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Episode 15
The Starry Messenger
Dr T V Venkateswaran
Main points
-Use of telescope by Galileo
- Discovery of Moon’s craters, Jupiter’s Moons, sunspots, phases of Venus, starry nature
of Milky Way
- Biography of Galileo and his contributions to mechanics, engineering, art
Emphasis of the episode
- First person to use telescope
- 2009 is the 400th anniversary of the first ever use of telescope by Galileo
- His famous experiment at Pisa
- His publication of Starry Messenger in 1610
- He died the same year Newton was born
- Galileo’s inquisition
Detailed content brief
Introduction
One small book not even 100 pages in length turned the astronomy during middle ages
upside down. It caused such a stir during its time and resulted in complete refashioning of
the way astronomy was done since then. The book is Galileo Galilee’s Sidereus nuncius
or The Starry Messenger was first published in 1610. This small but very important book
made him famous over night so to say.
The science of astronomy took a huge leap forward in the first decade of the 1600s with
the invention of the optical telescope and its use to study the night sky. Galileo Galilee
did not invent the telescope but was the first to use it systematically to observe celestial
objects and record his discoveries. In this booklet he reported on his observations of the
Moon, Jupiter, Venus and the Milky Way. These and subsequent observations and his
interpretations of them eventually led to the demise of the geocentric Ptolemaic model of
the universe and the adoption of a heliocentric model as proposed in 1543 by Copernicus.
Also it opened a new way for doing astronomy- by studying the night sky with telescope,
to discover many new celestial objects unimagined so far.
Galileo’s Telescope
It was in the late 13th century that the first eyeglasses with convex lenses to correct
presbyopia (eye disorder of the old, wherein one is not able to focus the eye easily) came
into use. The first known use of a magnifying glass to aid in reading was in the 1200s, by
Roger Bacon at Oxford. It proved a boon to aging scholars, many of whom had been
forced to retire while still relatively young. And toward the middle of the 15th century
the first eyeglasses with concave lenses to correct myopia made their appearance. During
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this century, Florence became one of the major centres for the production of eyeglasses,
rivalling the German cities of Nuremberg and Regensburg. From the end of the 13th
century, parallel to the diffusion of eyeglasses, an exceptionally transparent type of
Venetian glass, with few flaws and almost colourless, the so-called "crystal", came into
widespread use.
At first the lenses (lens means lentil in Latin, so called because of the shape of the pieces
of glass used) for the eyeglasses were made by cutting discs out of balls of blown glass.
The concave surface was ground to make lenses for the farsighted and the convex surface
to make lenses for the nearsighted. In the late 15th century the spectacle-makers of
Nuremberg began however to cut the discs out of plates of flat glass – crystals- and then
grind one of the surfaces to either a concave or a convex shape. Through this procedure,
lenses with more accurate spherical curvature could be produced. It was the spread of this
perfecting technique that made the invention of the telescope possible to those spectaclesmakers who thought of combining concave and convex lenses of high quality.
The first time, as far as we know, 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 called Lipperhey. He was unable to get a patent, however, because his
invention was deemed too easy to reproduce. Perhaps the reason it had not been done
before was that to get magnification, one needs a concave lens stronger than the convex
lens being used with it, and commonly the lenses in wide use were the other way around.
Thomas Harriott, tutor of Lord Raleigh is known to be the first to observe the moon
through telescope. He also produced a crude map of moon as it was seen through the
telescope in July 1609, well before Galileo. However he seems to have not followed up
his efforts and not to have understood the import of the telescope for astronomy.
Galileo found out about this invention in the spring of 1609, and immediately set about
improving it. Galileo was an excellent experimentalist, and working with different lenses,
he realized that the magnification was proportional to the ratio of the power of the
concave (eyepiece) lens to the convex (more distant) lens. In other words, to get high
magnification he needed a weak convex lens and a strong concave lens. The problem
was that the opticians only made glasses in a narrow range of strengths, and three or so
was the best magnification available with off the shelf lenses. Galileo therefore learned
to grind his own lenses, and by August, he had achieved about ninefold linear
magnification. This was an enormous improvement over everything else available at that
time. Galileo therefore approached the Senate of Venice to demonstrate his instrument.
Many senators climbed the highest bell towers in Venice to look through the telescopes at
ships far out at sea. All were impressed by power of the improved telescope of Galileo
and understood it’s obvious military potential. Galileo stated that “This (telescope) is of
inestimable benefit for all transactions and undertakings, maritime or terrestrial, allowing
us at sea to discover at a much greater distance than usual the hulls and sails of the
enemy, so that for two hours or more we can detect him before he detects us...”
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The basic tool that Galileo used was a crude refracting telescope. His initial version only
magnified 8x but was soon refined to the 20x magnification he used for his observations
for Sidereus nuncius. It had a convex objective lens and a concave eyepiece in a long
tube. The main problem with his telescopes was their very narrow field of view, typically
about half the width of the Moon.
We do not know what all Galileo explored in the night sky with his new found tool –
telescope. However we are sure of his observations of moon, Jupiter, Venus and Milky
way. All these observations and the results that he obtained by applying scientific method
to the observations resulted in a revolution of astronomy.
Moon:
According to Aristotelian principles the Moon was above the sub-lunar sphere and in the
heavens, 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."
Any crude telescope will show us the craters on the face of moon. However how did
Galileo come to the conclusion about the mountains and how did he calculate its height?
That is where his ingenuity and application of scientific method lies. Galileo’s first major
astronomical discovery with the telescope was that the Moon’s surface is mountainous,
and not a perfect sphere as had always been assumed. He could observe that some parts
of the moon were illuminated well before its surroundings were illuminated by the
sunlight. We know that mountain tops are illuminated at the dawn well before the valleys.
He built a convincing case for the reality of the mountains by sketching the appearance of
parts of the Moon’s surface at different times of the month, that is, under different angles
of lighting, and showing how the light and shadow seen could be simply and naturally
accounted by hypothesising mountains. Obviously this caused uproar. He was able to
estimate the height of the mountains on the moon by seeing how far into the dark part
bright spots could be discerned. At half moon, a little geometry is enough to calculate the
heights!
Not all were impressed. The orthodox were enraged, how can Galileo claim that moon,
an celestial body to be ‘imperfect’ they contented. Some with scientific bend of mind
pointed out that the edges of the moon were smooth and if there were indeed mountains it
should not be uniformly smooth. Galileo had two arguments to meet this challenge. First,
ranges behind those on the edge would tend to fill in the gaps. This is indeed correct.
Second, maybe things were fuzzed out by the Moon’s atmosphere said Galileo. However
we know that Moon does not have any appreciable atmosphere.
Jupiter
Galileo’s next major discovery began with his observation on January 7, 1610, of what he
took to be a rather odd set of three small fixed stars near Jupiter, and, in fact, collinear
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with the planet. These ‘stars’ were invisible to the naked eye. Observations of the planet
Jupiter over successive night revealed that actually four star-like objects in a line with it.
The objects moved from night to night, sometimes disappearing behind or in front of the
planet. Galileo correctly inferred that these objects were moons of Jupiter and orbited it
just as our Moon orbits Earth. By the 15th January 1610 had realized that he was looking
at moons of Jupiter, which were going around the planet with periods of the order of
days.
Significance of this observation is not just that Galileo found new moons around Jupiter.
For the first time ever a celestial object not stated in any religious canon was found. This
discovery caused even more consternation than the demystification of the Moon. Seven
was a sacred number, and there were seven planets, wanderers, or moving stars. Jupiter’s
moons spoiled this.
Moreover the Aristotelian cosmos permitted object to go around the center of the
Universe. But here were three ‘stars’- moons of Jupiter that were going around another
planet other than the center of the universe – Earth (as belived then). For the first time,
objects had been observed orbiting another planet, thus weakening the hold of the
Ptolemaic model. This made Copernicus’ argument, that the Moon goes around the Earth
and the Earth around the Sun, more plausible. Today these four moons are known as the
Galilean satellites; Io, Europa, Ganymede and Callisto.
Stars in the Milky Way
Galileo then turned his attention to most numerous objects in the night sky- stars. Rather
to his disappointment the stars showed no features- they were still point source, even
through a telescope the stars still appeared as points of light. Galileo suggested that this
was due to their immense distance from Earth. On turning his telescope to the band of the
Milky Way Galileo saw it resolved into thousands of hitherto unseen stars. In like manner
when ex explored the region of Pleiades, he found stars that were unseen to naked eye.
His exploration of the Orion nebula also revealed unseen stars.
Starry messenger
Thus in one single stroke Galileo made report of his observations of Moon, Jupiter, fixed
stars and milky way in his seminal work – Starry messenger. The publication caused
great upheaval in Europe. The observations appeared to attest the heliocentric model and
upset many of the notions held so far- like Earth is at the center, celestial objects are
perfect and smooth and so on. It was a epistemic break and once for all dissolved the
dichotomies – celestial and territorial
After Starry messenger
Subsequent to his publication, for many more years Galileo continued with his
observations. His observation of Venus, Sun spots and Saturn also resulted in many
startling revelations about the nature of the cosmos. Venus was observed to go through a
sequence of phases similar to the Moon. This could not be explained in the Ptolemaic
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model but could be accounted for by either the Sun-centered Copernican model or the
Earth-centered Tychonic model that had the other planets orbiting the Sun as it orbited
the Earth. Galileo rejected Tycho's model as an unnecessary hybrid and used the
discovery to consolidate his support of the Copernican model.
Along with contemporaries such as Thomas Harriot, David Frabicius and Christoph
Scheiner, Galileo observed dark regions that appeared to move across the surface of the
Sun. Debate centered on whether these were satellites of the Sun or actual spots on its
surface. Galileo, in his Letters on Sunspots supported the sunspot interpretation and used
it to show that the Sun was rotating. Its blemishes and imperfections again undermined
the Aristotelian ideal of a perfect cosmos.
In the crude telescope of Galileo Saturn appeared with two appendages on its sides . To
the utter confusion of Galileo these appendages disappeared then later reappeared. It was
not until 1656 that the Dutch scientist, Christiaan Huygens correctly described them as
rings around Starun.
Opposition to Galileo
In 1611, Galileo packed his brass telescope in his bag and decided to go to Rome. The
publication of Starry messenger had raised stir amongst the Catholic orthodoxy. Criticism
of Galileo's observations began immediately. Even when Galileo urged, the authorities at
Rome would not even look through his telescope. Why not? They had absolute faith in
Aristotle and their interpretation of the holy book. Not only that, if you think about it, the
telescope reveals the existence of things which are not ‘really there’. Look at the Saturn
with the naked eye. Do you see its rings? Of course not. In Galileo's day, seeing
something that could not be seen with the naked eye was the same thing seeing
apparitions or hearing voices - it was the work of the Devil!
The religious authorities at Rome were uneasy with the New Science. Copernicus, Kepler
and Galileo seemed to be turning the world upside down. The sun was the center of the
cosmos, the earth moved and the sky seemed to hold hidden visions. In effect, the
Scientific Revolution had created an invisible world behind the visible world and those
men of an older generation, weaned on Aristotle and Aquinas were fearful of it.
Astronomy and Cosmology
Until Galileo cosmology and astronomy were for all practical purposes two distinct
disciplines. The former concerned the definition of the physical structure of the Universe,
and was the province of philosophers. The latter aimed instead to record the positions of
the heavenly bodies, and was the realm of mathematicians and observational
astronomers. To maintain the dual social function of astronomy - determining the
calendar accurately and predicting the positions of the planets for drawing up horoscopes
- the mathematical astronomers had developed over the centuries a set of graduated
measurement instruments to be used by the naked eye. Such outstanding figures as
Nicolaus Copernicus (1473-1543) and Tycho Brahe (1546-1601) had already tried to
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resolve the dichotomy between cosmology and astronomy, and to define the structure of
the Universe through accurate observations. Tycho Brahe that still had Earth at the
center. In this model planets were going around the sun and all these together were going
around Earth. Tycho Brahe was at least tolerated. However Copernicus wanted to place
Sun at the center and built a model wherein all the planets including Earth will go around
it in Elliptical orbit. Copernicus was decried by the orthodoxy.
The invention of the telescope merged the two fields of knowledge materially. The
telescope introduced an entirely new mode of observing the sky. The telescopic
observations provided interesting insights and revealing deeper elements. It was no
longer tenable to hold a view of cosmos that could not accommodate these elements. The
old gave way to new eventually.
Biographical note on Galileo
Galileo was born in Pisa, Tuscany in 1564, the son of Florentine musician Vincenzo
Galilei. At age 17, Galileo went to the University of Pisa. He enrolled as a medical
student, following his father’s advice, but turned to math, after persuading his father that
he didn’t want to be a doctor. His father allowed him to be tutored by the Tuscan court
mathematician, Ricci, who designed fortifications, which no doubt impressed Galileo.
Galileo proved to be an extremely talented mathematician, and in his early twenties he
wrote some tracts extending results of Archimedes on centers of gravity of shapes. At
age 25, he was appointed to the Chair of Mathematics at Pisa. At age 28, in 1592, Galileo
moved to a better position at Padua, in the Venetian Republic, where he stayed until the
age of 46. Around 1610, when he was about 46 years old he developed the telescope,
secured tenure and a big raise at Padua, then went on to make all the discoveries
announced in Sidereus Nuncius: mountains on the moon, the moons of Jupiter, phases of
Venus, etc. Of course, Galileo’s belief that his discoveries with the telescope strongly
favored the Copernican world view meant he was headed for trouble with the Church. In
1611, Galileo went to Rome and met with the Jesuit astronomers. Probably he felt that if
he could win them over, he would smooth his path in any future problems with the
Church. Father Clavius, author of Gregorian Calendar and undisputed leader of Jesuit
astronomy had a hard time believing there were mountains on the moon, but he
surrendered with good grace on looking through the telescope . However he could hardly
convince him. When the clergy saw for themselves the mountains on the moon and other
new facts of the starry world unseen till now, they held it to be the work of devil, to
mislead them away from the path of the Lord.
Incidentally, Galileo was thinking about quite a different series of physics problems at
this same time-trying to understand when things will float and when they sink. He
believed Archimedes’ Principle, that denser objects than water sink in water. It was
pointed out to him that a ball made of ebony sinks in water, but a flat chip of ebony
floats. We now understand this in terms of surface tension, but that had not been
understood in Galileo’s time. Nevertheless, Galileo gave an essentially correct answer:
he observed that the chip floated somewhat below the previous level of the surface,
dragging the water down slightly around its edges, so one should consider the floating
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body to be the chip plus the thin sheet of air over it, and putting these together gives an
average density equal to that of water.
One more source of tension between Galileo and the Jesuits arose at this point. Since
1611, Galileo had been observing the motion of sunspots: small dark spots on the surface
of the sun, easily visible through a telescope at sunset. They were observed
independently at about the same time by Christopher Scheiner, a German Jesuit from
Ingoldstadt. While Scheiner thought they were small dark objects circling the sun at
some distance, Galileo correctly surmised they were actually on the sun’s surface,
another blow to the perfect incorruptibility of a heavenly body. Galileo published his
findings in 1613, with a preface asserting his priority of discovery. This greatly upset
Scheiner.
About this time, some members of another order of the Church, the Dominicans, were
becoming aware of the Copernican world view, and began to preach against it. In
February 1616, the Copernican System was condemned. The pope asked Bellarmine to
convey the ruling against the Copernican system to Galileo. Bellarmine had a meeting
with Galileo and subsequently Galileo was given an affidavit by Bellarmine stating that
he must no longer hold or defend the propositions that the earth moves and the sun
doesn’t. A week later (early March) books describing a moving earth were placed on the
Index of Prohibited Books, some pending correction.
In 1623, Galileo’s admirer the Florentine Maffeo Barberini becomes Pope Urban VIII.
The new pope saw himself as a widely educated man, who appreciated even Galileo’s
current theories. He had written a poem “In Dangerous Adulation” about Galileo’s ideas.
He also suggests his own pet theory to Galileo: even though the universe may be most
simply understood by thinking of the sun at rest, God could have arranged it that way, but
really with the earth at rest. Galileo felt that with his friend and admirer as pope, and his
affidavit from Bellarmine that didn’t actually forbid him from describing the Copernican
system, it was safe to write further about his world view. His ambition was to prove that
the Copernican system must be correct, even though the more cumbersome Ptolemaic
system might be fixed up to describe observations. (For example, the Danish Astronomer
Tycho Brahe suggested that the sun went around the earth, but all the other planets went
around the sun. That would account correctly for the phases of Venus.)
Having felt that Pope would not take action against him Galileo worked on his new book,
“Dialogue Concerning the Two Chief Systems of the World-Ptolemaic and Copernican”.
As usual, Galileo spared no-one in the book. He mocked the pope himself, by putting
Pope Urban’s suggestions in the mouth of Simplicio, then dismissing it contemptuously .
The book was published in March 1632 in Florence. In August, an order came from the
Inquisition in Rome to stop publication, and Galileo was ordered to stand trial. The trial
did not address the scientific merits of the case, it was about whether or not Galileo had
disobeyed an official order. It was suggested that he admit to some wrongdoing, and he
would get off lightly. He agreed to tone down the Dialogue, pleading that he had been
carried away by his own arguments. He was condemned to indefinite imprisonment, and,
after some negotiation, was confined to his villa until his death in 1642. The very same
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year Newton was born! During this period, he wrote Two New Sciences, a book on the
strength of materials and on the science of motion.
Galileo wrote in his old age, in his own copy of the Dialogue: “Take note, theologians,
that in your desire to make matters of faith out of propositions relating to the fixity of sun
and earth, you run the risk of eventually having to condemn as heretics those who would
declare the earth to stand still and the sun to change position-eventually, I say, at such a
time as it might be physically or logically proved that the earth moves and the sun stands
still”