A BRIEF OUTLINE OF GALILEO'S ASTRONOMY
Modern observational astronomy surfaced the very moment Galileo Galilei pointed his self-made telescope towards the starry Venetian night sky. Much to his dismay, Galileo started to discover inconceivable things such as the rugged and jagged surface characteristics of the Moon, mountains, valleys and plain, the phases of Venus and Mercury, Mars, the planet Jupiter and its four moons, Saturn, and not to mention the multitude of stars within the known constellations that remain otherwise invisible to the human eye.
Almost all of us are quite familiar with Galileo's life, his disputes with the Roman Inquisition for supporting the Copernican, i.e., the Heliocentric Model of the universe and challenging the Aristotelian view of the world and how he stood for the freedom of science and independent rational thinking. His greatest contribution is the development of the scientific method, wherein he isolated physics from theology and metaphysics and reasoned that the Book Of Nature is a book written in the language of mathematics. He argued that Nature is not whimsical but follows strict mathematical laws and is not under the obligation to suit our liking. Galileo's entire life has been a struggle to refute the Aristotelian doctrine of Natural Philosophy and establish his own ideas, which would almost 350 years later earn him the title the father of modern science bestowed none other upon by Albert Einstein.
Galileo was never the one to invent the telescope, which itself has a very long and controversial origin. From his famous astronomical treatise, the Sidereus Nuncius, published in 1610, we learn that almost nine months ago, he heard the news that a certain Fleming had constructed a spyglass using a combination of lenses through which distant objects appeared to come closer. Galileo's genius lies in the fact that without even taking a single look at the contraption and relying on his own understanding of the principle of refraction optics, he designed a better instrument using a long lead tube fitted with a plano-convex objective lens at one end and a plano-concave eyepiece lens at the other end. This simple arrangement could magnify objects up to three times. Hoping to achieve higher magnifications, Galileo kept on improving his designs and in his own words, "I succeeded in constructing for myself so excellent an instrument that objects seen by means of it appeared nearly one thousand times larger and over thirty times closer than when regarded with our natural vision''¹.
During Galileo's time, much of European thought was under the strong influence of the 1500-year-old Greek doctrines left-behind by philosophers like Aristotle and Ptolemy. Aristotle regarded the Earth as the motionless centre of the whole universe, surrounded by the concentric spherical orbs containing the Moon, the planets Mercury, Venus, the Sun and once again the planets Mars, Jupiter, Saturn, the background stars and the known constellations. The ninth sphere contained an immutable version of the zodiacal constellations. The tenth sphere, the last and final instalment of the Aristotelian cosmos, was imagined to be the divine prime mover or the unmoved mover, i.e., the celestial orb, which itself remained motionless but moved the lesser spheres. And beyond the tenth sphere was the so-called kingdom of God. The motion of the celestial objects in this geocentric universe was explained by Ptolemy using a complicated set of geometrical constructions such as cycles, epicycles, equants and deferents.
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| The Aristotelian Universe with the Earth as the centre, surrounded by the celestial orbs containing the respective planets Image Credits: Public Domain, via Wikimedia Commons |
When Galileo looked through his telescope, he realised that the Aristotelian picture did not quite conform to reality. When Galileo pointed his telescope towards the Moon, he observed a rough, undulating, rugged and jagged surface in contradistinction to the smooth and perfect surface as assumed by Aristotle. Aristotle was further of the opinion that the dark patches on the Moon were due to Her proximity to the Earth, whose corruptions and imperfections polluted Her divine grace. Upon close observation of the illuminated hemisphere of the Moon, while it faces the Earth, Galileo noted that the lunar surface is marked by two contrasting regions - the illuminated and the lighter portion that encompasses the totality of the lunar disc and intermittent dark patches. He named these patches the ancient spots to distinguish them from the smaller patches scattered in profusion throughout the lunar surface. Next, Galileo focused his telescope on the terminator line, i.e., the boundary between the lit and the unlit hemispheres. There he discovered that the terminator does not correspond to a solid oval line as it was supposed to be in the case of a perfectly spherical moon. Instead it has a rough, wavy and uneven characteristic. He also observed that the unlit side had quite a number of luminous protuberances while the illuminated side had a similar number of dark patches much farther from the terminator line. Galileo came to the conclusion that the lunar surface must be very similar to the Earth. What happens on Earth is that as the Sun rises, only the high mountain tops are at first illuminated, while the valleys remain dark. Now, as the Sun reaches higher elevations in the sky, all places, including the valleys, are uniformly illuminated. Galileo noticed the same thing happening on the Moon.
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| Image Credits: Public domain, via Wikimedia Commons |
Galileo made extensive observations of the Moon, drew sketches of the lunar surface, attempted to give rough estimates of the heights of the lunar mountains, the expanse of the dark lunar plains, their origin and composition, and tried explaining why even in the presence of all these prominences the periphery of the lunar disc appears to be a perfect circle. He is also reported to have noticed lunar libration, i.e., a dynamical phenomenon where the Moon does not always point exactly the same face towards the Earth but permits the observer to see a little more of the hemisphere at different times of the year.
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| It is a very beautiful thing and most gratifying to the sight, to behold the body of the Moon - Galileo Galilei Image Credits: Image from pixabay |
Being done with the Moon, Galileo turned his telescope towards the known constellations and was amazed to discover an unimaginable number of stars that lay hidden otherwise. To the best of his ability, he published star charts of the constellations of Orion and Taurus, Pleiades and Praesepe (now known as the Beehive) star clusters. He writes, ''In the first, I had intended to depict the entire constellation of Orion, but I was overwhelmed by the vast quantity of stars and by limitations of time...There are more than five hundred new stars distributed among the old ones...Hence to the three stars in the Belt of Orion and the six in the Sword which were previously known, I have added eighty adjacent stars....''³. Galileo notes that unlike the planets, which resolves themselves as a brilliantly lit circular disc in the telescope, the distant stars however, do not appear as spherical globes but continually flicker and scintillate, ''shooting out beams on all sides and very sparkling''⁴. The Milky Way galaxy, as a whole, was found to be full of densely packed star clusters and bright and faint nebulous regions.
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| The Pleiades Cluster as seen by Galileo Image Credits: Public Domain, via Wikimedia Commons |
While performing general observations of the Milky Way, Galileo notes that on the night of 7th January, 1610, he observed the planet Jupiter and three more stars in its immediate neighbourhood. Even though he first considered the neighbouring stars to belong to the host of the background stars, Galileo still had a hunch for the stars as they were positioned in almost a straight line with Jupiter. Galileo called them starlets due to their small size and greater brightness compared to the background stars. In the following night, he observed all the three starlets to be sitatedon the west, while on the previous night, two were to the East and one to the west of the said planet.This anomaly took his curiosity even further. In the following nights he observed that only two of the starlets would appear to the East while the third had disappeared. By the 13th night of the same month, Galileo found that there were actually four starlets that surrounded Jupiter. From intensive observations of the Jovian system, Galileo finally came to the conclusion that the starlets must be the moons orbiting Jupiter. These four moons were named, as The Medicean Stars, in honour of the Grand Duke of Tuscany, Cosimo II de' Medici and his three brothers.
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| Jupiter and its four moons as seen through an amateur telescope. Galileo must have seen something like this Image Credits: Public Domain, via Wikimedia Commons |
With the discovery of the Jovian system, Galileo became a stern supporter of the Copernican Heliocentric Model of the Universe. It made him realise that if four celestial objects could orbit Jupiter, then there is whatsoever no reason as to why Earth should not orbit the Sun. This line of thought gained further ground with the observation of the phases of Venus and Mercury. According to the Ptolemaic Model, Venus could never have phases other than the crescent and the new phase since it was assumed to be situated before the sphere of the Sun. But Galileo observed that both Mercury and Venus went through all the phases, i.e., crescent, gibbous, new and full. This could only be possible if they revolved around the Sun, just like the Moon revolves around Earth. The Ptolemaic system proposed that Venus can never move behind the Sun. But based on observations, Galileo argued that not just Venus, all planets revolve around the Sun because there is no plausible explanation regarding how the former could go through all the phases.
From Galileo's astronomical notes and other works, we learn that somewhere in 1610, he observed the planet Mars, resolved as a red disc through the telescope. Unfortunately, Galileo failed in his attempt to discover the changing phases of Mars. From similar records, we also learn that Galileo was almost on the verge of discovering the planet Neptune when he is reported to have observed a fixed blue star on the night of 28th December and then again on 27th January 1612.
Next, he turned his telescope towards the Sun and knowing that direct observation might damage his eyes, he projected an image of the Sun on a piece of paper and noticed that the solar disc had some dark spots. Galileo wrote a series of correspondences to Mark Welser, who served as the mode of communication between Father Christopher Scheiner, a Jesuit priest who made extensive observations of sunspots and asked for Galileo's opinion. Even though, at that time, no one had a dint of clue about the real nature of the sunspots, Galileo, to the best of his reasoning, argued that they must be features present on the Sun's surface or be simialr to the clouds on Earth. Galileo was not quite sure about the substance of the spots, but he knew this much that the spots were created on the surface and moved from west to east and slanting from south to north. Using simple arguments and analogies with the Earth, Galileo affirmed that the daily movement of the sunspots means the Sun itself moves about an axis. Galileo saw that the sunspots appear and disappear in definite periods; some condense, others expand, few are irregular, while at times, the number of sunspots diminished to almost none. The discovery of the sunspots served a severe blow to the Aristotelian doctrines, which assumed the Sun to be a pure and luminous body with no impurities or spots.
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| Sunspots as observed on October 18, 2014 Image Credits: NASA/SDO, via Wikimedia Commons |
From the letters on sunspots, we learn that in 1609 when Galileo turned his telescope to observe Saturn, he mistook it for a three-bodied planet. This happened because his instrument was not powerful enough to resolve the full planetary disc and the rings of Saturn. From what he could see, Galileo assumed the rings to be a pair of giant moons so close to the central star that they almost touched. Three years later, i.e., in 1612, Galileo once again looked at Saturn, and this time he did not see any rings. At that time, Galileo was perplexed by his discovery and did not want to make any definite comments. However, he proposed that the moons might appear after 1614. In 1616, when he observed Saturn for the third time, he saw the rings again and commented that they must be some kind of arms or handles or maybe ears.
At this juncture, Galileo's astronomy came to an end. Because of his illness, he was unable to make telescopic observations of the three comets that apeared over Europe, one in October and two in November. Galileo got into a violent dispute with Father Orazio Grassi, a Jesuit mathematician of the Collegio Romano, regarding the nature and the origin of comets, where the later argued that an absence of any detectable parallax meant that the comets must be situated far beyond the Moon. Galileo criticised Grassi's conclusions in a pamphlet published in 1619 with Mario Guiducci as the main author and colleague of Galileo. Galileo's dispute with Grassi was not on the matter of comets but was on the proper method of doing science. In fact Galileo had serious misconceptions regarding comets. In 1623, Galileo published The Assayer, where he severely attacked the Aristotelian doctrines. In 1632, Galileo published the Dialogues Concerning The Two Chief World Systems, through which he openly argued in favour of the Copernican Heliocentric Model and reasoned how the said model could explain the existence of tides, the retrograde motion of the planets, the motion of Earth and why only the Sun should be the centre of the universe.
References:
- Drake 1957. pp. 29
- Carlos 1880. pp. 15
- Drake 1957. pp. 47
- Carlos 1880. pp. 40
- https://en.wikipedia.org/wiki/Galileo_Galilei
- https://en.wikipedia.org/wiki/Sidereus_Nuncius
- https://plato.stanford.edu/entries/galileo/
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