Galileo and the Telescope

The invention of the telescope played an important role in advancing our understanding of Earth's place in the cosmos. While there is evidence that the principals of telescopes were known in the late 16th century, the first telescopes were created in the Netherlands in 1608. Spectacle makers Hans Lippershey & Zacharias Janssen and Jacob Metius independently created telescopes. The telescope emerged from a tradition of craftsmanship and technical innovation around spectacles and developments in the science of optics traced back through Roger Bacon and a series of Islamic scientists, in particular Al-Kindi (c. 801–873), Ibn Sahl (c. 940-1000) and Ibn al-Haytham (965–1040).

The story of Galileo's telescopic observations illustrates how a tool for seeing and collecting evidence can dramatically change our understanding of the cosmos.

Early telescopes were primarily used for making Earth-bound observations, such as surveying and military tactics. Galileo Galilei (1564-1652) was part of a small group of astronomers who turned telescopes towards the heavens. After hearing about the "Danish perspective glass" in 1609, Galileo constructed his own telescope. He subsequently demonstrated the telescope in Venice. His demonstration of the telescope earned him a lifetime lectureship.

After his initial success, Galileo focused on refining the instrument. The initial telescope he created (and the Dutch ones it was based on) magnified objects three diameters. That is, it made things look three times larger than they did with the naked eye. Through refining the design of the telescope he developed an instrument that could magnify eight times, and eventually thirty times.

This increased magnification of heavenly objects had a significant and immediate impact.  These new observations were by no means exclusive to Galileo.  The story of Galileo and the telescope is a powerful example of the key role that technologies play in enabling advances in scientific knowledge. With that said, the telescope isn't the only technology at play in this story. Galileo deftly used the printed book and the design of prints in his books to present his research to the learned community.  This is not a story of a lone thinker theorizing and piecing together a new model of the cosmos. Quite the contrary, an array of individuals in the early 17th century took the newly created telescopes and pointed them toward the heavens. Unlike those other observers, however, Galileo rapidly published his findings.  In some cases, Galileo understood the significance and importance of these observations more readily than his contemporaries. It was this understanding, and foresight to publish, that made Galileo's ideas stand the test of time.

Starry Messenger, Galileo's Rapidly Published Findings

Shortly after his first telescopic observations of the heavens, Galileo began sketching his observations. He wanted to get his findings out. His observations and interpretations of stars, the moon, Jupiter, the sun and the phases of the planet Venus, were critical in refining our understanding of the cosmos. In March of 1610, Galileo published the initial results of his telescopic observations in Starry Messenger (Sidereus Nuncius), this short astronomical treatise quickly traveled to the corners of learned society.

The Moon is not a Perfect Sphere

The engravings of the Moon, created from Galileo's artfully drawn sketches, presented readers with a radically different perspective on the Moon. Due to Galileo's training in Renaissance art and an understanding of chiaroscuro (a technique for shading light and dark) he quickly understood that the shadows he was seeing were actually mountains and craters. From his sketches, he made estimates of their heights and depths. These observations, only possible by the magnifying power of the telescope, clearly suggested that the Aristotelian idea of the Moon as a translucent perfect sphere (or as Dante had suggested an "eternal pearl") were wrong. The Moon was no longer a perfect heavenly object; it now clearly had features and a topology similar in many ways to the Earth. The notion that the moon had a topology like the Earth led to speculation on what life might be like on the Moon.

It's now understood that English astronomer Thomas Harriot, (1560-1621) made the first recorded observations of the Moon through a telescope, a month before Galileo in July of 1609. Moreover, the map Harriot created of the Moon in 1612 or 1613 is more detailed than Galileo's. Harriot observed the Moon first, and the maps he created included more information, but he did not broadly distribute his work. However, over 500 copies of the Starry Messenger were printed and sold, solidifying Galileo's legacy in astronomy. 

Jupiter has its Own Moons

When Galileo turned his telescope to observe Jupiter, he saw what he initially thought to be three previously unobserved fixed stars. After continued observations it became clear that they were not fixed, and in a matter of days he had come to the conclusion that these new stars were in fact orbiting Jupiter. He had discovered three of the largest moons of Jupiter.

The implications of this discovery, of objects orbiting a planet, were part of what pushed Galileo to argue for a sun-centered cosmos. Jupiter's moons countered a key argument against the Earth orbiting the sun. Critics of Copernicus' sun-centered cosmos asked, how could the Earth drag the moon across the heavens? Remember, the idea of the underlying mechanism of gravity wouldn't come until Newton's Principia Mathematica in 1687, which makes this both a reasonable and important question. Since there was wide agreement that Jupiter was already in motion, the fact that Jupiter clearly had its own moons offered a clear refutation of an important critique of the heliocentric system.

In Mundus Jovialis (1614), Simon Marius claimed that he, not Galileo, had first discovered the moons of Jupiter. In his times, Marius was publicly condemned as a plagiarist. Galileo had published his results already in 1610 and was rather well known and powerful in renaissance court. Only in the 19th century, would historians return to examine the evidence. It turns out that Marius had not plagiarized Galileo. Clearly his observations were different; in fact he had more accurately charted the orbits of Jupiter's moons. It's now broadly understood that Marius was an independent observer of Jupiter's moons.

A Spotted Rotating Sun

In observing the sun, Galileo saw a series of "imperfections". He had discovered sunspots. Monitoring these spots on the sun demonstrated that the sun in fact rotated. Furthermore, later observations by Francesco Sizzi in 1612 suggested that the spots on the sun actually changed over time. It would seem that the Sun, like the Moon, was not the perfect sphere that learned Europeans thought of as a key feature of their universe.

These sunspots were also independently observed by the Jesuit priest and astronomer Christoph Scheiner (1575-1650). Scheiner observed sunspots in 1611 and published his results in 1612. Over the course of their careers Galileo and Schiener feuded over who should get credit for the discovery. Unbeknownst to either of them, Thomas Harriot had observed them in 1610 and the German theologian, David Fabricius and his son Johanes likely beat both Scheiner and Galileo to the publication of the discovery with their Apparente earum cum Sole Conversione Narratio in June of 1611. However, their publication was not widely circulated and thus remained obscure in its times. Outside the western tradition of science. Chinese astronomers have long observed sunspots, going back to at least 165 BC.

Galileo's name became synonymous with the telescope. In this frontispiece to Galileo's collected works he is shown presenting the telescope and gesturing toward some of his discoveries in the heavens. The artist has depicted Jupiter and its satellites. The book did not include Galileo's most famous work, his Dialogue Concerning the Two Chief World Systems, which had been added to the Index of Forbidden Books. Still, the artist included visual reference to this work. Below the moons of Jupiter, one can clearly see a model of a Sun-centered system. Frontispiece to Opere di Galileo Galilei.1656. Rare Book & Special Collections division.

In Starry Messenger, Galileo shared detailed drawings of different phases of the Moon. The significant magnification provided evidence of the rocky nature of the Moon. Sidereus Nuncius, Image 23 (1610) Rare Book & Special Collections"

Here,  Galileo presents a sequence of his observations of the moons of Jupiter (the larger circle) and Jupiter's satellites over subsequent nights. The sequence shows the movement and disappearance of some of the satellites as they move behind Jupiter. Sidereus Nuncius, Image 47. (1610) Rare Book & Special Collections.

Here is Galileo's sketch of the sun, note the spots on it. In his times, it was widely believed that the sun was a perfect sphere, but clearly it had spots. Further, by watching the spots over time it became clear that the sun was actually rotating. Both of these observations seemed to be at odds with interpretations of Aristotelian cosmology. Three Letters on Sunspots. (1613) Rare Book & Special Collections