closed the windows against the light, I directed my telescope—previously adjusted to a focus—through the aperture towards the Sun, and received his rays at right angles upon the paper already mentioned. The Sun’s image exactly filled the circle, and I watched carefully and unceasingly for any dark body that might enter upon the disc of light.
‘Although the corrected computation of Venus’ motions which I had before prepared, and on the accuracy of which I implicitly relied, forbade me to expect anything before three o’clock in the afternoon of the 24th, yet since, according to the calculations of most astronomers, the conjunction should take place sooner—by some even on the 23rd—I was unwilling to depend entirely on my own opinion, which was not sufficiently confirmed, lest by too much self-confidence I might endanger the observation. Anxiously intent, therefore, on the undertaking through the greater part of the 23rd, and on the whole of the 24th, I omitted no available opportunity of observing her ingress. I watched carefully on the 24th from sunrise to nine o’clock, and from a little before ten until noon, and at one in the afternoon, being called away in the intervals by business of the highest importance, which for these ornamental pursuits I could notwith propriety neglect.[3]But during all this time I saw nothing in the Sun except a small and common spot, consisting as it were of three points at a distance from the centre towards the left, which I noticed on the preceding and following days. This evidently had nothing to do with Venus. About fifteen minutes past three in the afternoon, when I was again at liberty to continue my labours, the clouds, as if by divine interposition, were entirely dispersed, and I was once more invited to the grateful task of repeating my observations. I then beheld a most agreeable spectacle—the object of my sanguine wishes; a spot of unusual magnitude and of a perfectly circular shape, which had already fully entered upon the Sun’s disc on the left, so that the limbs of the Sun and Venus precisely coincided, forming an angle of contact. Not doubting that this was really the shadow of the planet, I immediately applied myself sedulously to observe it.
‘In the first place, with respect to the inclination, the line of the diameter of the circle being perpendicular to the horizon, although its plane was somewhat inclined on account of the Sun’s altitude, I found that the shadow of Venus at the aforesaid hour—namely, fifteen minutes past three—had entered the Sun’s disc about 62° 30', certainly between 60° and 65°, from the top towards the right. This was the appearance in the dark apartment;therefore, out of doors, beneath the open sky, according to the laws of optics, the contrary would be the case, and Venus would be below the centre of the Sun, distant 62° 30' from the lower limbs or the nadir, as the Arabians term it. The inclination remained to all appearances the same until sunset, when the observation was concluded.
‘In the second place, the distance between the centres of Venus and the Sun I found by three observations to be as follows:—
The true setting being 3·45, and the apparent about 5 minutes later, the difference being caused by refraction. The clock therefore was sufficiently correct.
‘In the third place I found after careful and repeated observation that the diameter of Venus, as her shadow was depicted on the paper, was larger indeed than the thirtieth part of the solar diameter, though not more so than the sixth, or at the utmost the fifth of such a part. Therefore let the diameter of the Sun be to the diameter of Venus as 30' to 1' 12''. Certainly her diameter never equalled 1' 30'', scarcely perhaps 1' 20'', and this was evident as well when the planet was near the Sun’s limb as when far distant from it.
VENUS ON THE SUN’S DISC.VENUS ON THE SUN’S DISC.
‘This observation was made in an obscure village where I have long been in the habit of observing, about fifteen miles to the north of Liverpool, the latitude of which I believe to be 53° 20', although by common maps it is stated at 54° 12', therefore the latitude of the village will be 53° 35', and longitude of both 22° 30' from the Fortunate Islands, now called the Canaries. This is 14° 15' to the west of Uraniburg in Denmark, the longitude of which is stated by Brahé, a native of the place, to be 36° 45' from these islands.
‘This is all I could observe respecting this celebrated conjunction during the short time the Sun remained in the horizon: for although Venus continued on his disc for several hours, she was not visible to me longer than half an hour on account of his so quickly setting. Nevertheless, all the observations which could possibly be made in so short a time I was enabled by Divine Providence to complete so effectually that I could scarcely have wished for a more extended period. The inclination was the only point upon which I failed to attain the utmost precision; for, owing to the rapid motion of the Sun it was difficult to observe with certainty to a single degree, and I frankly confess that I neither did nor could ascertain it. But all the rest is sufficiently accurate, and as exact as I could desire.’
Besides having ascertained that the diameter of Venus subtends an angle not much greater than one minute of arc, Horrox reduced the horizontal solar parallax from fifty-seven seconds as stated byKepler to fourteen seconds, a calculation within one and a half second of the value assigned to it by Halley sixty years after. He also reduced the Sun’s semi-diameter.
Crabtree, to whom Horrox refers as ‘his most esteemed friend and a person who has few superiors in mathematical learning,’ made preparations to observe the transit similar to those already described. But the day was unfavourable, dark clouds obscured the sky and rendered the Sun invisible. Crabtree was in despair, and relinquished all hope of being able to witness the conjunction. However, just before sunset there was a break in the clouds, and the Sun shone brilliantly for a short interval. Crabtree at once seized his opportunity, and to his intense delight observed the planet fully entered upon the Sun’s disc. Instead of proceeding to take observations, he was so overcome with emotion at the sight of the phenomenon, that he continued to gaze upon it with rapt attention, nor did he recover his self-possession until the clouds again hid from his view the setting Sun.[4]
Crabtree’s observation of the transit was, however, not a fruitless one. He drew from memory a diagram showing the exact position of Venus on the Sun’s disc, which corresponded in every respect with Horrox’s observation; he also estimated the diameter of the planet to be 7/200 that of the Sun, which when calculated gives one minute threeseconds; Horrox having found it to be one minute twelve seconds. This transit of Venus is remarkable as having been the first ever observed of which there is any record, and for this we are indebted to the genius of Horrox, who by a series of calculations, displaying a wonderfully accurate knowledge of mathematics, was enabled to predict the occurrence of the phenomenon on the very day, and almost at the hour it appeared, and of which he and his friend Crabtree were the only observers.
Having thought it desirable to write an account of the transit, Horrox prepared an elegant Latin treatise, entitled ‘Venus in Sole Visa’—‘Venus seen in the Sun;’ but not knowing what steps to take with regard to its publication, he requested Crabtree to communicate with his bookseller and obtain his advice on the matter.
In the meantime Horrox returned to Toxteth, and arranged to fulfil a long-promised visit to Crabtree, which he looked forward to with much pleasure, as it would afford him an opportunity of discussing with his friend many matters of interest to both. This visit was frustrated in a manner altogether unexpected. For we read that Horrox was seized with a sudden and severe illness, the nature of which is not known, and that his death occurred on the day previous to that of his intended visit to his friend at Broughton. He expired on January 3, 1641, when in the 23rd year of his age.
His death was a great grief to Crabtree, who, in one of his letters, describes it as ‘an irreparable loss:’ and it is believed that he only survived hima few years.[5]Of the papers left by Horrox, only a few have been preserved, and these were discovered in Crabtree’s house after his death. Among them was his treatise on the transit of Venus which, with other papers, was purchased by Dr. Worthington, Fellow of Emmanuel College, Cambridge, a man of learning, who was capable of appreciating their value. Ultimately, the treatise fell into the possession of Hevelius, a celebrated German astronomer, who published it along with a dissertation of his own, describing a transit of Mercury.
Horrox did not live to see any of his writings published, nor was any monument erected to his memory until nearly two hundred years after his death. But his name, though long forgotten except by astronomers, is now engraved on marble in Westminster Abbey. Had his life been spared, it would have been difficult to foretell to what eminence and fame he might have risen, or what further discoveries his genius might have enabled him to make. Few among English astronomers will hesitate to rank him next with the illustrious Newton, and all will agree with Herschel, who called him ‘the pride and the boast of British Astronomy.’
William Gascoignewas born in 1612, in the parish of Rothwell, in the county of York, and afterwards resided at Middleton, near Leeds.
He was a man of an inventive turn of mind, and possessed good abilities, which he devoted to improving the methods of telescopic observation.
At an early age he was occupied in observing celestial objects, making researches in optics, and acquiring a proficient knowledge of astronomy.
Among his acquaintances were Crabtree and Horrox, with whom he carried on a correspondence on matters appertaining to their favourite study.
The measurement of small angles was found at all times to be one of the greatest difficulties which astronomers had to contend with. Tycho Brahé was so misled by his measurements of the apparent diameters of the Sun and Moon, that he concluded a total eclipse of the Sun was impossible.
Gascoigne overcame this difficulty by his invention of the micrometer. This instrument, when applied to a telescope, was found to be of great service in the correct measurement of minute angles and distances, and was the means of greatly advancing the progress of practical astronomy in the seventeenth century. A micrometer consists of a short tube, across the opening of which are stretched two parallel wires; these being intersected at right angles by a third. The wires are moved to or from each other by delicately constructed screws, to which they are attached. Each revolution, or part of a revolution, of a screw indicates the distance by which the wires are moved.
This apparatus, when placed in the focus of a lens, gives very accurate measurements of the diameters of celestial objects. It was successfully used by Gascoigne in determining the apparentdiameters of the Sun, Moon, and several of the planets, and the mutual distances of the stars which form the Pleiades.
Crabtree, after having paid Gascoigne a visit in 1639, describes in a letter to Horrox the impression created on his mind by the micrometer. He writes: ‘The first thing Mr. Gascoigne showed me was a large telescope, amplified and adorned with new inventions of his own, whereby he can take the diameters of the Sun or Moon, or any small angle in the heavens or upon the earth, most exactly through the glass to a second.’
The micrometer is now regarded as an indispensable appliance in the observatory; the use of a spider web reticule instead of wire having improved its efficiency. Gascoigne was one of the earliest astronomers who recognised the value of the Keplerian telescope for observational purposes, and Sherburn affirms that he was the first to construct an instrument of this description having two convex lenses. Whether this be true or not, it is certain that he applied the micrometer to the telescope, and was the first to use telescopic sights, by means of which he was able to fix the optical axis of his telescope, and ascertain by observation the apparent positions of the heavenly bodies.
Crabtree, in a letter to Gascoigne, says: ‘Could I purchase it with travel, or procure it with gold, I would not be without a telescope for observing small angles in the heavens; or want the use of your device of a glass in a cane upon the movable rulerof your sextant, as I remember for helping to the exact point of the Sun’s rays.’
It was not known until the beginning of the eighteenth century that Gascoigne had invented and used telescopic sights for the purpose of making accurate astronomical observations. The accidental discovery of some documents which contained a description of his appliances was the means by which this became known.
Townley states that Gascoigne had completed a treatise on optics, which was ready for publication, but that no trace of the manuscript could be discovered after his death. Having embraced the Royalist cause, William Gascoigne joined the forces of Charles I., and fell in the battle of Marston Moor on July 2, 1644.
The early death of this young and remarkably clever man was a severe blow to the science of astronomy in England.
The invention of logarithms, by Baron Napier, of Merchistoun, was found to be of inestimable value to astronomers in facilitating and abbreviating the methods of astronomical calculation.
By the use of logarithms, arithmetical computations which necessitated laborious application for several months could with ease be completed in as many days. It was remarked by Laplace that this invention was the means of doubling the life of an astronomer, besides enabling him to avoid errors and the tediousness associated with long and abstruse calculations.
Thomas Harriot, an eminent mathematician, and an assiduous astronomer, made some valuable observations of the comet of 1607. He was one of the earliest observers who made use of the telescope, and it was claimed on his behalf that he discovered Jupiter’s satellites, and the spots on the Sun, independently of Galileo. Other astronomers have been desirous of sharing this honour, but it has been conclusively proved that Galileo was the first who made those discoveries.
The investigations of Norwood and Gilbert, the mechanical genius of Hooke, and the patient researches of Flamsteed—the first Astronomer Royal—were of much value in perfecting many details associated with the study of astronomy.
The Royal Observatory at Greenwich was founded in 1675. The building was erected under a warrant from Charles II. It announces the desire of the Sovereign to build a small observatory in the park at Greenwich, ‘in order to the finding out of the longitude for perfecting the art of navigation and astronomy.’ This action on the part of the King may be regarded as the first public acknowledgment of the usefulness of astronomy for national purposes.
Since its erection, the observatory has been presided over by a succession of talented men, who have raised it to a position of eminence and usefulness unsurpassed by any similar institution in this or any other country. The well-known names of Flamsteed, Halley, Bradley, and Airy, testify tothe valuable services rendered by those past directors of the Greenwich Observatory in the cause of astronomical science.
If we take a general survey of the science of astronomy as it existed from 1608 to 1674—a period that embraced the time in which Milton lived—we shall find that it was still compassed by ignorance, superstition, and mystery. Astrology was zealously cultivated; most persons of rank and position had their nativity or horoscope cast, and the belief in the ruling of the planets, and their influence on human and terrestrial affairs, was through long usage firmly established in the public mind. Indeed, at this time, astronomy was regarded as a handmaid to astrology; for, with the aid of astronomical calculation, the professors of this occult science were enabled to predict the positions of the planets, and by this means practised their art with an apparent degree of truthfulness.
Although over one hundred years had elapsed since the death of Copernicus, his theory of the solar system did not find many supporters, and the old forms of astronomical belief still retained their hold on the minds of the majority of philosophic thinkers. This can be partly accounted for, as many of the Ptolemaic doctrines were at first associated with the Copernican theory, nor was it until a later period that they were eliminated from the system.
Though Copernicus deserved the credit of having transferred the centre of our system from the Earth to the Sun, yet his theory was imperfect in itsdetails, and contained many inaccuracies. He believed that the planets could only move round the Sun in circular paths, nor was he capable of conceiving of any other form of orbit in which they could perform their revolutions. He was therefore compelled to retain the use of cycles and epicycles, in order to account for irregularities in the uniformly circular motions of those bodies.
We are indebted to the genius of Kepler for having placed the Copernican system upon a sure and irremovable basis, and for having raised astronomy to the position of a true physical science. By his discovery that the planets travel round the Sun in elliptical orbits, he was enabled to abolish cycles and epicycles, which created such confusion and entanglement in the system, and to explain many apparent irregularities of motion by ascribing to the Sun his true position with regard to the motions of the planets.
After the death of Kepler, which occurred in 1630, the most eminent supporter of the Copernican theory was the illustrious Galileo, whose belief in its accuracy and truthfulness was confirmed by his own discoveries.
Five of the planets were known at this time—viz. Mercury, Venus, Mars, Jupiter, and Saturn; the latter, which revolves in its orbit at a profound distance from the Sun, formed what at that time was believed to be the boundary of the planetary system. The distance of the Earth from the Sunwas approximately known, and the orb was observed to rotate on his axis.
It was also ascertained that the Moon shone by reflected light, and that her surface was varied by inequalities resembling those of our Earth. The elliptical form of her orbit had been discovered by Horrox, and her elements were computed with a certain degree of accuracy.
The cloudy luminosity of the Milky Way had been resolved into a multitude of separate stars, disclosing the immensity of the stellar universe.
The crescent form of the planet Venus, the satellites of Jupiter and of Saturn, and the progressive motion and measurement of light, had also been discovered. Observations were made of transits of Mercury and Venus, and refracting and reflecting telescopes were invented.
The law of universal gravitation, a power which retains the Earth and planets in their orbits, causing them year after year to describe with unerring regularity their oval paths round the Sun, was not known at this time. Though Newton was born in 1642, he did not disclose the results of his philosophic investigations until 1687—thirteen years after the death of Milton—when, in the ‘Principia,’ he announced his discovery of the great law of universal gravitation.
Kepler, though he discovered the laws of planetary motion, was unable to determine the motive force which guided and retained those bodies in their orbits. It was reserved for the genius of Newtonto solve this wonderful problem. This great philosopher was able to prove ‘that every particle of matter in the universe attracts every other particle with a force proportioned to the mass of the attracting body, and inversely as the square of the distance between them.’ Newton was capable of demonstrating that the force which guides and retains the Earth and planets in their orbits resides in the Sun, and by the application of this law of gravitation he was able to explain the motions of all celestial bodies entering into the structure of the solar system.
This discovery may be regarded as the crowning point of the science of astronomy, for, upon the unfailing energy of this mysterious power depend the order and stability of the universe, extending as it does to all material bodies existing in space, guiding, controlling, and retaining them in their several paths and orbits, whether it be a tiny meteor, a circling planet, or a mighty sun.
The nature of cometary bodies and the laws which govern their motions were at this time still enshrouded in mystery, and when one of those erratic wanderers made its appearance in the sky it was beheld by the majority of mankind with feelings of awe and superstitious dread, and regarded as a harbinger of evil and disaster, the precursor of war, of famine, or the overthrow of an empire.
Newton, however, was able to divest those bodies of the mystery with which they were surrounded by proving that any conic section may bedescribed about the Sun, consistent with the law of gravitation, and that comets, notwithstanding the eccentricity of their orbits, obey the laws of planetary motion.
Beyond the confines of our solar system, little was known of the magnitude and extent of the sidereal universe which occupies the infinitude of space by which we are surrounded. The stars were recognised as self-luminous bodies, inconceivably remote, and although they excited the curiosity of observers, and conjectures were made as to their origin, yet no conclusive opinions were arrived at with regard to their nature and constitution, and except that they were regarded as glittering points of light which illumine the firmament, all else appertaining to them remained an unravelled mystery. Even Copernicus had no notion of a universe of stars.
Galileo, by his discovery that the galaxy consists of a multitude of separate stars too remote to be defined by ordinary vision, demonstrated how vast are the dimensions of the starry heavens, and on what a stupendous scale the universe is constructed. But at this time it had not occurred to astronomers, nor was it known until many years after, that the stars are suns which shine with a splendour resembling that of our Sun, and in many instances surpassing it. It was not until this truth became known that the glories of the sidereal heavens were fully comprehended, and their magnificence revealed.It was then ascertained that the minute points of light which crowd the fields of our largest telescopes, in their aggregations forming systems, clusters, galaxies, and universes of stars, are shining orbs of light, among the countless multitudes of which our Sun may be numbered as one.
It would be reasonable to imagine that Milton’s knowledge of astronomy was comprehensive and accurate, and superior to that possessed by most scientific men of his age. His scholarly attainments, his familiarity with ancient history and philosophy, his profound learning, and the universality of his general knowledge, would lead one to conclude that the science which treats of the mechanism of the heavens, and especially the observational part of it—which at all times has been a source of inspiration to poets of every degree of excellence—was to him a study of absorbing interest, and one calculated to make a deep impression upon his devoutly poetical mind. The serious character of Milton’s verse, and the reverent manner in which celestial incidents and objects are described in it, impress one with the belief that his contemplation of the heavens, and of the orbs that roll and shine in the firmament overhead, afforded him much enjoyment and meditative delight. For no poet, in ancient or in modern times, has introduced into his writings with such frequency, or with such pleasing effect, so many passages descriptiveof the beauty and grandeur of the heavens. No other poet, by the creative effort of his imagination, has soared to such a height; nor has he ever been excelled in his descriptions of the celestial orbs, and of the beautiful phenomena associated with their different motions.
In his minor poems, which were composed during his residence at Horton, a charming rural retreat in Buckinghamshire, where the freshness and varied beauty of the landscape and the attractive aspects of the midnight sky were ever before him, we find enchanting descriptions of celestial objects, and especially of those orbs which, by their brilliancy and lustre, have always commanded the admiration of mankind.
For example, in ‘L’Allegro’ there are the following lines:—