Galileo had named the satellites the Medicæan stars in honor of his patron, Cosmo di Medici, and one antagonistic philosopher gravely denied the willingness of nature to give Jupiter four moons simply for the sake of immortalizing the name of Medici, and said that the whole thing was an idle dream.

Another declared solemnly that he did not more surely know that he had a soul in his body, than that the moons were caused entirely by reflected rays of light, and claimed that Galileo's "thirst for gold" had alone led him to such an announcement.

And still another astronomer seriously demonstrated that it was contrary to the law of nature to have more than seven planets, and that therefore more than seven could not exist. He argued tha there were seven windows given to animals in the domicile of the head, to admit the air to the rest of the body to warm and nourish it, and that likewise, in the heavens there were two favorable stars, Venus and Jupiter; two unfavorable stars, Mars and Saturn; two luminaries, the sun and the moon; and alone undecided and indifferent. Also, that there were but seven metals, seven days in the week, and innumerable similar phenomena to prove that there could only be seven planets; summing up with the conclusion that the satellites were invisible to the naked eye, that they therefore could exercise no influence on the earth, that they were therefore useless, and therefore did not exist.

To this Galileo only replied that, however weighty the reasons might be that no more than seven planets could exist, they scarcely seemed sufficient to destroy the new ones when actually seen, and went on observing Jupiter.

His friends supported his theories as warmly as ever, and the controversy was kept up until the existence of the satellites was established beyond a doubt, when his enemies went to the other extreme and claimed that Galileo's observations were most imperfect, as there were really twelve satellites instead of four; and it was only when Jupiter moved to another part of the heavens, carrying his four moons with him, that they admitted that the original announcement was correct.

Galileo's observations of the moon also led to a fierce discussion, and philosophers again spent a great time in arguing and denying, with the usual results.

From its nearness to the earth, and the interesting phenomena connected with the various changes that it passed through every month, the moon had from the earliest times been an object of the greatest interest to man, who attributed mysterious power to its influence, and placed it among the divinities. And it still held its subtle attraction long after the old religions had passed away, for with the exception of the sun, it alone of all the heavenly bodies exercised an important influence in the concerns of daily life. Filling the heavens with its wonderous beauty long after the great god of day had set, it seemed like a beneficent spirit sent by some protecting power to guard the lonely watches of the night; while to the traveller on desert or mountain or sea, its beams came with friendly assurance of help and in braving the unseen perils of the darkness.

In the time of Galileo the popular belief concerning the moon was that it was a perfectly spherical body, with a surface as smooth and polished as a mirror, and that the dark parts of its surface were either the reflections of the forests and mountains of the earth, or caused by the interposition of opaque bodies floating between it and the sun, or, because of its nearness to the earth, the result of contact with certain terrestrial elements which marred its beauty and made it less pure than the bodies in the more remote heavens.

But Galileo's observations led him to the belief that the moon resembled the earth in structure, and that its dark portions were the shadows reflected from mountains and other inequalities in its surface; while he also claimed that it was probable that there were continents and oceans distributed over the surface similar to those on the earth, and that the faint shadow which was attached to the crescent moon, and filled out that part of the surface unlighted by the sun, was caused by the reflection of the earth's light, or earthshine. These theories were at once attacked by his opponents, who said that Galileo took delight in ruining the fairest works of nature, and utterly denied the existence of mountains on the moon, as their presence there would destroy its spherical shape.

Galileo replied that to conceive of the moon and the earth as perfectly spherical bodies would only detract from their use, in the plan of nature, for absolute smoothness and sphericity would make the earth only a vast, unblessed desert, void of animals, of plants, and of men; the abode of silence and inaction; senseless, lifeless, soulless, and stripped of all those ornaments which made it so beautiful. But this argument was derided by his enemies, who replied that the moon's surface was really smooth and unalterable in spite of all that Galileo could say, and that the parts which appeared hollow or sunken were in reality filled up with a crystal substance perfectly imperceptible to the senses, but still serving the purpose of giving to the moon her true spherical shape.

Galileo agreed to accept the theory of a crystal substance filling all irregularities, provided the philosophers would allow him to raise crystal mountains ten times higher than those he had actually seen and measured, and this nonsense effectually put an end to the crystalline theory.

In regard to Galileo's theory of earthshine his critics averred that it was untenable, because the earth was not a planet and did not revolve around the sun, or shine like the other planets, and ascribed the shadow to Venus or the fixed stars, or the rays of the sun shining through the moon. And thus the endless dispute went on, and all of Galileo's wonderful discoveries were received with scorn and unbelief by the enemies of progress, who bent all the powers of their minds to the refutation of the Copernican theory. But Galileo went on with his observations undisturbed by this opposition, and constantly announced new wonders.

He examined the Milky Way, and was the first to prove that its nebulous appearance was caused by the presence of myriads of stars, whose light reached to infinite distances beyond the system of the earth; and although this theory was of course disputed, it was firmly established by repeated observation, and this confirmed beyond a doubt the conjecture of Pythagoras that countless millions of stars circled continuously through their distant courses far beyond the vision of man.

Galileo subjected all of the planets in turn to his scrutinizing gaze, and one discovery followed another with astounding rapidity, so that there never ceased to be a new marvel to wonder at.

He detected the presence of Saturn's rings, although his glass was not strong enough to show him their real nature, and he supposed the planet to have two attendant stars; and a month later he announced the discovery of the phases of Venus, deducing from this fact another proof of the Copernican system. He also examined the fixed stars, and by careful comparison of their light with that of the planets decided that they did not receive their light from the sun, and he added still another argument to the doctrine of Copernicus by the discovery of the spots on the sun and their motion across its disc.

As early as 807 dark spots had been observed on the face of the sun, and for centuries after this phenomenon attracted the attention of astronomers. But all the curiosity was satisfied by the supposition that the dark body was simply caused by the passage of Mercury or some other small object across the sun's surface.

But Galileo claimed that the spots were in actual contact with the sun, and that they had a common and regular motion with which they revolved around the sun, which turned upon his axis once a month.

Here was another argument for the Copernican theory, and in consequence the new explanation of sun spots was received with little favor by the followers of Aristotle.

And thus in the midst of opposition and discouragement Galileo kept on his way, continually adding to the sum of scientific knowledge, and unwearying in his efforts to place natural science upon a more reasonable and comprehensible plane than it had before reached.

His observations included not only the phenomena of the heavens, but also those connected more intimately with the earth, and his essays extended over a great variety of subjects which had hitherto been treated only with ignorance or indifferent success.