IN March 1610, a book containing fewer than 100 pages caused such a stir that its author was placed under house arrest for the rest of his life. The book itself made a profound impact in its own time and went on to refashion astronomy completely to a form that marked the beginning of modern astronomy. To the author, its contents and his own fate ensured a permanent place in the hall of fame of science.
The book in question is Sidereus Nuncius (The Starry Messenger) and the author, Galileo Galilei. The short text, with drawings by Galileo himself, reported the observations made by him when he famously turned his telescope towards the night sky. The observations of the moon, Jupiter, the Milky Way and the stars, reported in the slender volume, and other observations and their interpretations eventually led to the demise of the earth-centred Ptolemaic model of the universe. It opened the way to the adoption of the suncentred, heliocentric model of the solar system, originally proposed by Copernicus in 1543.
The science of astronomy took a huge leap forward into the modern era with the invention of the optical telescope and its use to study the night sky and discover new celestial objects.
Contrary to popular belief, Galileo was not the discoverer of the telescope. Nor was he the first to use the telescope to study the universe. Galileos significance in the history of astronomy, and indeed of all sciences, does not arise just from the fact that he used the telescope innovatively or simply saw the wonders of the night sky. He made a revolutionary change in the way we see the universe; he ruptured the epistemic difference between the celestial and the terrestrial and engendered the birth of a modern science.
Early telescopes were refractors made out of two lenses: convex as the objective (the end pointed towards the heavens) and concave as the eyepiece (through which one looked at the sky). Thus, the invention of the telescope required the development of lenses, which in turn was related to the evolution of the humble spectacles superbly described in Renaissance Vision from Spectacles to Telescopes by Vincent Ilardi (published by American Philosophical Society, 2007).
The first time that anyone put two lenses together to make a telescope-like optical instrument was in 1608, in Holland. The inventor of an opera-glass-like telescope was Hans Lipperhey, an optician-craftsman. Legend has it that his children and their friends in the neighbourhood playfully experimented with the lenses that were lying around in his workshop and accidentally discovered that when they held two lenses against each other they could see far-away objects magnified. Hans Lipperhey learned of this and assembled the first crude telescopes. Zacharias Janssen, a neighbour and a fellow optician, too claimed that he was the first to invent the telescope. The Dutch royal court decided not to award a patent for the contraption to either of them. What perhaps was a loss for Lipperhey and Janssen was a gain for science and progress; if the patent had been granted, who knows it may even have hindered Galileo and held back the progress of sciences for several years.
Soon telescopes were a rage and many scientists and amateurs craved to possess one. It is but natural that one of them would turn the telescope skywards. Thomas Harriot, an English physicist, ventured to use the telescope to study the heavens. Having obtained a telescope from the Netherlands, he made numerous telescopic observations from 1609 to 1613. His telescopic drawing of the moon of early August 1609 is the first on record and preceded Galileos study of the moon by several months. Several of Harriots observations of sunspots in December 1610 are also the first on record. Nevertheless, there was one significant difference that set Galileo apart from Harriot.
Galileo found out about this invention in the spring of 1609 and by then very many spectacle-makers were producing their own contraptions. However, Galileo was not just interested in purchasing one off the shelf and using it. He inquired into the physics of telescopes and wondered how they worked. Soon he realised that the magnification was proportional to the ratio of the power of the concave lens to that of the convex lens. In other words, he needed a weak convex lens and a strong concave lens; commonly the lenses in general use were the other way round. Furthermore, opticians only made glasses in a narrow range of strengths. Using available off-the-shelf lenses, three or thereabouts, was the best magnification possible. Galileo learned to grind his own lenses, and by August 1609 he had achieved about 9x magnification. He perfected it further and made a telescope with 20x magnification, which he used for his telescopic study of the universe.
The magnificent moon naturally attracted Galileos attention. He observed the moon for several nights sometime around December 1609. According to Aristotelian principles, the moon was above the sublunar sphere and in the heavens, and hence should be perfect. Galileo found the surface of the moon to be not smooth, even and perfectly spherical ... but on the contrary, to be uneven, rough, and crowded with depressions and bulges. And it is like the face of the earth itself, which is marked here and there with chains of mountains and depths of valleys.
Even a crude telescope would show the craters on the moon, formed, as we now know, owing to the impact of asteroids and meteorites. However, Galileos investigation of the mountains on the moon remains even today a classic lesson in scientific method. It can be observed at the time of dawn, even before the ground is illuminated, the clouds above catch the sunlight. Similarly, sunlight reaches the valleys much later than it reaches the peaks of mountains. Galileo noticed that some points in the night (dark) side of the moon were illuminated much before the sunlight reached the ground beneath and came to the conclusion that they must be mountain peaks. From the shadow these peaks cast, he even set out to calculate the height of those peaks. Thus it was evident that celestial bodies were indeed just like the earth, another object in the solar system, and not heavenly in nature.
Galileos next major discovery began with his observation of Jupiter on January 7, 1610. His telescopes unmistakably showed an odd set of three small fixed stars near Jupiter, collinear with the planet. A few days later he saw yet another one, making the total four. These stars were invisible to the naked eye. Observations of Jupiter over successive nights revealed that these objects sometimes disappeared from view as they moved behind or in front of the planet. By January 15, 1610, Galileo correctly inferred that these objects were indeed moons of Jupiter and that they orbited Jupiter in the same manner as the moon orbits the earth.
The discovery of the moons of Jupiter doubly jeopardised the prevailing dogmas. Here were four stellar objects not stated in any religious cannon. Furthermore, contrary to the belief that all heavenly bodies must go around the earth, for the first time stellar objects had been observed orbiting another planet. This weakened the hold of the Ptolemaic model and made Copernicus argument that the moon goes around the earth and the earth around the sun more plausible. Today, the four moons of Jupiter Io, Europa, Ganymede and Callisto are known as Galilean satellites.
Galileo then turned his attention to the most numerous objects in the night sky the stars. To his disappointment, the stars showed no features, they were just a speck of light. Galileo suggested that it was owing to their immense distance from the earth that stars appeared as point source. On turning his telescope to the band of the Milky Way, Galileo saw it resolved into thousands of hitherto unseen stars. Similarly, when he explored the region of the Pleiades (Krittika), he found stars that were unseen by the naked eye. His exploration of the Orion nebula also revealed unseen stars and nebulas. Thus the universe seemed to be more than what met the eye. All these were the subject matter of his first treatise, The Starry Messenger, published in 1610.
His subsequent observations of the sky revealed that Saturn has an appendage-like structure around it (later clarified by Christiaan Huygens as the rings of Saturn). His painstaking observation of Venus revealed that it too exhibits waxing and waning crescents just like the moon (again establishing the heliocentric model). He convincingly proved that comets were indeed placed well above the moon, thus making the unchanging realm of the heavens as chaotic as the terrestrial. The proverbial last straw came when he established that the sun was blemished: he observed sunspots through his telescope and the feat cost him his eyesight.
For the earth-bound observers, while rain, wind and erosion kept changing the features of the earths terrain, hardly anything changed in the heavens. Day after day the sun rose in the east; month after month the moon waxed and waned in the same manner; year after year the sun appeared to go around the earth; and all the while stars appeared to be fixed and immobile. Aristotle thus made a clear distinction between the heavenly (celestial) and terrestrial (sublunar) realms, the former being unchanging, perfect, and so on and the latter being changeable, imperfect, and so on. Hence, it was assumed that laws that were applicable to one realm would not apply to another and this cosmic divide lasted for centuries. This was held by the religious orthodoxy as an immutable dogma to be taken as faith.
Galileos observations and the Copernican model of cosmos favoured by him contradicted the Aristotelian view of the cosmos and challenged the then ruling orthodoxy. Moreover, he also established that the unruly comets were true celestial bodies. With these observations, the long-standing distinction between the celestial and terrestrial in physics was dissolved. The same laws were seen to apply in the heavens as on the earth there was just one world; a unitary cosmos.
Reasonable people would have found such discoveries astounding, but the religious orthodoxy in Europe thought otherwise. It was alarmed with Galileos discoveries, so much so that when he went to Rome, it is said that many of the orthodox clerics were not even willing to look through the telescope lest they should be waylaid by the design of the Devil. They had such absolute faith in Aristotle and their own interpretation of the holy book that they claimed that the rings around Saturn and the moons around Jupiter, revealed by the telescope, were the work of the Devil, meant to tempt them to stray from the path of the Lord.
Consequently, Galileo had to suffer at the hands of the orthodoxy and was forced to renounce his scientific views. He was condemned to house arrest and some of his works were proscribed.
Not long after, improving upon Galileos telescope, Johann Kepler designed a new refractor telescope, and Isaac Newton came up with a reflecting telescope. With these advancements, far more details and new objects in the universe could be seen.
Four centuries later, radio telescopes, X-ray telescopes and Infrared telescopes were invented. Not only do we study the planets that go around the sun, we have now catalogued more than 200 planets that go around other stars exoplanets. Today we are able to fathom the far reaches of the universe and the farthest object that we have identified (using the Hubble space telescope) is so far away that it takes 1,300 crore years for the light to reach us from there. A long way from Galileo, indeed. But any long journey has to begin with the first step. Galileo took that step 400 years ago, and it has been established beyond doubt, that it was in the right direction.
T.V. Venkateswaran is Scientist, Vigyan Prasar, Department of Science and Technology, New Delhi.
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