1610: The Year that Improved our Vision

Transcription

1 1610: The Year that Improved our Vision H R Madhusudan Madhusudan works at J N Planetarium, Bangalore. He teaches at the various academic programmes conducted by the planetarium. Keywords Galileo, satellites of Jupiter, heliocentric model, phases of Venus. The telescopic observations by Galileo Galilei in 1610 provided compelling evidence of a heliocentric system of the universe, as forwarded by Nicholas Copernicus in the earlier century. Firstly, Galileo established that the Earth was not the only centre of revolution. Therefore, the Earth did not enjoy a special place, as such. Next, his observations of Venus showed that the planet Earth clearly revolved around the Sun. Vision is the art of seeing things invisible to others. Jonathan Swift The year 2009 was declared as The International Year of Astronomy to (IYA) to celebrate the introduction of a path-breaking innovation in observing the heavens namely, the telescope, four hundred years ago. As opposed to the popular notion, Galileo was not the inventor of this device. Nor was he the first to train a telescope to observe the happenings in the heavens. Thomas Harriot in England had used a telescope for observing the moon in July His sketch of the moon dated July 26, 1609 is the first documentation of an astronomical object observed through a telescope. Galileo s telescopic observations began almost a month later. His serious observations of the moon were carried out towards the end of As we shall see in this article, the telescope improved only the sight for most people. In Galileo, however, it raised new visions about the arrangement of planets in our solar system. In that short period, he observed that the moon was neither perfectly spherical nor smooth. Considering the head-start that Harriot had over Galileo, why is it that IYA had greater focus on Galileo and his use of the telescope for astronomical purposes? The answer lies in the impact that Galileo had over the masses as well as over the authorities who mattered. Had it been just observing and recording the features on the 948 RESONANCE October 2010

2 moon, Galileo s contributions to astronomy would not be worth the IYA. It is the telescopic observations that he made towards the beginning and the end of 1610 that really changed our perception of the arrangement of planets in the heavens a problem that challenged the greatest thinkers of Nature for nearly three thousand years. The lunar observations of 1609 served to usher in a silent revolution on a sedate note quickly metamorphosing into a storm that would leave Galileo in turmoil. For that reason, the year 1610 assumes significance that is incomparable in the annals of astronomy at least. So, let us see what happened in 1610, a full four hundred years ago. During the first week of January, Galileo observed the largest planet of our solar system Jupiter. He observed three stars arranged in a straight line with Jupiter. Two of them were to the east of Jupiter and one of them to the west. This was on January 7. The following night, as if led by destiny, he observed the same part of the sky. Galileo saw that all the three stars were on the western side of Jupiter. He recorded the positions meticulously as shown in Figure 1. This must have startled him, for Jupiter was moving westward then. If any, the three stars must have been displaced more eastward, away from Jupiter. Again, on January 10, he could see only two stars now positioned to the east of Jupiter while the third was hidden behind Jupiter. The hidden star reappeared the following night. Galileo s keen eyes unfailingly recorded that whereas the brightnesses were equal two days earlier, now they differed! Galileo noted, Figure 1. Galileo s observations of the satellites of Jupiter. Courtesy: Sidereus Nuncius translated by Albert Van Helden; TheUniversity of Chicago Press, I had now decided beyond all question that there existed in the heavens three stars wandering about Jupiter as do Venus and Mercury around the Sun, and this became plainer RESONANCE October

3 Accepting heliocentrism, meant a surrender of the idea unquestioned for several centuries that the Earth is seated in the middle around which all other celestial bodies revolved. than daylight from observations on similar occasions that followed. This is typical of Galileo s tongue-in-cheek remark when he says as do Venus and Mercury around the Sun. Galileo was already a Copernican among Ptolemians. His observational evidence for a Sun-centred (heliocentric) system had not gathered as yet. For us, a generation following a well-established tradition (this is another way of saying that we are all brainwashed into believing an idea), indeed, it is plainer than daylight. But, not in Galileo s time. Accepting heliocentrism, however, meant a surrender of the idea unquestioned for several centuries that the Earth is seated in the middle around which all other celestial bodies revolved. And, that was an uncomfortable thought for the Church. Soon, Galileo observed a fourth star, the largest one, wandering about Jupiter on January 13. Galileo was not yet done with the Medician Stars, as Galileo had christened the four stars after Cosimo II de Medici, Grand Duke of Tuscany of Florence and his three brothers. Galileo noticed that the four stars moved around Jupiter at different speeds with the nearest one moving around the swiftest and the farthest one moving the slowest. The details of Galileo s observations appear like a mundane description of what Galileo saw night after night. His deduction arising out of his observations questioned the religious doctrines of the day. The Medician Stars, now popularly known as Galilean Satellites, went around an object other than the Earth. This was totally unacceptable to the religious heads who firmly believed that the Earth is the only centre of revolution. Now, Galileo had discovered one more. The Earth was not a special place anymore. This clearly went against theological principles of the day. A revolution was brewing in Galileo s mind who believed in Copernicus s system. Without delay and perhaps, to invite public participation in a great debate, Galileo published his telescopic observations of the moon and the planet Jupiter in a small book entitled Sidereus Nuncius meaning Starry Messenger in March While his observations of the Earth s moon led to noting 950 RESONANCE October 2010

4 down topographical details and carrying out some calculations involving the heights of lunar mountains, the observations of the Medician Stars demanded a fair bit of imagination coupled with courage to think on the contrary to popular notions in order to set them into revolution. And, what a revolution (and, a revelation, too) that was! Galileo s assertion that the Moon was not a perfect body was a small tap on the knuckles whereas his discovery of another centre of revolution was a body-blow to the staunch followers of a geocentric system. He penned Sidereus Nuncius in Italian, the common man s language. All the 550 copies of the 24- page booklet sold out within a week. Everyone who read and understood had an opinion about Galileo s findings just as willed by Galileo himself. Now, the debate about heavenly happenings was not confined to the elite and the religious authorities alone. The silent revolution that started in one man s mind had now multiplied very quickly by several times over. Silence gave way to murmurs. Galileo s assertion that the moon was not a perfect body was a small tap on the knuckles whereas his discovery of another centre of revolution was a body-blow to the staunch followers of a geocentric system. Galileo continued his telescopic forays into the heavens. In July 1610, he studied planet Saturn. He found it to be a sphere with small lobes on either ends. He thought that Saturn was threebodied. The question of mysterious lobes was resolved with more powerful telescopes and nearly fifty years later. As an exception, Galileo s study of Saturn did not cause any discomfort in religious circles! The period between October and December 1610 is one of the most significant periods in the annals of astronomical observations. Firstly, three astronomers independently observed Jupiter s satellites, confirming what Galileo had earlier observed about them. Considering that several people around Europe had failed to see what Galileo had discerned, this was very important. After all, any kind of scientific result must be reproducible elsewhere under identical conditions. Science demands this. That is the only way to garner support and credentials to one s works. The next important event was Galileo s observation of planet Venus one that went on to conclusively dislodge the Earth from RESONANCE October

5 Figure 2. Phases of Venus as observed by Galileo. Redrawn from Galileo, the Assayer. its exalted central position. In his book, De Revolutionibus, Copernicus had mentioned that if his Sun-centred theory were indeed correct, then planet Venus ought to display all the phases waxing crescent, gibbous, full, waning gibbous and crescent during its motion around the Sun. If Venus were to orbit the Earth, it would never be seen in full phase. Also, if it were orbiting the Sun, Venus must be brightest when it is in the crescent phase because it is also nearest to the Earth then (Figure 2). Copernicus had claimed that one could see these effects if one could somehow improve the power of vision. Galileo s telescope had accomplished just that. On December 30, 1610, Galileo wrote a letter to his pupil Benedetto Castelli describing his observations of planet Venus: when Venus began to be visible in the evening sky, I started observing it and saw that its figure was circular, though extremely small. Afterwards, I saw [Venus] growing in magnitude significantly, though always maintaining its circular shape. Approaching maximum elongation, [Venus] began to lose its circular shape on the other side from the Sun and within a fewdays had acquired a semicircular shape. This shape is maintained for a number of days. More precisely, it maintained [this shape] until it began to move towards the Sun, slowly abandoning the tangent. It now begins to assume a notable corniculate shape. Thus, it will continue to decrease during the period in which it remains visible in the evening sky. Reportedly, Galileo sent a letter to Johannes Kepler that concealed his observation of the phases of Venus in the form of an anagram which he later unscrambled to mean The mother of Love (Venus) emulates the figures of Cynthia (Moon). That is, Venus goes through the phases just as the moon does. The fact that the shape of Venus changed from markedly gibbous (nearly full phase) to crescent is extremely significant (Figure 3). For, this is the one that demands planet Venus to orbit around the Sun. If Venus were to execute its revolutions below the Sun, as had been advanced in the Ptolemaic system, Venus would never show 952 RESONANCE October 2010

6 Figure 3. A comparison of the phases of Venus in the Ptolemaic (top) and the Copernican (bottom) systems. gibbous or full phase. It is not compatible with Aristotle s idea of Venus revolving in an orbit above the Sun. Venus would, then, never show the crescent phase. But, Galileo had observed the progress of Venus from nearly full phase (later, in 1611, Galileo did observe the full phase) to crescent, clearly indicating that Venus went around the Sun. Galileo s observations had undeniably verified Copernicus s prediction. That is the hallmark of great theories. They do not just describe or explain. They predict phenomena that have never been observed! Venus, the Roman Goddess of Love had yielded the supreme secret about the heavens that, the Sun is the centre of revolution around which planets sail in space. Galileo s support for the Copernican Theory did not stem from the simplicity of the theory or the geometrical beauty it offered. His belief was strongly based on what his eyes saw through his telescope a modest and a meek instrument by today s standards. It enabled him to see the invisible and through that, arrive at the not-so-obvious deductions. Truly, the mind sees beyond the limits of the eyes. It is now exactly 400 years since observational evidence of a heliocentric system was first provided. Let us rejoice this momentous occasion. Suggested Reading [1] Galileo Galilei, Sidereus Nuncius or The Sidereal Messenger, (Translated by Albert Van Helden), The University of Chicago Press, [2] Gingerich Owen, Phases of Venus in 1610, Journal for the History of Astronomy, xv pp , [3] G Galileo, Resonance, Vol.6, No.8, [4] J Kepler, Vol.14, No.12, Address for Correspondence H R Madhusudan J N Planetarium High Grounds Bangalore , India. RESONANCE October