While pursuing these important researches, Hipparchus resolved upon a work of extraordinary difficulty, which had never before been attempted, and which fully attests the grandeur and sagacity of his views. This enterprise was nothing less than numbering the stars, and fixing their positions in the heavens. This he actually accomplished, and his catalogue of 1081 of the principal stars, is perhaps the richest treasure which the Greek school has transmitted to posterity. We cannot too much admire the disinterested devotion to science, which prompted this great undertaking, and the firmness of purpose which sustained the solitary observer, through long years of toil. It was a work for posterity, and could yield to its author no reward during his life. Conscious of this, his resolution never faltered, and grateful posterity crowns his memory with the well-earned title of Father of Astronomy. The noble example thus set by Hipparchus, was not lost on Ptolemy, justly the most distinguished among his immediate successors. An ardent student, a close observer, a patient and candid reasoner, Ptolemy collected and digested the discoveries and theories of his predecessors, and transmitted them, in connection with his own, successfully to posterity. Rejecting the absurd doctrine of the solid crystal spheres of Eudoxus, and the unsustained notions of Pythagoras, this bold Greek undertook the resolution of the great problem, which Plato had long before presented, and to accomplish which, so many unsuccessful efforts had been made.

After a careful examination of all the facts and discoveries, which the world then possessed, adding his own extensive observations, Ptolemy promulged a system which bears his name, and which endured for more than fourteen hundred years. He fixed the earth as the great centre, about which

the sun, the moon, the planets, and the starry heavens, revolved. Retaining the doctrine of uniform circular motion, he accounted for the irregularity in the movements of the sun and moon by the eccentric position of the earth in their orbits. To explain the anomalous movement of the planets, he devised the system of cycles and epicycles. Every planet moved uniformly in the circumference of a small circle, whose centre moved uniformly in the circumference of a large circle, near whose centre the earth was located. By this ingenious theory, it was shown that a planet moving in the circumference of its small circle might appear to retrograde, to become stationary, and finally to advance among the fixed stars. Thus were all the phenomena known to the Greek astronomer, so satisfactorily accounted for, that it even became possible from this singular theory, to compute tables of the planetary motions, from which their places could be predicted with such precision, that the error, if any existed, escaped detection by the rude instruments then in use.

While the explanation of the celestial phenomena had constituted the principal object of the Greek astronomers, some rude efforts were commenced to determine the magnitude of the earth, and relative distances of the sun and moon. The process adopted by Eratosthenes, two thousand years ago, to determine the circumference of the earth, and its diameter, is essentially the same now employed by modern science. The results reached by the Greek astronomer, owing to an ignorance of the exact value of his unit, are lost to the world.

When astronomy was banished from Greece, it found a home among the Arabs. When darkness and gloom wrapped the earth through ten long centuries, and human knowledge languished, and art died, and genius slumbered, it is a remarkable

fact, that astronomy during that long period of ignorance, instead of being lost, was actually slowly advancing, and when the dawn of learning once more broke on Europe, the astronomy of the Greeks, improved by the Arabs and Persians, was preserved in the great work of Ptolemy, and transmitted to posterity.

It is true that no change had been wrought in the Greek theory, but observations had been multiplied and slow changes measured, which prepared the way for the discoveries which were soon to succeed. On the revival of learning in Europe, the literature and science of the Greeks and Romans rapidly spread, and gained an astonishing ascendancy over the human mind. Indeed, theirs was the only science, the only wisdom. Time honoured, and venerable with age, the philosophy of Aristotle, the geometry of Euclid, and the astronomy of Ptolemy, filled the colleges and universities, and fastened itself upon the age, with a tenacity, which permitted no one to question or doubt, and which seemed to defy all further progress. Such was the state of science and the world, when Copernicus consecrated his genius to the examination of the heavens.

To a mind singularly bold and penetrating, Copernicus united habits of profound study and severe observation. Deeply read in the received doctrines of science, he examined with the keenest interest, every hint which the philosophers of antiquity had left on record concerning the system of nature. For more than thirty years he watched, with unceasing perseverance, the movements of the heavenly bodies. By the construction of superior instruments, he compared the observed places of the sun, moon and planets, with their positions computed from the best tables founded on the theory of Ptolemy. The hypothesis of

uniform circular motion, had originally been adopted, to preserve the simplicity of nature, and with true philosophy. But as one irregularity after another had been discovered in the movements of the heavenly bodies, each of which must be explained on the circular hypothesis, one circle had been successively added to another, eccentrics and epicycles, equants and differents, until to preserve simplicity, the system had grown to the most extravagant complexity. The primitive idea of simplicity was a just one, founded in nature, and adopted in reason. But after thirty years of vain effort to harmonize the phenomena of the heavens with the theory of Ptolemy, after entangling himself in a maze of complexity in his effort to preserve simplicity, Copernicus was at last driven to doubt, and doubt soon grew into disbelief. By a close examination of the motions of Mercury and Venus, he found that these planets always accompanied the sun, participated in its movements, and never receded from it except to limited distances. The uniformity of their oscillations, from the one side to the other of the sun, suggested their revolution about that luminary, in orbits, whose planes passed nearly through the eye of the observer. The Egyptians had reached to this doctrine, had communicated it to Pythagoras, who taught it to his countrymen, nearly two thousand years before the time of Copernicus.

If then simplicity imperiously demanded the abandonment of the earth as the great centre of motion, in the search of a new centre, a multitude of circumstances pointed to the sun. It was the

largest and most brilliant of all the heavenly bodies. It gave light to the moon and planets. It gave life to the earth and its inhabitants. It was certainly accompanied by two satellites, and above all, it was so related to the earth, that if

motion in the one was abandoned, it must instantly and without a moment's hesitation, be transferred to the other. Long did the philosopher hesitate, perplexed with doubts, surrounded by prejudice, embarrassed with difficulties, but finally rising superior to every consideration save truth, he quitted the earth, swept boldly through space, and planted himself upon the sun. With an imagination endowed with the most extraordinary tenacity, he carried with him all the phenomena of the heavens, which were so familiar to his eye, while viewed from the earth. A long train of investigation was now before him. He commences with his now distant earth. Its immobility is gonehe beholds it sweeping round the heavens in the precise track once followed by the sun. The same constellations mark its career, the same periodic time, the same inequalities of motion; all that the sun has lost the earth has gained.

Thus far the change had been without results. He now gives his attention to the planets. Here a most beautiful scene broke npon his senses. The complex wanderings of the planets, their stations, their retrograde motions, all disappeared, and he beheld them sweeping harmoniously around him. The earth, deprived of her immobility, started in her orbit, joined her sister planets, and gave perfection to the system. The oscillations of Mercury and Venus were converted into regular revolutions, still holding their places nearest to the sun; then came the earth, next Mars, and Jupiter, and last of all Saturn away in the distance, slowly pursuing his mighty orbit. All were moving in the same direction, their paths filling the same belt of the heavens.

Charmed with this beautiful scene, the philosopher turns to an examination of the moon. Was she, too, destined to take her place among the