Galileo's telescope: the small moons that changed astronomy
Galileo's telescope made small points of light beside Jupiter into objects with a new astronomical importance. In January 1610, Galileo identified four bodies accompanying the planet and concluded that they were its moons. In March, he announced the discovery in Sidereus nuncius, the Starry Messenger, published in Venice. The book also described the Moon's uneven surface and the multitude of stars within the Milky Way. Its first printing of 550 copies sold rapidly, carrying observations made with a recently developed instrument across Europe.
Galileo had heard about a Dutch spyglass in 1609. Although he did not invent the telescope, he understood its principle and built substantially more powerful versions. Early Dutch instruments magnified objects only two or three times; Galileo's telescope soon provided about twentyfold magnification. His workshop experience with optical lenses helped him develop the instrument without first examining a Dutch example. By late autumn, he was using his improved telescope for systematic observations of the sky, applying a device useful for distant terrestrial objects to astronomy.
The optical arrangement used two lenses. The objective, at the end facing the object, was convex, while the eyepiece was concave. This combination produced an upright image, useful when looking across a landscape or harbor. It also had limitations: the field of view was extremely narrow, and brightness diminished toward the edges. Galileo's telescope could reveal objects invisible to the unaided eye while showing only a small part of the sky at once. Its later replacement in astronomy by other designs reflected these practical constraints.
Galileo studied changes in light and shadow on the Moon. The uneven boundary between illuminated and dark areas indicated mountains and valleys rather than a perfectly smooth celestial surface. Through the instrument, hazy patches in the sky resolved into stars. These observations challenged an established distinction between an imperfect, changing terrestrial world and perfect, unchanging heavens. A mountainous Moon resembled Earth in a way that traditional celestial theory had not allowed. The new instrument supplied observations that made comparisons between the two worlds much harder to dismiss.
The moons of Jupiter created a different problem for an Earth-centered universe. Here were bodies orbiting another planet. Galileo named them the Medicean stars to honor Tuscany's ruling family, linking the discovery to his search for patronage. The Starry Messenger helped secure his appointment as chief mathematician and philosopher to the grand duke, and he moved from Padua to Florence. Other observers investigated his results. During a visit to Rome in 1611, Jesuit astronomers confirmed his observations, and he was admitted to the Accademia dei Lincei.
Tracking the four moons required more than recognizing their existence. They appeared with nearly equal brightness, making it difficult to identify each one consistently. Their orbital speeds differed, with the inner moons moving faster than the outer ones. Galileo sought their periods and developed a diagrammatic calculator called a Jovilabe. It represented Jupiter and the four orbits on a grid. He could estimate a moon's apparent distance in units of Jupiter's radius and use the diagram to work out its orbital position.
The changing viewpoint from Earth complicated the calculation. As Earth and Jupiter moved around the Sun, the observed position of a moon differed from the position that would be seen from the Sun. Galileo devised another diagram showing the orbits of Earth and Jupiter, with a scale for their relative positions. The combined Jovilabe used a rotating disk and a movable pointer to account for the changing line of sight. It connected repeated observations with a practical method of calculating the moons' positions without repeating lengthy arithmetic each time.
Further observations followed the Starry Messenger. Later in 1610, Galileo observed the phases of Venus, showing that it traveled around the Sun. Saturn presented a puzzling appearance that he described as three-bodied; Galileo's telescope did not let him recognize its rings. Sunspots supplied another example of change in the heavens. He published on them in 1613, openly supporting Copernican heliocentrism. The discoveries did not all appear in the first book, and their interpretation developed alongside continuing work with the telescope.
The wider dispute over Earth's motion eventually brought Galileo into conflict with Church authorities. His Dialogue Concerning the Two Chief World Systems appeared in 1632, and his trial followed in 1633. Under house arrest, he continued working, and the Two New Sciences was published in 1638. The observations begun in 1609 remained central to his astronomical influence: an uneven Moon, a star-filled Milky Way, changing planets, and moons moving around Jupiter had transformed the objects that European astronomers could examine.
- Museo Galileo, Galileo’s Observations — brunelleschi.imss.fi.it
- Stanford Encyclopedia of Philosophy, Galileo Galilei — plato.stanford.edu
- Museo Galileo, Telescope — catalogue.museogalileo.it
- Museo Galileo, Jovilabe — brunelleschi.imss.fi.it