Venus rivals the phases of the moon.
Hitherto, Galileo had observed Venus when her disc was largely illuminated; but having directed his telescope to her when she was not far removed from the sun, he saw her in the form of a crescent, resembling exactly the moon at the same elongation from the sun. He continued to observe her night after night, during the whole time that she could be seen in the course of her revolution round the sun, and he found that she exhibited the very same phases which resulted from her motion round that luminary.
Galileo had long contemplated a visit to the metropolis of Italy, and he accordingly carried his intentions into effect in the early part of the year 1611. Here he was received with that distinction which was due to his great talents and his extended reputation. Princes, cardinals, and prelates hastened to do him honour; and even those who discredited his discoveries, and dreaded their results, vied with the true friends of science in their anxiety to see the first wonder of the age.
In order to show the new celestial phenomena to his friends at Rome, Galileo took with him his best telescope; and as he had discovered the spots on the sun's surface in the month of March, 1611, he had the gratification of exhibiting this new wonder to his admiring disciples. He accordingly erected his telescope in the Quirinal garden, belonging to cardinal Bandini; and in April, 1611, he exhibited them to his friends in many of their most interesting variations. From their change of position on the sun's disc, Galileo at first inferred, either that the sun revolved about an axis; or that other planets, like Venus and Mercury, revolved so near the sun as to appear like black spots when they were opposite to his disc. Upon continuing his observations, however, he saw reason to abandon this last opinion. He found, that the spots must be in contact with the surface of the sun; that their figures were irregular; that they had different degrees of darkness; that one spot would often divide itself into three or four; that three or four spots would often unite themselves into one; and that all the spots revolved regularly with the sun, which appeared to complete its revolution in about twenty-eight days.
Previous to the invention of the telescope, spots had been more than once seen on the sun's disc with the unassisted eye. But even if these were of the same character as those which Galileo and others observed, we cannot consider them as anticipations of their discovery by the telescope. As the telescope was now in the possession of several astronomers, Galileo began to have many rivals in discovery; and it is now placed beyond the reach of doubt, that he was not the first discoverer of the solar spots. From the communication which I received from the late Dr. Robertson, of Oxford[13], it appears that Thomas Harriot had discovered the solar spots on or before the 8th of December, 1610. His manuscripts, in lord Egremont's possession, incontestably prove that his regular observations on the spots commenced on the 8th of December, 1610,—at least three months before Galileo discovered them; and that they were continued till the 18th of January, 1613. The observations which he has recorded are 199 in number; and the accounts of them are accompanied with rough drawings representing the number, position, and magnitude of the spots.[14]
Another candidate for the honour of discovering the spots of the sun, was John Fabricius, who undoubtedly saw them previous to June, 1611. The dedication of the work[15] in which he has recorded his observation, bears the date of the 13th of June, 1611; and it is obvious, from the work itself, that he had seen the spots during the year 1610: but as there is no proof that he saw them before the 8th of December, 1610, and as it is probable that Harriot had seen them before that date, we are compelled to assign the priority of the discovery to our distinguished countryman.
The claim of Scheiner, professor of mathematics at Ingolstadt, is more intimately connected with the history of Galileo. This learned astronomer having, early in 1611, turned his telescope to the sun, necessarily discovered the spots which at that time covered his disc. Light flying clouds happened, at the time, to weaken the intensity of his light, so that he was able to show the spots to his pupils. These observations were not published till January, 1612; and they appeared in the form of three letters, addressed to Mark Velser, one of the magistrates of Augsburg, under the signature of Apelles post Tabulam. Scheiner, who, many years afterwards, published an elaborate work on the subject, adopted the same idea which had at first occurred to Galileo,—that the spots were the dark sides of planets revolving round and near the sun.[16]
On the publication of Scheiner's letters Velser transmitted a copy of them to his friend Galileo, with the request that he would favour him with his opinion of the new phenomena. After some delay, Galileo addressed three letters to Velser, in which he combated the opinions of Scheiner on the cause of the spots. These letters were dated the 4th of May, 1612; but though the controversy was carried on in the language of mutual respect and esteem, it put an end to the friendship which had existed between the two astronomers. In these letters, Galileo showed that the spots often dispersed like vapours or clouds; that they sometimes had a duration of only one or two days, and at other times of thirty or forty days; that they contracted in their breadth when they approached the sun's limb, without any diminution of their length; that they describe circles parallel to each other; that the monthly rotation of the sun again brings the same spots into view; and that they are seldom seen at a greater distance than 30° from the sun's equator. Galileo, likewise, discovered on the sun's disc faculœ, or luculi, as they were called, which differ in no respect from the common ones but in their being brighter than the rest of the sun's surface.[17]
In the last of the letters which our author addressed to Velser, and which was written in December, 1612, he recurs to his former discovery of the elongated shape, or rather the triple structure, of Saturn. The singular figure which he had observed in this planet had entirely disappeared; and he evidently announces the fact to Velser, lest it should be used by his enemies to discredit the accuracy of his observations. "Looking on Saturn," says he, "within these few days, I found it solitary, without the assistance of its accustomed stars, and, in short, perfectly round and defined like Jupiter; and such it still remains. Now, what can be said of so strange a metamorphosis? Are the two smaller stars consumed like the spots on the sun? Have they suddenly vanished and fled? or has Saturn devoured his own children? or was the appearance indeed fraud and illusion, with which the glasses have for so long a time mocked me, and so many others who have often observed with me? Now, perhaps, the time is come to revive the withering hopes of those who, guided by more profound contemplations, have followed all the fallacies of the new observations, and recognised their impossibilities. I cannot resolve what to say in a chance so strange, so new, and so unexpected; the shortness of the time, the unexampled occurrence, the weakness of my intellect, and the terror of being mistaken, have greatly confounded me." Although Galileo struggled to obtain a solution of this mystery, yet he had not the good fortune of succeeding. He imagined that the two smaller stars would reappear, in consequence of the supposed revolution of the planet round its axis; but the discovery of the ring of Saturn, and of the obliquity of its plane to the ecliptic, was necessary to explain the phenomena which were so perplexing to our author.
The ill health to which Galileo was occasionally subject, and the belief that the air of Florence was prejudicial to his complaints, induced him to spend much of his time at Selve, the villa of his friend Salviati. This eminent individual had ever been the warmest friend of Galileo, and seems to have delighted in drawing round him the scientific genius of the age. He was a member of the celebrated Lyncæan Society, founded by Prince Frederigo Cesi; and though he is not known as the author of any important discovery, yet he has earned, by his liberality to science, a glorious name, which will be indissolubly united with the immortal destiny of Galileo.