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22 i.e. Venus imitates the phases of the moon. It had been regarded as an objection to the theory of Copernicus that Venus and Mercury ought to show phases but did not, and also that Venus ought to appear much larger when nearest the earth. Galileo's new discovery disposed of these objections, making it clear that Venus did show phases and that it did appear much larger when nearer the earth, but that as it was then nearly "new" most of its disc was not illuminated, so that it did not look so bright and consequently not so large to the naked eye.

Galileo found little trustworthy evidence of phases of Mars or Mercury, as the former does not reach crescent shape, and the latter is always close to the sun, but at any rate he found no reason to doubt that both these planets shared the motion of revolution round the sun. There seems to have been a suspicion in his mind that his credit might be assailed at Rome by the many enemies whom he had made, not only by his opposition to the Aristotelian tradition, but by his great skill in argument, his pungent satire, and his capacity for turning the opposition into ridicule after, adducing ingenious points in its favour. It was not altogether unlike the case of Socrates, whose system of debate must have been excessively annoying to anyone who dared seriously to maintain any proposition against him. Be that as it may, Galileo now considered it expedient to go to Rome to make His discoveries known at first hand to the leaders of the Church and forestall any possible accusation. The Grand Duke not only granted him leave of absence but provided for his journey and arranged for him to lodge at the Tuscan Embassy in Rome. Under such auspices and armed with many letters of introduction, including one to Cardinal Barberini (afterwards Pope Urban VIII.), he had no difficulty in obtaining a hearing