The beautiful conjunction of Jupiter with the crescent moon earlier this evening reminds us that this week marks precisely the 400th anniversary of the night when Galileo first ascended to the rooftop of his villa in Florence, Italy, and pointed his telescope toward Jupiter, the brightest planet in the nighttime sky. What he saw that night revolutionized our view of ourselves and our place in the world.
Galileo Galilei was born in 1564 in Pisa Italy. His father was a musician, and encouraged his son’s predilection for mathematics, but hoped he would pursue the more lucrative field of medicine. After Galileo had completed 4 years of study at the University of Pisa, including some courses in the medical curriculum to please his father, he chose the more intellectually challenging though lower paying position of lecturer in mathematics at his alma mater. As the father of a son who has chosen to make his living by being a philosopher, I can relate to the dad.
Two world views
At the time of the European Renaissance, there were two competing world views of the Universe. The first was the establishment position advanced by Aristotle and taught with mathematical rigor by Claudius Ptolemy. This was an Earth-centered world in which all the celestial bodies circled about the Earth – the moon and sun and each of the “wandering stars” or planets moving in concentric planes at increasing distances from a stationary Earth, with the fixed stars occupying the highest, most distant realm.
The second, less orthodox but equally old view, dating from the time of Pythagoras, viewed the sun as the center of all worlds – with the Earth and other planets, then the fixed stars, rotating around it. Both views recognized that the Earth, moon, and sun were spheres. Both assumed that orbits were perfectly circular, and, the Aristotelian view in particular, emphasized the imperfection and mutability of the Earth, but held that all celestial objects, being nearer to the spiritual realm, were manifestations of perfect harmony and unchanging constancy.
In 1543, the year of his death, the Polish cleric Nicolai Copernicus had published his monumental tome demonstrating with mathematical rigor the logical superiority of the sun-centered, or Pythagorian system, as it was then labeled, over the Ptolemaic view.
When Galileo began to lecture at the University of Pisa, he held to the party line, teaching the Ptolemaic structure of the universe. He enjoyed almost immediate success in applying mathematical calculations and an experimental approach, something his Aristotelian colleagues frowned upon, to the study of mechanics and motion. His reputation spread, and he was offered a Lectureship at the University of Padua in the city-state of Venice. At this point – he was 28 at the time – you could say he had become the equivalent of a tenure-track Assistant Professor.
At Padua, his ground-breaking advances in physics continued. As they did so, he became increasingly impressed by the strengths of Copernican astronomy, and the weaknesses of the Ptolemaic system. An excess of humility not being one of his virtues, he was outspoken and eager to debate and defend the Copernican world view at every opportunity, often with biting humor, devastating logic, and more than an occasional touch of arrogance. This did little to promote his popularity.
In the summer of 1609, he learned of a toy-like curiosity that was all the rage in France. A Dutch lense grinder, Hans Lippershey, the previous year had fabricated a tube with lenses arranged so that they made distant objects look much closer. Galileo was in Venice when he heard that a foreigner had shown up with a version of the instrument, in hopes of interesting the Venetian Senate in its purchase. Fortunately, Galileo had a friend in Venice who persuaded the Senate to hold off its purchasing decision until Galileo could provide a much superior instrument, which he did. Then, as now, it pays to have contacts in high places.
The Senators were astounded and amazed at Galileo’s demonstration that one could see the approach of an enemy ship two hours before it appeared on the horizon to the naked eye. The Senators promptly put in an order for the gadget, and more importantly, doubled Galileo’s salary AND gave him a life-time appointment at the University of Padua. Tenure at last.
What Galileo saw
By the fall of 1609, Galileo had improved the instrument to 20 times greater than its original power. As night was coming earlier by January, he turned his telescope – the term by then being used – toward the sky. His first surprising discovery was that the moon was littered with mountains, valleys, and rough terrain – far from the perfectly smooth orb expected by Aristotelian theory.
Another discovery was that all the planets appeared to be spheres, in marked contrast to the fixed stars, which remained only points of light. Furthermore, the nearest planet, Venus, was seen to pass through phases ranging from full to partial to very little illumination – a series of transitions that made no sense in the Ptolemaic formulation. At last, Galileo felt he had direct evidence in favor of the Copernican view.
On a night in the second week of January, 1610, he turned his telescope toward Jupiter, the brightest of the distant planets. Already accustomed to seeing many more stars in the sky than the naked eye could perceive, he noted that Jupiter seemed to sit in a cluster of newly-discovered stars, curiously arranged in almost a straight line. When he looked the next night, though, the stars had changed their positions radically, relative to Jupiter. As he repeated his observations over a succession of nights, he realized that the “stars” kept changing their relative positions. This was not possible, if they were indeed part of the pantheon of fixed distant stars. In fact, what he was observing, he came to realize, was a group of small moons circling around Jupiter.
This was huge, because it showed definitively that not all bodies rotate around a fixed Earth at the center of the universe. Furthermore, if the moon is not a perfect sphere, then maybe the moons of Jupiter aren’t either. Not only was the terracentric view of the world scientifically inaccurate, but the Greek notion of harmony and perfection in the celestial sphere did not hold up, bringing into question the distinction between the earthly and celestial provinces, and maybe even blurring the line between the physical world below and the metaphysical world above.
Galileo would go on to many new revelations, including discovery of double stars, the fact that the Milky Way itself is a collection of discrete stars, and that all the “fixed” stars are much further away than anyone had imagined.
Ships to sail among the stars
As a 16th Century visionary wrote “When ships have been built to sail among the stars, men will come forth to sail them.” And so we have, to the moon for the first time 40 years ago this past July, and in a seemingly endless shuttle back and forth between low-Earth orbiting capsules and space stations and laboratories since the 1980s.
So what of those Galilean moons first seen four centuries ago, and the wonders of other worlds discovered by later explorers of the nighttime sky? While space craft with humans aboard have not ventured past the moon – we seemed to lose our vision and our lust for adventure in the self-indulgent ‘80s, the booming ‘90s, and the angst-riddled and partisan plagued decade we’re now bringing to a close. But a form of sailing ship has persisted, not one that ferries human passengers, but as craft that carry an extension of our senses in the form of cameras, and particle sensors, and chemical analyzers. These robotic explorers have continued the quest that Galileo began, revealing wonders beyond even his fertile imagination.
Among the robotic fleet of ships we’ve sent into space, none has proved more revealing than the Voyager spacecraft, an instrument package about the size of a living room couch. There were two of them, launched in tandem 16 days apart in the late summer of 1977. The mission of the voyagers was to fly past, photograph, and collect data from all four of the gas giant planets and their moons. Galileo himself could not have known what wonders they would reveal.
When Voyager 1 photographed the innermost of Jupiter’s moons, Io, in March of 1979, it revealed a moon looking more like a pizza than a planet, with a surface overrun by calderas, lava fields, and technicolor plains of ash and sulfur.
Would Europa, the next moon to be encountered, look like Io? Voyager’s message back to Earth was an emphatic NO. Europa was already known to be the brightest object of its size in the solar system. As Voyager approached, Europa appeared not only bright, but far smoother than either the Earth’s moon or Io had shown to be. At closest approach, the reality turned out to be stranger still. Sure enough, Europa was pretty smooth, but it looked like a highly fractured crystal ball – crisscrossed by an incredible network of strange linaments, with ample evidence of fractured plates, chaotic terrain, and frequent resurfacing. Measurements of electrical field distortions, and the appearance of chemical deposits erupted onto the surface, suggested the possibility of a global ocean beneath the frozen surface – a conclusion now agreed to by the majority of planetary scientists.
With Io and Europa turning out to be about as similar as an apple and an orange, what would Ganymede and Callisto, the third and fourth moons out from Jupiter be like? Voyager showed Ganymede to look a lot like Europa, but with broader streaks and more craters, suggesting Europa-like geology, but with an older, more stable surface. And Callisto appeared to be the oldest and least active of all, with many more craters and almost no linaments or fractured plains.
Life on the Galilean Satellites
What about the possibility of life on any of these far away worlds?
Is it possible that the undersea worlds of Europa, Ganymede, and Callisto teem with marine-like ecosystems, evolving in the same region of the solar system, but each in its own way? Could there be boundaries between torrid lava and frozen substrate on Io, where microbes at least huddle at the midpoint between fire and ice, much as thermophilic bacteria ring the hot springs found on our own planet?
We don’t know, of course. But the point is, we can find out if we have the will to do so.
Already, the creative visionaries of our own time are thinking of ways we can reach into the depths of those enticing moons Galileo first glimpsed four centuries ago. The limits to our endeavor are not technological, though the challenges are formidable, and not conceptual, though our understanding is still quite limited. The only thing stopping us from reaching for the stars is the will to do so.