Galileo’s Telescope – The What, When and How

Galileo Galilei is often hailed as the father of modern astronomy. From humble beginnings to his ultimate fame, remembered in rock anthems and celebrated by space probes dispatched to the vast reaches of Jupiter, Galileo’s contributions to science have solidified his place in history. Delving into his life reveals the makings of an extraordinary man who managed to transform the way we perceive the universe. Who was Galileo Galilei? What’s a Galileo telescope?

Painting of Galileo Galilei
This painting of Galileo dates back to about 1640.
Image Credit: National Maritime Museum, Greenwich

Born in Pisa, Italy, on February 15th, 1564, Galileo’s early life was shaped by his family and societal expectations. He was the son of a musician, and initially, the pull of spiritual fulfillment beckoned him. Raised in a devout Roman Catholic household, the young Galileo had aspirations of joining the priesthood. Yet destiny had different plans for him. At the behest of his father, at the age of 16, he enrolled in studies geared towards obtaining a medical degree.

However, a pivotal moment at the University of Pisa changed the trajectory of Galileo’s academic pursuits. An ordinary observation of a swaying chandelier sparked his burgeoning curiosity in physics. This newfound interest was further fueled after attending a lecture on geometry. Recognizing his true passion, Galileo pivoted his academic focus, immersing himself in the studies of mathematics, natural philosophy, and the other fine arts and humanities popular in Renaissance Italy.

By the age of 24, Galileo’s expertise was already in demand. He briefly took on a role as an instructor at the esteemed Academy of Arts in Drawing in Florence. But Pisa beckoned once more, and he soon found himself occupying the prestigious chair of mathematics. This was a stepping stone to greater heights, and in 1592, Galileo relocated to Padua. Here, he held the esteemed title of Professor of Mathematics, imparting knowledge on subjects like geometry, mechanics, and, of course, astronomy.

The origins of Galileo’s intrigue with astronomy remain shrouded in mystery, but it’s likely that his detailed knowledge of mathematics and physics may have led him to the study of optics and, thus, to building his own telescope to gaze at the heavens.

Did Galileo Invent the Telescope?

Galileo’s legacy is a testament to the breadth of his genius. More than just an astronomer, he was a pioneering scientist whose methods and discoveries laid foundational stones across multiple disciplines, forever altering the trajectory of scientific inquiry.

No one seems to know what drew Galileo to astronomy in the first place, and while he made a number of inventions (including an early thermometer and a water pump), it’s not true to say he invented the telescope.

Again, no one quite knows for sure, but it’s thought the telescope may have been invented by a German-Dutch spectacle maker called Hans Lippershey. It was Lippershey who submitted the earliest known patent for a refracting telescope in October 1608.

Once Galileo heard about it, he soon built his first telescope, and throughout 1609, he worked to improve his creations. By August that year, Galileo had built an 8-power telescope, and just two or three months later, he had built another with a magnification two and a half times greater. Less than six months later, he had made discoveries that would alter our view of the universe forever.

What is a Galilean Telescope?

Galileo's telescope at Museo Galileo
Original Galileo’s telescope at Museo Galileo
A replica of one of Galileo's earliest telescopes
A replica of one of Galileo’s earliest telescopes. Credit: Jim and Rhoda Morris

The Galilean telescope, based on the designs of Lippershey and possibly others before him, marked the inception of telescopic observation of the cosmos. It operates using a convergent or convex objective lens, which gathers and focuses light, and a divergent or concave eyepiece. The simple concave lens of the eyepiece spreads out the light before it reaches the eye, unlike a modern telescope eyepiece. The resultant effect is an upright image, which is particularly useful for terrestrial viewing. However, Galileo’s telescope has a relatively narrow field of view.

Galileo Galilei’s telescopes, by today’s standards, would indeed be considered rudimentary. The first yielded only 3x, which seems to have disappointed Galileo. He did not use this one for astronomy. The second and most famous of Galileo’s telescopes was a 37mm lens stopped down to a mere 15mm of aperture (for comparison, your eyeball has an aperture of 7-9mm when your pupil dilates). Initially, it was equipped with an 8- or 9-power eyepiece yielding a field of about half a degree, or similar in angular diameter to the full moon or Sun in the sky. He eventually upgraded this instrument to yield 20x with a new eyepiece, with which its field of view spanned only about 15 minutes of arc, approximately half the width of the full Moon or Sun in the sky. This was the main telescope he used for most of his observations. Despite its limitations, it was with this very instrument that Galileo began to challenge centuries-old beliefs and unveil the universe’s hidden secrets.

Later telescopes built by Galileo featured some performance improvements. One unit had a 26mm aperture, yielding 14x. This one and his original 15mm telescope are exhibited at the Museo Galileo in Florence, Italy. Galileo almost certainly built more telescopes for clients such as his patrons as well as teaching students or other scientists how to make them.

What Did Galileo Observe First With His Telescope?

Before Galileo’s groundbreaking observations, the prevailing worldview placed Earth at the center of the universe. This geocentric model posited that celestial bodies, including the Sun, Moon, and planets, orbited the Earth. The stars were perceived as minute specks of “aether” embedded in crystalline spheres enveloping our planet.

Further, celestial objects like the Sun, Moon, and planets were regarded as divine and flawless creations. This belief was rooted in the idea that they were crafted by a divine being and, as such, had to be impeccable. So, when Galileo trained his telescope on the Moon in late November 1609, his observations were nothing short of revelatory.

Sketches of the Moon by Galileo, as it appeared in his book Sidereus. redit: Wellcome Trust

To most observers, the Moon’s surface appeared relatively smooth, with its darker “seas” offering some contrast. Yet, through Galileo Galilei’s telescope, a different picture emerged: craters, mountain ranges, and shadows cast by the rising Sun, debunking the notion of a pristine lunar surface.

Galileo’s curiosity then led him to the Milky Way, a luminous band stretching across the night sky whose nature was largely misunderstood at the time. Some theories suggested it marked the juncture between the northern and southern celestial hemispheres, while others proposed it as a belt of compressed fire. However, the ancient Greek philosopher Democritus had a different perspective. He postulated that the Milky Way comprised countless tiny stars, too diminutive to discern with the naked eye. After meticulously observing the Milky Way, Galileo concluded that Democritus’ hypothesis held water.

But Galileo’s astronomical journey was far from over. Eager to unlock more of the universe’s enigmas, he turned his attention to constellations such as Orion and the Pleiades. While only six or seven stars from the Pleiades cluster are visible to the unaided eye, Galileo’s telescope revealed around thirty-five. Additionally, he observed other “nebulous” celestial objects documented by the ancient astronomer Ptolemy, including the Praesepe, or Beehive Cluster, located in the Cancer constellation. Through these observations, Galileo discerned that these “nebulae” were, in reality, conglomerates of numerous stars, thereby shedding light on the true nature of star clusters.

Galileo’s Telescope Discoveries: What Did Galileo Discover With His Telescope?

When Did Galileo Discover the Moons of Jupiter?

A page from Sidereus Nuncius, showing Galileo’s observations of Jupiter and its moons. Credit: History of Science Collections, the University of Oklahoma Libraries

In early January 1610, the bright planet Jupiter was high in the sky in the late evening, making its celestial journey through the eastern stars of the constellation Taurus, the Bull. It was in the early days of the year that Galileo Galilei embarked on a series of observations that would revolutionize our understanding of the universe. On the evening of January 7th, Galileo, peering through his telescope, identified “three fixed stars, totally invisible by their smallness,” aligned near Jupiter — two flanking one side of the giant planet and the third on the opposite.

Intrigued, he continued observing over subsequent nights. By January 8th, he noted a shift: all three stars congregated on one side of Jupiter. On January 10th, only two stars were visible, leading Galileo to hypothesize that the third was obscured by Jupiter itself. This celestial dance culminated on the 13th, when a fourth star emerged. Witnessing these consistent shifts, Galileo deduced that these weren’t distant stars but celestial bodies in orbit around Jupiter. This was groundbreaking: it marked the first-ever observation of celestial entities revolving around something other than Earth, challenging centuries of astronomical orthodoxy.

The prevailing paradigm for nearly two millennia was the geocentric model, which posited Earth as the universe’s center. This model, championed by the ancient Greek philosopher Aristotle in his treatise “On the Heavens” circa 350 BC, had faced little serious contention.

While Nicolaus Copernicus, the renowned Polish astronomer, had introduced the heliocentric, or Sun-centered model of the Universe roughly seven decades prior to Galileo’s observations, this groundbreaking perspective had yet to gain universal acceptance. Galileo, an ardent heliocentrist, recognized that the behavior of Jupiter’s moons served as empirical evidence countering the geocentric view. It offered a clear demonstration: not everything orbits Earth. He swiftly disseminated his observations and conclusions in his seminal work, “Sidereus Nuncius” or “Starry Messenger,” published in March 1610. While many in the scientific community hailed his findings, skeptics dismissed the moons’ existence, attributing the observations to flaws in Galileo’s telescope.

Yet, as the truth often does, Galileo’s findings gained traction. Notably, Johannes Kepler, a towering figure in astronomy, penned a letter endorsing Galileo’s observations in the subsequent month. Independent verifications by other astronomers solidified the legitimacy of his claims. It’s worth noting that the German astronomer Simon Marius also professed to have identified these moons prior to Galileo. And while Marius would later bestow names on them, the annals of history credit Galileo with their discovery. Today, these four moons — Io, Europa, Ganymede, and Callisto — stand collectively celebrated as the Galilean Satellites.

Even amid the grandeur of his celestial observations, Galileo Galilei’s mind was attuned to the practical applications of his discoveries. One such realization was that the moons of Jupiter, with their predictable orbits, could function as a universal clock. This concept was not merely abstract but held real-world implications, particularly for the challenge of determining longitude at sea—a problem that had vexed sailors and navigators for generations.

Galileo theorized that if one could accurately note the times of the Jovian moons’ eclipses, it could serve as a universal reference point to determine the local time wherever they were. Comparing this local time with the standard time of a known location (based on the predicted times of the moon’s eclipses) would, in turn, provide the longitude of the observer.

Though profoundly innovative, using such a method presented challenges. The main hindrance was the difficulty of observing the moons from the deck of a moving ship through a telescope, followed by, of course, the need for a clear sky and Jupiter to be above the horizon. Despite this, Galileo’s vision of the Jovian moons as a celestial clock illustrated his ability to bridge pure observation with tangible applications, and in the years to come, his idea was actually adopted as a practical method until ocean-going mechanical clocks could be made sufficiently reliable.

Did Galileo Discover the Rings of Saturn?

Two sketches of Saturn by Galileo, showing the differing views from 1610 to 1616
Two sketches of Saturn by Galileo, showing the differing views from 1610 to 1616

By the time July 1610 rolled around, Galileo’s insatiable curiosity had led him to explore planets farther out in the solar system. At this juncture, astronomers were only aware of five planets, with Saturn, the furthest and most elusive, meandering through the eastern regions of the constellation Capricornus and moving towards Aquarius.

Considering Saturn’s location in the celestial sphere, and the fact that the planet was months away from reaching opposition, Galileo would have likely been observing deep into the night, possibly close to midnight. His observations of Saturn left him both puzzled and fascinated. Instead of seeing a singular planet, his telescope revealed what appeared to be a planetary trio!

Eager to share this astounding observation, Galileo communicated his discovery in a letter dated July 30th, 1610. He penned, “It appears that the planet Saturn is not a solitary entity but is composed of three close-knit bodies, which remain steadfast in relation to one another.”

Fresh from his groundbreaking observation of Jupiter’s four largest moons earlier that year, Galileo hypothesized that these celestial bodies around Saturn were akin to large moons orbiting swiftly around the planet, just like those he had discovered around Jupiter.

However, Galileo’s deduction, though innovative, was off the mark. He was, in actuality, glimpsing Saturn’s magnificent rings. Due to the limitations of his telescope’s optics, the rings appeared as indistinct, blurry circles flanking the planet rather than the smooth, continuous structures we understand them to be today. Keep in mind that Galileo’s best telescope could only magnify by 20x – similar to high-power astronomical binoculars – which is barely sufficient to resolve the planet’s rings with perfect optics, let alone the primitive instruments he was confined to using.

Intriguingly, when Galileo returned to his observations of Saturn nearly two and a half years later, the mysterious “moons” seemed to have disappeared. Documenting this perplexing phenomenon in a letter dated December 4th, 1612, he expressed, “How can we explain such an odd transformation?” He pondered whimsically if the planet, echoing the tales of its mythological counterpart, had consumed its own offspring.

On a more pragmatic note, Galileo considered that perhaps the imperfections in his telescope’s optics were to blame. Steadfast in his belief that the “moons” would re-emerge, he diligently continued his surveillance. As anticipated, over the following years, he noticed the planet undergoing changes, with the “moons” gradually becoming more prominent and luminous. By 1616, although Galileo recognized the rings for their distinct structure, their true nature eluded him. He depicted and described them as “ears”, “handles”, or appendages of the planet, and the enigma persisted.

Galileo’s telescope observations, spanning almost seven years, captured the varying appearances of Saturn as its rings shifted from being edge-on (as in 1612) to gradually widening due to their tilt as viewed from Earth, as they orbited the Sun.

The puzzle of Saturn’s “ears” remained unsolved during Galileo’s lifetime. It was only in 1655, more than a decade after Galileo’s demise, that Christiaan Huygens finally resolved the planet’s rings using a more powerful telescope equipped with a more complex eyepiece design than Galileo’s or Kepler’s telescopes that now bear his name.

Galileo’s Discovery of the Phases of Venus

Fresh on the heels of his observations of Saturn, by late 1610, the planet Venus was making its luminous ascent back into the evening horizon, serving as a beckoning canvas for Galileo’s next great discovery.

To Galileo’s astonishment, Venus through a telescope presented characteristics reminiscent of Earth’s own Moon. Just as the Moon cycles through phases – from new to crescent, to half, to full, and back again – Venus, too, showcased a dynamic interplay of light and shadow. When Galileo initially trained his gaze upon the planet, Venus manifested as a diminutive, nearl-fully illuminated disc like a gibbous Moon. However, as the weeks progressed and the nights turned colder, Venus underwent a captivating transformation right before Galileo’s eyes.

Over the subsequent three-month period, Venus’s appearance evolved in a manner that was both enthralling and enigmatic. The planet’s disc, initially full and compact, began to swell in apparent size. Yet, intriguingly, as it expanded, it concurrently grew more svelte, transitioning from a nearly round orb to a semi-illuminated half and eventually to a slender, delicate crescent nearly an arc minute tall.

This revelation was not merely an astronomical curiosity; it bore profound implications. The shifting phases of Venus could only be logically explained if the planet was orbiting the Sun, moving both between the Earth and the Sun and then to the other side of the Sun relative to Earth. Combined with the recent discovery of Jupiter’s moons, Galileo and many others at the time saw insurmountable evidence that the Earth was just like the other planets and orbited around the Sun.

Did Galileo Discover Neptune?

While Galileo is renowned for many of his astronomical observations, one that is often overlooked is his incidental and unrecognized recording of the planet Neptune, over 230 years before its official discovery and just a few years after he first pointed his telescope to the heavens.

In the winter of 1612-1613, while Galileo was keenly observing Jupiter and its moons with his telescope, he noted a faint star near Jupiter on his observational charts. This “star” was, in fact, the planet Neptune. On the night of January 28, 1613, Galileo drew a static chart showing Jupiter and a few of its moons, and on this chart, he marked the position of Neptune, not realizing its significance. Another observation of Neptune seems to have also been recorded earlier in the month, on the 6th.

Galileo’s telescope was far too small in aperture, not to mention low-quality optically, to resolve Neptune’s disk – even large amateur telescopes often have a hard time doing so – but even at its fairly low magnification, Neptune’s movement over a period of a few weeks was enough to notice, and Galileo seems to have been aware that the object moved between his observations.

However, there is a crucial detail to remember: Galileo never recognized it as a planet. The idea that new planets could be discovered with a telescope may not have even crossed his mind. It is likely that Galileo believed himself to have made a slight mistake in his observations and thus did not feel the need to investigate further.

It wasn’t until 1846 that Neptune was officially discovered by Johann Galle, based on mathematical predictions by Urbain Le Verrier. After its discovery, historical records were revisited, and it was noted that Galileo had inadvertently observed Neptune long before anyone recognized it as a planet.

Did Galileo Discover Sunspots?

To safely observe sunspots, Galileo projected the image of the Sun through his telescope onto a white screen or piece of paper. This method, known as solar projection, allowed him to view and sketch the sunspots without directly exposing his eyes to the intense sunlight. The projected image provided a detailed, magnified view of the Sun’s surface, revealing the intricate movements and evolutions of the sunspots. While it is true that Galileo suffered from blindness in the later years of his life, there’s no concrete evidence linking his visual impairment directly to his solar observations. Moreover, Galileo was well aware of the dangers of looking directly at the Sun, even with the aid of a telescope, which is why he projected the image. The causes of his blindness in later years are believed to be more attributable to cataracts and glaucoma than to solar observations.

Galileo embarked on a series of solar observations around 1610, and by 1612, he began meticulously documenting an intriguing phenomenon on the Sun’s surface: dark, irregularly shaped patches that we now recognize as sunspots. These spots, contrary to their seemingly transient nature, were not mere optical artifacts or fleeting shadows, but actual features on the Sun. Moving gradually across the solar disc, they would change in size, shape, and number over time.

Galileo’s observations were groundbreaking in the context of the Western scientific tradition. While ancient civilizations like the Chinese and Greeks had records of sunspot observations dating back millennia, such observations in the West since the end of the classical era have been scarce, fragmented, and often misinterpreted as transits of Mercury by the few monks who bothered to take note of any especially large ones that were visible without optical aid.

Galileo’s methodical approach to studying sunspots involved sketching their daily positions and evolution. This allowed him to infer that the Sun rotates on its axis as he observed the movement of sunspots across its face and their eventual reappearance after a set period. Such a discovery was of significant importance, not only for its direct astronomical implications but also for its broader philosophical and theological ramifications. A rotating Sun, marked with imperfections, stood in contrast to the prevailing Aristotelian and Ptolemaic beliefs of a perfect, unblemished celestial realm.

Galileo’s sunspot observations were not without controversy. Around the same time, the Jesuit scholar Christoph Scheiner had also observed sunspots with his own telescope and proposed that they were satellites of the Sun, i.e., planets of some sort, since he refused to back down on the Aristotelian belief that the Sun was perfect and unchanging.

The ensuing debate between Galileo and Scheiner was rigorous and public, ultimately leading Galileo to publish his observations and conclusions in 1613 in “Istoria e Dimostrazioni Intorno Alle Macchie Solari e Loro Accidenti” (History and Demonstrations Concerning Sunspots and their Properties), often simplified in English to “Letters on Sunspots”. Galileo was ultimately right, and these letters show Galileo’s genius in proving that the spots had to be more or less on the Sun’s surface and were obviously not static, separate objects.

Galileo’s Studies of Comets

Galileo’s contributions to the realm of astronomy did not stop with planets, moons, and sunspots; his studies also encompassed the mysterious celestial visitors known as comets. However, his perspectives on comets sparked one of the most contentious debates of his career, placing him at odds with some contemporary thinkers and today’s scientific body of knowledge.

In 1618, three bright comets appeared in the night sky, drawing significant attention from astronomers and sparking discussions about their nature and origin. Comets, with their ephemeral and brilliant displays, have always been subjects of intrigue and speculation. Were they atmospheric phenomena or distant celestial objects? Did they move in regular orbits, or were they unpredictable in their motion?

Galileo, primarily based on his telescopic observations, posited that comets were nothing more than optical illusions—merely refractions of light without substance. He believed them to be near the Earth, located in the Earth’s atmosphere, and not celestial objects in the outer cosmos.

To articulate his views, Galileo inspired one of his disciples, Mario Guiducci, to write a treatise in 1619. Though Guiducci was the named author, it was widely understood that Galileo was the driving intellectual force behind the content. This publication was somewhat dismissive of other theories of the day.

This perspective ran counter to the observations and beliefs of Jesuit astronomer Orazio Grassi. Grassi, under the pseudonym Lotario Sarsi, responded to Guiducci’s (and by extension, Galileo’s) assertions in a publication that argued in favor of comets being genuine celestial objects with substance, moving in the heavens beyond our atmosphere.

Galileo then responded directly with his own work, “Il Saggiatore” (The Assayer), in 1623. This publication was not just a defense of his views on comets but a broader philosophical treatise on the scientific method. Galileo argued in favor of a more empirical approach to scientific inquiry, asserting that physical reality should be interrogated through experiments rather than relying on established authority.

However, history proved Galileo wrong on the matter of comets. Grassi’s perspective, that comets were actual celestial objects with substantial bodies, was the correct one. Despite this error in his comet theory, Galileo’s “Il Saggiatore” remains a foundational text in the philosophy of science, emphasizing the importance of direct observation and empirical evidence over pure philosophical speculation.

Did Galileo get into trouble with the Catholic Church?

Galileo’s revolutionary observations and conclusions didn’t merely reshape the scientific landscape; they also stirred significant controversy with the Roman Catholic Church. The heart of this dispute lay in the heliocentric model’s apparent conflict with certain scriptural interpretations, which seemed to support a geocentric universe.

While Galileo’s clash with the Church over heliocentrism is well-documented, it’s essential to recognize that his relationship with the Church was multifaceted. In fact, before the confrontation over the heliocentric model, the Church was, in many respects, supportive of Galileo and his work. Throughout his life, Galileo maintained friendships with several influential churchmen. Some of his work was financed by church officials, and he was frequently hosted by cardinals and other leading figures within the Church. His scientific endeavors were, for the most part, respected, and his devout Catholic faith was recognized.

The Jesuit Order, a significant branch of the Catholic Church, housed some of the leading scientists of the time. They appreciated Galileo’s work, especially in the early stages of his astronomical discoveries. For instance, after his publication of “Sidereus Nuncius” in which he detailed his observations of the moon’s surface and the moons of Jupiter, Jesuit astronomers confirmed his findings and lauded him for them.

In 1616, the Catholic Church issued a decree that heliocentrism was “formally heretical” because it appeared to contradict the Scriptures. Consequently, Galileo was warned not to support or teach the Copernican system as anything more than a theory, and one of his books was banned. Pope Urban VIII, who was the pope during Galileo’s later years, had been a friend and admirer of the scientist before his papacy. They shared mutual respect, with the Pope even writing verses in praise of Galileo’s work. It was only later that their relationship became strained. Even then, it’s believed that Pope Urban VIII did not wish for Galileo’s severe punishment but was forced into action by political and societal pressures.

Galileo continued teaching heliocentric theory at the time. The Pope and the church let him get away with this since he was technically presenting it as a hypothetical rather than an established fact.  However, in 1632, Galileo published “Dialogue Concerning the Two Chief World Systems,” which was a clear defense of the heliocentric model, albeit presented as a hypothetical dialogue. More importantly, however, the character depicted as arguing for the geocentric view was clearly made to seem more incompetent than the one arguing for helicentrism. This pro-geocentrism character, Simplicio, was seen at the time as obviously meant to be some sort of allegory to Pope Urban VIII. The Pope was not happy with this, and the church proclaimed the book a violation of its 1616 decree. This led to Galileo’s infamous trial in 1633.

Contrary to popular misconceptions, Galileo was never tortured or imprisoned in a dark dungeon. Instead, he was tried by the Roman Catholic Church’s Inquisition. The outcome? He was found “vehemently suspect of heresy,” asked to recant, which he did, and then spent the remainder of his life under house arrest. It wasn’t until October 31st, 1992—nearly 460 years after his trial—that the Church finally pardoned Galileo. By that time, a Jupiter space probe named in his honor was on its way. Its discoveries would prove to be as astonishing as those of Galileo himself, and this time, no one dared to doubt them.

The Galieo space probe at Jupiter. Credit – NASA

A widespread myth is that Galileo defiantly muttered “And yet it moves” (“E pur si muove” in Italian) after being forced to recant his heliocentric views. While this tale is evocative of a rebellious spirit facing dogma, there’s no solid evidence to suggest he actually uttered these words.

2 thoughts on “Galileo’s Telescope – The What, When and How”

  1. How did Galileo hold his telescope while he made his observations? At 20X did he have some means of stabilizing it? If he held it with his hands then his sketches would have required his ability to make quick visual and mental transfer.


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