The account of his discoveries, which he entitled Sidereus Nuncius, the Sidereal Messenger," was received by the astronomers of the old school with insults and incredulity. Some exclaimed against the impiety of scooping out valleys from the fair face of the moon; some attempted to explain away the satellites of Jupiter as mere appearances caused by reflected light. A professor in the university of Padua argued that as there were only seven metals, seven days in the week, and seven apertures in a man's head, so there could be but seven planets; and when forced to admit the visibility of the satellites through the telescope, he reasoned that, being invisible to the naked eye, they were useless, and consequently did not exist. Several persons claimed a prior discovery of the "Jovian planets," and the astronomer Zach, as late as 1788, claimed for Thomas Harriot the credit of having observed them on Jan. 16, 1610, some time before Galileo's discovery was made known. According to Sir David Brewster, however, Harriot did not see them till Oct. 17. Viviani, in his life of Galileo, tells us that the telescope led him to the construction of the compound microscope, and that he presented one to the king of Poland. The grand duke of Tuscany gave Galileo 1,000 florins, and made him his philosopher and mathematician with a liberal salary and nominal duties.

He now removed to Florence. To guard against future attempts to steal his laurels, he published his subsequent discoveries in enigmas, and thus in the course of the same year he announced that Saturn was "triple," an appearance which Huygens subsequently showed was caused by that planet's rings. Galileo was the first to notice that Venus exhibits phases like those of the moon; and if not the first to descry spots on the sun's disk, he was at least the first to note their peculiarities, and to infer from them the sun's rotation. Some of these observations were made in 1611 at Rome, which he then visited for the first time, where he erected his telescope in the Quirinal garden belonging to Cardinal Bandini. He was received with the highest honors, and became a member of the famous Lincean academy. In 1612 he combated in his work on the laws of floating bodies the common opinion that the tendency of substances to sink or swim in water depends on their shape. With this period in his life the philosopher may be said to have reached the zenith of his prosperity, while at the same time the malice of his enemies began to acquire a dangerous intensity. The Copernican system, which he had long taught in public, afforded a good pretext for attacking him.

The sun's revolution round the earth was thought to be a truth of Scripture. Certain Tuscan ecclesiastics began to preach against the wickedness of sending our world spinning through space, and a sarcastic Dominican hurled a sermon at Galileo from the text: Viri Galilaei, quid statis adspicientes in caelum ? Ye men of Galilee, why stand ye looking up into heaven ?" In 1613 Galileo addressed a letter to his pupil Castelli, showing that the language of the Bible should be interpreted according to popular ideas, and that the.Ptolemaic system is really as much at variance with it as the Copernican.

This was followed by one to Christina, grand duchess dowager of Tuscany, reiterating his views, and supporting them by quotations from the writings of the fathers. A Dominican, Lorini, laid a copy of the Castelli letter before the Roman inquisition in February, 1615, but the inquisitors refused to act in the matter, remarking that by confining himself to the system and its demonstration, and letting alone the Scriptures, Galileo would be secure from molestation. His enemies, however, continued their intrigues, and about the end of 1615 he went to Rome, either to obtain a formal sanction of his opinions, or in obedience to a summons. His case came again before the holy office in February, 1616. He was charged with teaching that the sun is the centre of the planetary system, and interpreting Scripture to suit his own theory. The qualifiers of the inquisition pronounced the obnoxious doctrines "formally heretical, because expressly contrary to Holy Scriptures." Galileo's .letters to Castelli and the grand duchess, Copernicus's work on the revolution of the heavenly bodies, and Kepler's epitome of the Copernican theory, were placed on the Index Expurgatorius, whence they were not removed until the time of Benedict XIV.; and Galileo himself was forbidden ever again to teach the motion of the earth and the stability of the sun.

Thenceforward he was not permitted to express himself as though Copernicanism were, in the words of the Roman curia,an actually grounded hypothesis." But he was permitted and encouraged to use the hypothesis most actively as his clue to fresh scientific results, and to treat with the most ample justice the scientific arguments for and against. He was permitted to maintain that Copernicanism was scientifically likely in the highest possible degree; but he was not at liberty to teach expressly that it had received absolute and irrefragable proof. He had an audience of the pope, however, who assured him of his protection, and in 1617 he returned to Florence. Sickness prevented him from observing the three comets which appeared in 1618, but he entered warmly into discussions about them, and is supposed to have had the chief share in a lecture delivered by his friend Guiducci and printed in 1619, in which they are held to be only meteors. This discourse was attacked by the Jesuit Grassi under the pseudonyme of Lotario Sarsi, and defended by Galileo in his Saggiatore ("As-sayer), one of the most beautifully written of his works.

On the accession of his friend Cardinal Barberini to the pontificate under the title of Urban VIII., he went to Rome to offer his congratulations, arriving in the spring of 1624, and receiving during the two months that he remained every mark of esteem and liberality. The pope granted him a pension of 100 crowns, and one of 60 crowns to his son. He now set about composing a work in which he might sum up all the arguments for and against his favorite theory. It was written in the form of dialogues, and accompanied by a preface in which he protested ironically against the idea that the decision of the inquisition in 1616 was rendered through ignorance or passion. He says that, on the contrary, its officers listened with attention to his statement of the scientific arguments on which his theory was based, and maintains that the grounds upon which this decision was justified were entirely religious. The book was published at Florence in 1632 under the title of "Dialogue on the two Principal Systems of the World, the Ptolemaic and Copernican." This being regarded as a violation of the injunction, Galileo was ordered to appear in person at Rome, where he arrived in February, 1633, and took up his quarters with the Tuscan ambassador. His trial was short.