One of the most sacred beliefs of the day was the Ptolemaic theory that the earth was the centre of the universe, and that the sun, moon, planets, and stars all revolved around it, outside of the atmospheres of air and fire which immediately surrounded it.

Many absurd reasons were given to prove the truth of this theory, and philosophers seemed willing to accept anything as fact, provided it coincided with this popular superstition, and even gravely acquiesced when it was asserted that the earth must  be the centre of the universe because it was the only planet that had a moon. This theory took its name from Claudius Ptolemy, an old astronomer and geographer, who lived at Alexandria about the middle of the second century Ptolemy gave innumerable reasons for his belief, and said that it would be impossible and absurd to believe otherwise. About four hundred and fifty years Pythagoras, a Greek philosopher, who spent many years studying in Egypt, and who was familiar with the astronomical theories of the Chaldeans and Egyptians, proclaimed to his disciples that the earth had a motion and revolved periodically around a great central fire, and this theory met with the warmest approbation of some other Greek philosophers, who also believed in two motions of the earth, an annual and daily, and claimed that the heavens only appeared to move because the earth turned on its axis with such rapidity.

But this belief was rejected with scorn by Ptolemy, who said that it was impossible to believe that the earth turned on its axis from west to east during twenty-four hours; for if it were true, then bodies lighter than the earth and suspended in the air, would have an opposite movement, and that it would thus be impossible for clouds or birds, and any object thrown in the air to go toward the east, as the earth would be constantly going before them and make it seem as if everything were going toward the west. And for two thousand years the world clung to the Ptolemaic theory, in part because it seemed reasonable and convincing, but chiefly because it had received the sanction of Aristotle, the greatest of the Greek philosophers, whose influence upon thought was so unlimited that even his most absurd theories of mechanics were received without question.

But in 1543 Nicholas Copernicus, a Prussian astronomer, published his great work "De Revolutionibus"—concerning the revolutions—in which he entirely refuted the Ptolemaic theory, and asserted that the earth was not the centre of the universe, and that it had a daily rotation on its axis and an annual revolution around the sun, which two motions accounted for all the other phenomena of the heavens, and satisfactorily explained all the hitherto unexplainable mysteries in regard to the motions of the heavenly bodies.

The opinions of Copernicus were received with disdain by the philosophers of the old school, and his work was derided as the wildest nonsense; but the more thoughtful minds gave his writings careful attention, and came gradually to accept his incontrovertible arguments, and among these was Galileo, who found it impossible to hold the Ptolemaic theory after becoming familiar with the works of Copernicus.

His conversion to the true theory was not, however, made publicly known at once, either because he felt that he had not yet sufficiently studied it, or because he feared that the opposition of his enemies might do the new system more harm than it would be in his power to overbalance.

But in 1604 the scientific world was startled by the sudden appearance of a new star, whose splendor at once attracted the attention of all astronomers. Night after night its brilliant light, changing from orange to yellow, purple, red, and white successively, illumined the heavens with new glory, and records were searched and old treatises pored over in order to see how often similar appearances had been noticed before.

Galileo studied the star with the greatest interest, and his lecture-rooms were crowded when it was announced that he would give a public explanation of the wonder; but the crowds who had come to agree with old theories or idly speculate over ancient astronomical history, were rudely startled by Galileo's original views, which swept away many of the fondest illusions of the age, and proclaimed clearly a new and unwelcome advance in the study of the heavens.

It was generally believed that the new star was a meteor having its origin in the atmosphere, and that it was nearer the earth than the moon; but Galileo claimed that this was impossible, and proved, by exact calculations from the situation and appearance, that the star must be placed among the most distant of the heavenly bodies, and that the belief in its motion around the earth was contrary to true theory of the earth's revolution around the sun.

This view was received with scorn by the followers of Aristotle, who held that the sky was unchangeable, and that the stars were carried in hollow crystalline spheres around the earth, thus making it impossible to account for the new star in this manner. They also declared their opposition to the theory of the motion of the earth, and Galileo was called upon to defend the Copernican system. He did this with such zeal that the university was at once divided into two parties, one agreeing with the Aristotelians, and the other following Galileo and accepting the new doctrines with delight. The dispute went on for some years, and Galileo omitted no chance to proclaim his belief in the Copernican system, and to add new proofs to strengthen its hold upon the minds of others; and in 1609 an event occurred which enabled him to completely vindicate the truth of his new belief, and to convince all but the most obstinate that it would be no longer possible to hold to the old theories. This was the invention of the telescope, the use of which revealed the most startling wonders in the heavens, and demonstrated the truth of Galileo's belief to the fullest extent.

Previous to this astronomers had been obliged to depend entirely upon the naked eye for making all observations; and although the world had advanced in almost every other way, in this respect the Italian star-gazer of the sixteenth century had no advantage over the Chaldean shepherds who, ages before, had studied the mysteries of the heavens during their lonely night-watches. But the telescope changed all this, and revolutionized the study of astronomy. It brought to light unsuspected possibilities for research, and laid bare the secrets that had eluded man from the earliest times. Not only were the planets and stars that were already known brought nearer and rendered more familiar by closer observation, but even the most distant of the heavenly bodies shone with a new glory, that was not diminished by the discovery that, farther still beyond their circles, other stars even yet more beautiful swept through their limitless courses, and that what had before seemed only empty space was in reality filled with vast systems of worlds, which waited only the proper moment to reveal themselves in all their bewildering splendor.

It is claimed by some that Galileo's invention of the telescope was not strictly original, and that he only applied and improved upon an idea that had already been used to some extent in the manufacture of optical instruments.

But, however this may be, it is certain that the first telescope which Galileo made and pointed to the heavens created the greatest wonder in the scientific world, and was considered almost as much of a marvel as the discovery of a new world would have been.

This first telescope, which was called Galileo's tube, aroused public curiosity to the greatest height, and Galileo's house was thronged with visitors eager to satisfy their curiosity; the most extravagant and absurd stories were circulated, and all through Venice, where Galileo happened to be staying at the time, there was no talk of anything but the wonderful instrument which was thought to be possessed of almost magical powers. The news spread rapidly from place to place, and all the astronomers set themselves to making telescopes, though it was long before anyone could produce an instrument equal in excellence to those made by Galileo. And so great was the excitement over the new invention, that small telescopes were sold in the streets as curiosities, and the observatories were besieged with people who gave the astronomers no peace until they satisfied their incredulous wonder.

In the meantime Galileo ascended his tower night after night, and pointed his telescope towards the heavens which had so suddenly assumed such a new and intense interest. And the results showed that, although he had given his whole life to the study, he had really only just begun to learn anything of the marvels of creation. One mystery after another was unfolded to his wondering gaze, and even the objects that had once seemed familiar to him now disclosed such new characteristics as to appear almost strange.

This was especially true of his observations on Jupiter, a planet which, from its great size and brilliant light, had always attracted the attention of astronomers. Regarded at first by mankind simply as a splendid star whose beauty added another glory to the sky, it was studied with unusual care, and even when later philosophers denied its stellar character, it was still an object of intense interest to astronomers, who looked upon it as a mysterious presence wandering among the familiar stars, awing them by its majesty, and yet as little understood as the flittings of the will-o'-the-wisp among the fireflies in the meadow. And although its planetary character was fully established in the time of Galileo, the wonder in it had not yet ceased. Galileo brought it night after night under the range of the telescope, and was soon rewarded by the most startling discovery in astronomical science.

He noticed, at first, that there seemed to be three new stars situated very near to Jupiter, and further observation led to the discovery of a fourth. Careful study of that part of the heavens soon led to the astonishing disclosure that these small stars revolved around Jupiter, in the same way that the moon revolved around the earth; and Galileo, after verifying his theory by elaborate and continuous observations, announced the undreamed-of fact that Jupiter was attended by four moons.

This intelligence was received with undisguised amazement by all classes. The friends of Galileo and the advoctates of the Copernican system, at once joyfully accepted this new proof of the harmonious motions of the heavenly bodies, while his opponents were equally bitter in their denunciation, refusing to look through the telescope for fear it would convince them of their error, and, as usual, bringing forth the most absurd arguments in favor of their own obstinancy.