The Astronomy of Milton's 'Paradise Lost' by Thomas Nathaniel Orchard (easy books to read in english TXT) 📕

in a piece of organ pipe, which served the purpose of a tube, the glasses being distant from each other by the difference of their focal lengths. An exactly similar principle is adopted in the construction of an opera-glass, which can be accurately described as a double Galilean telescope. Galileo must be regarded as the inventor of this kind of telescope, which in one respect differed very materially from the one constructed by the Dutch optician. If what has been said with regard to the inverted weather-cock be true, then Lippershey’s telescope was made with two convex lenses, distant from each other by the sum of their focal lengths, and all objects observed with it were seen inverted. Refracting astronomical telescopes are now constructed on this principle, it having been discovered that for observational purposes they possess several advantages over the Galilean instrument. When Galileo had completed his first telescope he returned with it to Venice, where he exhibited it to his friends. The sensation created by this small instrument, which magnified only three times, was most extraordinary, and almost amounted to a frenzy. Crowds of the principal citizens of Venice flocked to Galileo’s house in order that they might see the magical tube about which such wonderful reports were circulated; and for upwards of a month he was daily occupied in describing his invention to attentive audiences. At the expiration of this time the Doge of Venice, Leonardo Deodati, hinted that the Senate would not be averse to receive the telescope as a gift. Galileo readily acquiesced with this desire, and, as an acknowledgment of his merits, a decree was issued confirming his appointment as professor at Padua for life, and increasing his salary from 500 to 1,000 florins. The public excitement created by the telescope showed no signs of abatement. Sirturi mentions that, having succeeded in constructing an instrument, he ascended the tower of St. Mark’s at Venice, hoping to be able to use it there without interruption. He was, however, detected by a few individuals, and soon surrounded by a crowd, which took possession of his telescope, and detained him for several hours until their curiosity was satisfied. Eager inquiries having been made as to where he lodged, Sirturi, fearing a repetition of his experience in the church tower, decided to quit Venice early next morning, and betake himself to a quieter and less frequented neighbourhood.

The instrument was at first called Galileo’s tube; the double eye-glass; the perspective; the trunk; the cylinder. The appellation telescope was given it by Demisiano.

Galileo next directed his attention to the construction of telescopes, and applied his mechanical skill in making instruments of a larger size, one of which magnified eight times. ‘And at length,’ he writes, ‘sparing neither labour nor expense, he completed an instrument that was capable of magnifying more than thirty times.’

Galileo now commenced an exploration of the celestial regions with his telescope, and on carefully examining some of the heavenly bodies, made many wonderful discoveries which added greatly to the fame and lustre of his name.

The first celestial object to which Galileo directed his telescope was the Moon. He was deeply interested to find how much her surface resembled that of the Earth, and was able to perceive lofty mountain ranges, the illumined peaks of which reflected the sunlight, whilst their bases and sides were still enveloped in dark shadow; great plains which he imagined were seas, valleys, elevated ridges, depressions, and inequalities similar to what are found on our globe. Galileo believed the Moon to be a habitable world, and concluded that the dark and luminous portions of her surface were land and water, which reflected with unequal intensity the light of the Sun. The followers of Aristotle received the announcement of these discoveries with much displeasure. They maintained that the Moon was perfectly spherical and smooth—a vast mirror, the dark portions of which were the reflection of our terrestrial mountains and forests—and accused Galileo ‘of taking a delight in distorting and ruining the fairest works of Nature.’ He appealed to the unequal condition of the surface of our globe, but this was of no avail in altering their preconceived notions of the lunar surface.

Perhaps the most important discovery made by Galileo with the telescope was that of the four moons of Jupiter. On the night of January 7, 1610, when engaged in observing the planet, his attention was attracted by three small stars which appeared brighter than those in their immediate neighbourhood. They were all in a straight line and parallel with the ecliptic; two of them were situated to the east, and one to the west of Jupiter. On the following night he was surprised to find all three to the west of the planet, and nearer to each other. This caused him considerable perplexity, and he was at a loss to understand how Jupiter could be east of the three stars, when on the preceding night he was observed to the west of two of them. Galileo was unable to reconcile the altered positions of those bodies with the apparent motion of Jupiter among the fixed stars as indicated by the astronomical tables. The next opportunity he had of observing them was on the 10th, when two stars only were visible, and they were to the east of the planet. As it was impossible for Jupiter to move from west to east on January 8 and from east to west on the 10th, he concluded that it was the motion of the stars and not that of Jupiter which accounted for the observed phenomena. Galileo watched the stars attentively on successive evenings and discovered a fourth, and on observing how they changed their positions relatively to each other he soon arrived at the conclusion that the stars were four moons which revolved round Jupiter after the manner in which the Moon revolves round the Earth. Having assured himself that the four new stars were four moons that with periodical regularity circled round the great planet, Galileo named them the Medicean Stars in honour of his patron, Cosmo de’ Medici, Grand Duke of Tuscany. He also published an essay entitled ‘Nuncius Sidereus,’ or the ‘Sidereal Messenger,’ which contained an account of this important discovery.

The announcement of Galileo’s discovery of the four satellites of Jupiter created a profound sensation, and its significance became at once apparent. Aristotelians and Ptolemaists received the information with much disfavour and incredulity, and many persons positively refused to believe Galileo, whom they accused of inventing fables. On the other hand, the upholders of the Copernican theory hailed it with satisfaction, as it declared that Jupiter with his four moons constituted a system of greater magnitude and importance than that of our globe with her single satellite, and that consequently the Earth could not be regarded as the centre of the universe.

When Kepler heard of this remarkable discovery, he wrote to Galileo and expressed himself in the following characteristic manner: ‘I was sitting idle at home thinking of you, most excellent Galileo, and your letters, when the news was brought me of the discovery of four planets by the help of the double eye-glass. Wachenfels stopped his carriage at my door to tell me, when such a fit of wonder seized me at a report which seemed so very absurd, and I was thrown into such agitation at seeing an old dispute between us decided in this way, that between his joy, my colouring, and the laughter of both, confounded as we were by such a novelty, we were hardly capable, he of speaking, or I of listening.... I am so far from disbelieving in the existence of the four circumjovial planets, that I long for a telescope to anticipate you, if possible, in discovering two round Mars (as the proportion seems to me to require), six or eight round Saturn, and perhaps one each round Mercury and Venus.’ The intelligence of Galileo’s discoveries was received by his opponents in a spirit entirely different from that manifested by Kepler. The principal professor of philosophy at Padua, when requested to look at the Moon and planets through Galileo’s glass, persistently declined, and did his utmost to persuade the Grand Duke that the four satellites of Jupiter could not possibly exist. Francesco Sizzi, a Florentine astronomer, argued that, as there are seven apertures in the head, seven known metals, and seven days in the week, so there could only be seven planets. To these absurd remarks Galileo replied by saying that, ‘whatever their force might be as a reason for believing beforehand that no more than seven planets would be discovered, they hardly seemed of sufficient weight to destroy the new ones when actually seen.’ Another individual, named Christmann, writes: ‘We are not to think that Jupiter has four satellites given him by Nature in order, by revolving round him, to immortalize the name of the Medici, who first had notice of the observation. These are the dreams of idle men, who love ludicrous ideas better than our laborious and industrious correction of the heavens. Nature abhors so horrible a chaos, and to the truly wise such vanity is detestable.’ Martin Horky, a protégé of Kepler’s, issued a pamphlet in which he made a violent attack on Galileo. He says: ‘I will never concede his four new planets to that Italian from Padua though I die for it.’ He then asks the following questions, and replies to them himself: (1) Whether they exist? (2) What they are? (3) What they are like? (4) Why they are? ‘The first question is soon disposed of by Horky’s declaring positively that he has examined the heavens with Galileo’s own glass, and that no such thing as a satellite about Jupiter exists. To the second, he declared solemnly that he does not more surely know that he has a soul in his body than that reflected rays are the sole cause of Galileo’s erroneous observations. In regard to the third question, he says that these planets are like the smallest fly compared to an elephant; and, finally, concludes on the fourth, that the only use of them is to gratify Galileo’s “thirst of gold,” and to afford himself a subject of discussion.’[7] Galileo did not condescend to take any notice of this scurrilous production; but Horky, who imagined that he had done something clever, sent a copy of his pamphlet to Kepler. In a few days after he called to see him, and was received with such a storm of indignation that he begged for mercy and implored his forgiveness. Kepler forgave him, but insisted on his making amends. He writes: ‘I have taken him again into favour upon this preliminary condition, to which he has agreed—that I am to show him Jupiter’s satellites, and he is to see them, and own that they are there.’

The evidence in support of the existence of Jupiter’s satellites became so conclusive that the opponents of Galileo were compelled to renounce their disbelief in those bodies, whether real or pretended. The Grand Duke, preferring to trust to his eyes rather than believe in the arguments of the professor at Padua, observed the satellites on several occasions, along with Galileo, at Pisa, and on his departure bestowed upon him a gift of one thousand florins. Several of Galileo’s enemies, as a result of their observations, now arrived at the conclusion that his discovery was incomplete, and that Jupiter had more than four satellites in attendance upon him. Scheiner counted five, Rheita nine, and other observers increased the number to twelve. But it was found to be quite as hazardous to exceed the number stated by Galileo as it was to deny the existence of any; for, when Jupiter had traversed a short distance of his