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 descriptive of 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:—

and in ‘Il Penseroso’—

In the happy choice of his theme, and by the comprehensive manner in which he has treated it, Milton has been enabled by his poetic genius to give to the world in his ‘Paradise Lost’ a poem which, for sublimity of thought, loftiness of imagination, and beauty of expression in metrical verse, is unsurpassed in any language.

It is, however, our intention to deal only with those passages in the poem in which allusion is made to the heavenly bodies, and to incidents and occurrences associated with astronomical phenomena. In the exposition and illustration of these it has been considered desirable to adopt the following general classification:—

1. To ascertain the extent of Milton’s astronomical knowledge.

2. To describe the starry heavens and the celestial objects mentioned in ‘Paradise Lost.’

3. To exemplify the use which Milton has made of astronomy in the exercise of his imaginative and descriptive powers.

In the earlier half of the seventeenth century the Ptolemaic theory—by which it was believed that the Earth was the immovable centre of the universe, and that round it all the heavenly bodies completed a diurnal revolution—still retained its ascendency over the minds of men of learning and science, and all the doctrines associated with this ancient astronomical creed were still religiously upheld by the educated classes among the peoples inhabiting the different civilised regions of the globe. The Copernican theory—by which the Sun is assigned the central position in our system, with the Earth and planets revolving in orbits round him—obtained the support of a few persons of advanced views and high scientific attainments, but its doctrines had not yet seriously threatened the supremacy of the older system. Though upwards of one hundred years had elapsed since the death of Copernicus, yet the doctrines associated with the system of which he was the founder were but very tardily adopted up to this time. There were several reasons which accounted for this. The Copernican system was at first imperfect in its details, and included several of the Ptolemaic, doctrines which rendered it less intelligible, and retarded its acceptance by persons who would otherwise have been inclined to adopt it. Copernicus believed that the planets travelled round the Sun in circular paths. This necessitated the retention of cycles and epicycles, which gave rise to much confusion; nor was it until Kepler made his great discovery of the ellipticity of the planetary orbits that they were eliminated from the system.

As the Ptolemaic system of the universe held complete sway over the minds of men for upwards of twenty centuries, it was difficult to persuade many persons to renounce the astronomical beliefs to which they were so firmly attached, in favour of those of any other system; so that the overthrow of this venerable theory required a lengthened period of time for its accomplishment.

It was thus in his earlier years, when Milton devoted his time to the study of literature and philosophy, which he read extensively when pursuing his academic career at Christ’s College, Cambridge, and afterwards at Horton, where he spent several years in acquiring a more proficient knowledge of the literary, scientific, and philosophical writings of the age, that he found the beliefs associated with the Ptolemaic theory adopted without doubt or hesitation by the numerous authors whose works he perused. His knowledge of Italian enabled him to become familiar with Dante—one of his favourite authors, whose poetical writings were deeply read by him, and who, in the elaboration of his poem, the ‘Divina Commedia,’ included the entire Ptolemaic cosmology.

In England the Copernican theory had few supporters, and the majority of those who represented the intellect and learning of the country still retained their adherence to the old form of astronomical belief. We therefore find that Milton followed the traditional way of thinking by adopting the views associated with the Ptolemaic theory.

According to the Ptolemaic system, the Earth was regarded as the immovable centre of the universe, and surrounding it were ten crystalline spheres, or heavens, arranged in concentric circles, the larger spheres enclosing the smaller ones; and within those was situated the cosmos, or mundane universe, usually described as ‘the Heavens and the Earth.’ To each of the first seven spheres there was attached a heavenly body, which was carried round the Earth by the revolution of the crystalline.

1st sphere: that of the Moon.

2nd sphere: that of the planet Mercury. 3rd sphere: that of the planet Venus.

4th sphere: that of the Sun; regarded as a planet.

5th sphere: that of the planet Mars.

6th sphere: that of the planet Jupiter.

7th sphere: that of the planet Saturn.

8th sphere: that of the fixed stars.

The eighth sphere included all the fixed stars, and was called the firmament, because it was believed to impart steadiness to the inner spheres, and, by its diurnal revolution, to carry them round the Earth, causing the change of day and night. The separate motions of the spheres, revolving with different velocities, and at different angles to each other, accounted for the astronomical phenomena associated with the orbs attached to each. According to Ptolemy’s scheme, the eighth sphere formed the outermost boundary of the universe; but later astronomers added to this system two other spheres—a ninth, called the Crystalline, which caused Precession of the Equinoxes; and a tenth, called the Primum Mobile, or First Moved, which brought about the alternation of day and night, by carrying all the other spheres round the Earth once in every twenty-four hours. The Primum Mobile enclosed, as if in a shell, all the other spheres, in which was included the created universe, and, although of vast dimensions, its conception did not overwhelm the mind in the same manner that the effort to comprehend infinitude does.

Beyond this last sphere there was believed to exist a boundless, uncircumscribed region, of immeasurable extent, called the Empyrean, or Heaven of Heavens, the incorruptible abode of the Deity, the place of eternal mysteries, which the comprehension of man was unable to fathom, and of which it was impossible for his mind to form any conception. Such were the imaginative beliefs upon which this ancient astronomical theory was founded, that for a period of upwards of two thousand years held undisputed sway over the minds of men, and exercised during that time a predominating influence upon the imagination, thoughts, and conceptions of all those who devoted themselves to literature, science, and art. Of the truthfulness of this assertion there is ample evidence in the poetical, philosophical, and historical writings of ancient authors, whose ideas and conceptions regarding the created universe were limited and circumscribed by this form of astronomical belief. In the works of more recent writers we find that it continued to assert its influence; and among our English poets, from Chaucer down to Shakespeare, there are numerous references to the natural phenomena associated with this system, and most frequently expressed by poetical allusions to ‘the music of the spheres.’

The ideas associated with the Ptolemaic theory were gratifying to the pride and vanity of man, who could regard with complacency the paramount importance of the globe which he inhabited, and of which he was the absolute ruler, fixed in the centre of the universe, and surrounded by ten revolving spheres, that carried along with them in their circuit all other celestial bodies—Sun, Moon, and stars, which would appear to have been created for his delectation, and for the purpose of ministering to his requirements. But when the Copernican theory became better understood, and especially after the discovery of the law of universal gravitation, this venerable system of the universe, based upon a pile of unreasonable and false hypotheses, after an existence of over twenty centuries, sank into oblivion, and was no more heard of.

Milton’s Ptolemaism is apparent in some of his shorter pieces, and also in his minor poems, ‘Arcades’ and ‘Comus.’ His ‘Ode on the Nativity’ is written in conformity with this belief, and the expression,

indicates a poetical allusion to this theory. But as Milton grew older his Ptolemaism became greatly modified, and there are good reasons for believing that in his latter years he renounced it entirely in favour of Copernicanism. When on his continental tour in 1638, he made the acquaintance of eminent men who held views different from those with which he was familiar; and in his interview with Galileo at Arcetri, the aged astronomer may have impressed upon his mind the superiority of the Copernican theory, in accounting for the occurrence of celestial phenomena, as compared with the Ptolemaic.

On his return to England from the Continent, Milton took up his residence in London, and lived in apartments in a house in St. Bride’s Churchyard. Having no regular vocation, and not wishing to be dependent upon his father, he undertook the education of his two nephews, John and Edward Phillips, aged nine and ten years respectively. From St. Bride’s Churchyard he removed to a larger house in Aldersgate, where he received as pupils the sons of some of his most intimate acquaintances. In the list of subjects which Milton selected for the purpose of imparting instruction to those youths he included astronomy and mathematics, which formed part of the curriculum of this educational establishment. The text-book from which he taught his nephews and other pupils astronomy was called ‘De SphÆra Mundi,’ a work written by Joannes Sacrobasco (John Holywood) in the thirteenth century. This book was an epitome of Ptolemy’s ‘Almagest,’ and therefore entirely Ptolemaic in its teaching. It enjoyed great popularity during the Middle Ages, and is reported to have gone through as many as forty editions.

The selection of astronomy as one of the subjects in which Milton instructed his pupils affords us evidence that he must have devoted considerable time and attention to acquiring a knowledge of the facts and details associated with the study of the science. In the attainment of this he had to depend upon his own exertions and the assistance derived from astronomical books; for at this time astronomy received no recognition as a branch of study at any of the universities; and in Britain the science attracted less attention than on the Continent, where the genius of Kepler and Galileo elevated it to a position of national importance.

We shall find as we proceed that Milton’s knowledge of astronomy was comprehensive and accurate; that he was familiar with the astronomical reasons by which many natural phenomena which occur around us can be explained; and that he understood many of the details of the science which are unknown to ordinary observers of the heavens. It is remarkable how largely astronomy enters into the composition of ‘Paradise Lost,’ and we doubt if any author could have written such a poem without possessing a knowledge of the heavens and of the celestial orbs such as can only be attained by a proficient and intimate acquaintance with this science.

The arguments in favour of or against the Ptolemaic and Copernican theories were well known to Milton, even as regards their minute details; and in Book viii. he introduces a scientific discussion based upon the respective merits of those theories. The configuration of the celestial and terrestrial spheres, and the great circles by which they are circumscribed, he also knew. The causes which bring about the change of the seasons; the obliquity of the ecliptic; the zodiacal constellations through which the Sun travels, and the periods of the year in which he occupies them, are embraced in Milton’s knowledge of the science of astronomy. The motions of the Earth, including the Precession of the Equinoxes; the number and distinctive appearances of the planets, their direct and retrograde courses, and their satellites, are also described by him. The constellations, and their relative positions on the celestial sphere; the principal stars, star-groups, and clusters, and the Galaxy, testify to Milton’s knowledge of astronomy, and to the use which he has made of the science in the elaboration of his poem.

The names of fourteen of the constellations are mentioned in ‘Paradise Lost.’ These, when arranged alphabetically, read as follows:—

Andromeda, Aries, Astrea, Centaurus, Cancer, Capricornus, Gemini, Leo, Libra, Ophiuchus, Orion, Scorpio, Taurus, and Virgo. Milton’s allusions to the zodiacal constellations are chiefly associated with his description of the Sun’s path in the heavens; but with the celestial sign Libra (the Scales) he has introduced a lofty and poetical conception of the means by which the Creator made known His will when there arose a contention between Gabriel and Satan on his discovery in Paradise.

Orion, the finest constellation in the heavens, did not escape Milton’s observation, and there is one allusion to it in his poem. It arrives on the meridian in winter, where it is conspicuous as a brilliant assemblage of stars, and represents an armed giant, or hunter, holding a massive club in his right hand, and having a shield of lion’s hide on his left arm. A triple-gemmed belt encircles his waist, from which is suspended a glittering sword, tipped with a bright star. The two brilliants Betelgeux and Bellatrix form the giant’s shoulders, and the bright star Rigel marks the position of his advanced foot. The rising of Orion was believed to be accompanied by stormy and tempestuous weather. Milton alludes to this in the following lines:—

Andromeda is described as being borne by Aries, and in ‘Ophiuchus huge’ Milton locates a comet which extends the whole length of the constellation. It is evident that Milton possessed a precise knowledge of the configuration and size of the constellations, and of the positions which they occupy relatively to each other on the celestial sphere.

Though Milton was conversant with the Copernican theory, and entertained a conviction of its accuracy and truthfulness, and doubtless recognised the superiority of this system, which, besides conveying to the mind a nobler conception of the universe and of the solar system—though it diminished the importance of the Earth as a member of it—was capable of explaining the occurrence of celestial phenomena in a manner more satisfactory than could be arrived at by the Ptolemaic theory. Notwithstanding this, he selected the Ptolemaic cosmology as the scientific basis upon which he constructed his ‘Paradise Lost,’ and in its elaboration adhered with marked fidelity to this system. There were many reasons why Milton, in the composition of an imaginative poem, should have chosen the Ptolemaic system of the universe rather than the Copernican. This form of astronomical belief was adopted by all the authors whose works he perused and studied in his younger days, including his favourite poet, Dante; and his own poetic imaginings, as indicated by his early poems, were in harmony with the doctrines of this astronomical creed, a long acquaintance with which had, without doubt, influenced his mind in its favour. This system of revolving spheres, with the steadfast Earth at its centre, and the whole enclosed by the Primum Mobile, constituted a more attractive and picturesque object for poetic description than the simple and uncircumscribed arrangement of the universe expressed by the Copernican theory. It also afforded him an opportunity of localising those regions of space in which the chief incidents in his poem are described—viz. Heaven, or the Empyrean, Chaos, Hell, and the Mundane Universe. Milton’s Ptolemaism, with its adjuncts, may be understood by the following:

All that portion of space above the newly created universe, and beyond the Primum Mobile, was known as Heaven, or The Empyrean—a region of light, of glory, and of happiness; the dwelling-place of the Deity, Who, though omnipresent, here visibly revealed Himself to all the multitude of angels whom He created, and who surrounded his throne in adoration and worship. Underneath the universe there existed a vast region of similar dimensions to the Empyrean, called Chaos, which was occupied by the embryo elements of matter, that with incessant turmoil and confusion warred with each other for supremacy—a wild abyss—

The lower portion of this region was divided off from the remainder, and embraced the locality known as Hell—the place of torment, where the rebellious angels were driven and shut in after their expulsion from Heaven.

The New Universe, which included the Earth and all the orbs of the firmament known as the Starry Heavens, was created out of Chaos, and hung, as if suspended by a golden chain, from the Empyrean above; and although its magnitude and dimensions were inconceivable, yet, according to the Ptolemaic theory, it was enclosed by the tenth sphere or Primum Mobile.

By this partitioning of space Milton was able to contrive a system which fulfilled the requirements of his great poem.

The annexed diagram explains the relative positions of the different regions into which space was divided.

Though there are traces of Copernicanism found in ‘Paradise Lost,’ yet Milton has very faithfully adhered to the Ptolemaic mechanism and nomenclature throughout his poem.

In his description of the Creation, the Earth is formed first, then the Sun, followed by the Moon, and afterwards the stars, all of which are described as being in motion round the Earth. Allusion is also made to this ancient system in several prominent passages, and in the following lines there is a distinct reference to the various revolving spheres.

The seven planetary spheres are first mentioned; then the eighth sphere, or that of the fixed stars; then the ninth, or crystalline, which was believed to cause a shaking, or trepidation, to account for certain irregularities in the motions of the stars; and, lastly, the tenth sphere, or Primum Mobile, called the ‘first moved’ because it set the other spheres in motion.

To an uninstructed observer, the apparent motion of the heavenly bodies round the Earth would naturally lead him to conclude that, of the two theories, the Ptolemaic was the correct one. We therefore find that Milton adopted the system most in accord with the knowledge and intelligence possessed by the persons portrayed by him in his poem; and in describing the natural phenomena witnessed in the heavens by our first parents, he adheres to the doctrines of the Ptolemaic system, as being most in harmony with the simple and primitive conceptions of those created beings.

To their upward gaze, the orbs of heaven appeared to be in ceaseless motion; the solid Earth, upon which they stood, was alone immovable and at rest. Day after day they observed the Sun pursue his steadfast course with unerring regularity: his rising in the east, accompanied by the rosy hues of morn; his meridian splendour, and his sinking in the west, tinting in colours of purple and gold inimitable the fleecy clouds floating in the azure sky, as he bids farewell for a time to scenes of life and happiness, rejoicing in the light and warmth of his all-cheering beams. With the advent of night they beheld the Moon, now increasing, now waning, pursue her irregular path, also to disappear in the west; whilst, like the bands of an army marshalled in loose array, the constellations of glittering stars, with stately motion, traversed their nocturnal arcs, circling the pole of the heavens.

By referring to Book viii., 15-175, we find an account of an interesting scientific discussion, or conversation, between Adam and Raphael regarding the merits of the Ptolemaic and Copernican systems, and of the relative importance and size of the heavenly bodies. By it we are afforded an opportunity of learning how accurate and precise a knowledge Milton possessed of both theories, and in what clear and perspicuous language he expresses his arguments in favour of or against the doctrines associated with each.

We may, with good reason, regard the views expressed by Adam as representing Milton’s own opinions, which were in conformity with the Copernican theory; and in the Angel’s reply, though of an undecided character, we are able to perceive how aptly Milton describes the erroneous conclusions upon which the Ptolemaic theory was based.

In this scientific discussion, it would seem rather strange that Adam, the first of men, should have been capable of such philosophic reasoning, propounding, as if by intuition, a theory upon which was founded a system that had not been discovered until many centuries after the time that astronomy became a science. By attributing to Adam such a degree of intelligence and wisdom, the poet has taken a liberty which enabled him to carry on this discussion in a manner befitting the importance of the subject.

In the following lines Adam expresses to his Angel-guest, in forcible and convincing language, his reasons in support of the Copernican theory:—

We are enabled to perceive that Milton had formed a correct conception of the magnitude and proportions of the universe, and also of the relative size and importance of the Earth, which he describes as ‘a spot, a grain, an atom,’ when compared with the surrounding heavens. He expresses his surprise that all the stars of the firmament, whose distances are so remote, and whose dimensions so greatly exceed those of this globe, should in their diurnal revolution have ‘such a sumless journey of incorporeal speed imposed upon them’ merely to officiate light to the Earth, ‘this punctual spot;’ and reasoning, wonders how Nature, wise and frugal in her ways, should commit such disproportions, by adopting means so great to accomplish a result so small, when motion imparted to the sedentary Earth would with greater ease produce the same effect.

The inconceivable velocity with which it would be necessary for those orbs to travel in order to accomplish a daily revolution round the Earth might be described as almost spiritual, and beyond the power of calculation by numbers.

The Angel, after listening to Adam’s argument, expresses approval of his desire to obtain knowledge, but answers him dubiously, and at the same time criticises in a severe and adverse manner the Ptolemaic theory.

When, with the advancement of science, astronomical observations were made with greater accuracy, it was discovered that uniformity of motion was not always maintained by those bodies which were believed to move in circles round the Earth. It was observed that the Sun, when on one side of his orbit, had an accelerated motion, as compared with the speed at which he travelled when on the other side. The planets, also, appeared to move with irregularity: sometimes a planet was observed to advance, then become stationary, and afterwards affect a retrograde movement. Those inequalities of motion could not be explained by means of the revolution of crystalline spheres alone, but were accounted for by imagining the existence of a small circle, or epicycle, whose centre corresponded with a fixed point in the larger circle, or eccentric, as it was called. This small circle revolved on its axis when carried round with the larger one, and round it the planet also revolved, which when situated in its outer portion would have a forward, and when in its inner portion a retrograde, motion.

The theory of eccentrics and epicycles was sufficient for a time to account for the inequalities of motion already described, and by this means the Ptolemaic system was enabled to retain its ascendency for a longer period than it otherwise would have done. But more recent discoveries brought to light discrepancies and difficulties which were explained away by adding epicycle to epicycle. This created a most complicated entanglement, and hastened the downfall of a system which, after an existence of many centuries, sank into oblivion, and is now remembered as a belief of bygone ages.

The devices which the upholders of this system were compelled to adopt, in order ‘to save appearances,’ with ‘centric and eccentric,’ cycle and epicycle, ‘orb in orb,’ are in this manner appropriately described by Milton, as indicating the confusion arising from a theory based upon false hypotheses.

Continuing his reply, the Angel says:—

Notwithstanding the Angel’s severe criticism of the Ptolemaic system, he does not unreservedly support the conclusions arrived at by Adam, but endeavours to show that his reasoning may not be altogether correct. He questions the validity of his argument that bodies of greater size and brightness should not serve the smaller, though not bright, and that heaven should move, while the Earth remained at rest. He argues that great or bright infers not excellence, and that the Earth, though small, may contain more virtue than the Sun, that ‘barren shines,’ whose beams create no beneficial effect, except when directed on the fruitful Earth. He reminds Adam that those bright luminaries minister not to the Earth, but to himself, ‘Earth’s habitant,’ and directs his attention to the magnificence and extent of the surrounding universe, of which he occupies but a small portion. The diurnal swiftness of the orbs that move round the Earth he attributes to God’s omnipotence, that to material bodies ‘could add speed almost spiritual.’

The Angel, after alluding to his rapid flight through space, suggests that God placed heaven so far from Earth that man might not presume to inquire into things which it would be of no advantage for him to know. He then suddenly changes to the Copernican system, which he lucidly describes in the following lines:—

The Copernican theory, which is less complicated and more easily understood than the Ptolemaic, is described by Milton with accuracy and methodical skill.

The Sun having been assigned that central position in the system which his magnitude and importance claim as his due, the planets circling in orbits around him have their motions described in a manner indicative of the precise knowledge which Milton acquired of this theory. At this time the law of gravitation was unknown, and, although the ellipticity of the orbits of the planets had been discovered by Kepler, the nature of the motive force which guided and retained them in their paths still remained a mystery. It was believed that the planets were whirled round the Sun, as if by the action of magnetic fibres; a mutual attractive influence having been supposed to exist between them and the orb, similar to that of the opposite poles of magnets.

Milton alludes to this theory in the following lines:—

An important advance upon this theory was made by Horrox, who, in his study of celestial dynamics, attributed the curvilineal motion of the planets to the influence of two forces, one projective, the other attractive. He illustrated this by observing the path described by a stone when thrown obliquely into the air. He perceived that its motion was governed by the impulse imparted to it by the hand, and also by the attractive force of the Earth. Under these two influences, the stone describes a graceful curve, and in its descent falls at the same angle at which it rose. Hence arises the general law: ‘When two spheres are mutually attracted, and if not prevented by foreign influences, their straight paths are deflected into curves concave to each other, and corresponding with one of the sections of a cone, according to the velocity of the revolving body. If the velocity with which the revolving body is impelled be equal to what it would acquire by falling through half the radius of a circle described from the centre of deflection, its orbit will be circular; but if it be less than that quantity, its path becomes elliptical.’

Newton afterwards embraced this law in his great principle of gravitation, and demonstrated that the force which guides and retains the Earth and planets in their orbits resides in the Sun. By the orb’s attractive influence a planet, after having received its first impulse, is deflected from its original straight path, and bent towards that luminary, and by the combined action of the projective and attractive forces is made to describe an orbit which, if elliptical, has one of its foci occupied by the Sun. So evenly balanced are those two forces, that one is unable to gain any permanent ascendency over the other, and consequently the planet traverses its orbit with unerring regularity, and, if undisturbed by external influences, will continue in its path for all time.

Milton describes the position of the planets in the sky as—

and their motions—

It is evident that Milton was familiar with the apparently irregular paths pursued by the planets when observed from the Earth. He knew of their stationary points, and also the backward loopings traced out by them on the surface of the sphere.

If observed from the Sun, all the planets would be seen to follow their true paths round that body; their motion would invariably lie in the same direction, and any variation in their speed as they approached perihelion or aphelion would be real. But the planets, when observed from the Earth, which is itself in motion, appear to move irregularly. Sometimes they remain stationary for a brief period, and, instead of progressing onward, affect a retrograde movement. This irregularity of motion is only apparent, and can be explained as a result of the combined motions of the Earth and planets, which are travelling together round the Sun with different velocities, and in orbits of unequal magnitude.

In his allusion to the Copernican system the ‘planet’ ‘Earth’ is described by Milton as seventh. This is not strictly accurate, as only five planets were known—viz. Mercury, Venus, Mars, Jupiter, and Saturn; but to make up the number Milton has included the Moon, which may be regarded as the Earth’s planet.

The three motions ascribed to the Earth are—(1) The diurnal rotation on her axis; (2) her annual revolution round the Sun; (3) Precession of the Equinoxes.

The rotation of the Earth on her axis may be likened to the spinning motion of a top, and is the cause of the alternation of day and night. This rotatory motion is sustained with such exact precision that, during the past 2,000 years, it has been impossible to detect the minutest difference in the time in which the Earth accomplishes a revolution on her axis, and therefore the length of the sidereal day, which is 3 minutes 56 seconds shorter than the mean solar day, is invariable. In this motion of the Earth we have a time-measuring unit which may be regarded as absolutely correct.

The Earth completes a revolution of her orbit in 365¼ days. In this period of time she accomplishes a journey of 580 millions of miles, travelling at the average rate of 66,000 miles an hour. The change of the seasons, and the lengthening and shortening of the day, are natural phenomena, which occur as a consequence of the Earth’s annual revolution round the Sun. Precession is a retrograde or westerly motion of the equinoctial points, caused by the attraction of the Sun, Moon, and planets on the spheroidal figure of the Earth. By this movement the poles of the Earth are made to describe a circular path in that part of the heavens to which they point; so that, after the lapse of many years, the star which is known as the Pole Star will not occupy the position indicated by its name, but will be situated at a considerable distance from the pole. These motions, Milton says, unless attributed to the Earth, must be ascribed to several spheres crossing and thwarting each other obliquely; but the Earth, by rotating from west to east, will of herself fetch day, her other half, averted from the Sun’s rays, being enveloped in night. Thus saving the Sun his labour, and the ‘primum mobile,’ ‘that swift nocturnal and diurnal rhomb,’ which carried all the lower spheres along with it, and brought about the change of day and night. Milton’s allusion to the occurrence of natural phenomena in the Moon similar to those which happen on the Earth is in keeping with the opinions entertained regarding our satellite, Galileo having imagined that he discovered with his telescope continents and seas on the lunar surface, which led to the belief that the Moon was the abode of intelligent life.

Milton in these lines refers to Jupiter and Saturn, and their satellites, which had been recently discovered; those of the former by Galileo, and four of those of the latter by Cassini. The existence of male and female light was an idea entertained by the ancients, and which is mentioned by Pliny. The Sun was regarded as a masculine star, and the Moon as feminine; the light emanating from each being similarly distinguished, and possessing different properties.

Milton supposes that, as the Earth receives light from the stars, she returns light back to them. But in his time little was known about the stars, nor was it ascertained how distant they are.

The Angel, in bringing to a conclusion his conversation with Adam, deems it unadvisable to vouchsafe him a decisive reply to his inquiry regarding the motions of celestial bodies, and in the following lines gives a beautifully poetical summary of this elevated and philosophic discussion:—

In this scientific discourse between Adam and Raphael, in which they discuss the structural arrangement of the heavens and the motions of celestial bodies, we are afforded an opportunity of learning what exact and comprehensive knowledge Milton possessed of both the Ptolemaic and Copernican theories. The concise and accurate manner in which he describes the doctrines belonging to each system indicates that he must have devoted considerable time and attention to making himself master of the details associated with both theories, which in his time were the cause of much controversy and discussion among philosophers and men of science.

The Ptolemaic system, with its crystalline spheres revolving round the Earth, the addition to those of cycles and epicycles, and the heaping of them upon each other, in order to account for phenomena associated with the motions of celestial bodies, are concisely and accurately described.

The unreasonableness of this theory, when compared with the Copernican, is clearly delineated by Milton where Adam is made to express his views with regard to motion in the heavens. His argument, declared in logical and persuasive language, demonstrates how contrary to reason it would be to imagine that the entire heavens should revolve round the Earth to bring about a result which could be more easily attained by imparting motion to the Earth herself. The inconceivable velocity with which it would be necessary for the celestial orbs to travel in order to accomplish their daily revolution is described by him as opposed to all reason, and entailing upon them a journey which it would be impossible for material bodies to perform. None the less accurate is Milton’s description of the Copernican system. He describes the Sun as occupying that position in the system which his magnitude and supreme importance claim as his sole right, having the planets with their satellites,

circling in majestic orbits around him, acknowledging his controlling power, and bending to his firm but gentle sway. Their positions, their paths, and their motions, real and apparent, are described in flowing and harmonious verse.