We briefly discussed Galileo and his relationship to the Catholic Church a few weeks ago. Galileo was an advocate of the Copernican system. He argued that the sun is at the center of the cosmos and that Earth and the other planets orbit the sun.
The church advocated the Ptolemaic system. Ptolemy’s system involved a complex system of orbits within orbits called epicycles that allowed the Earth to stay at the center of things.
The Ptolemaic system is far too ornate to describe here. I invite you to check out https://people.highline.edu/iglozman/classes/astronotes/ptolemy.htm to study its intricacies.
From a modern perspective, Ptolemy’s model sounds ridiculous, but intellectuals believed it was the gospel truth for almost 1,500 years.
Galileo was certainly not the only contemporary advocate of the Copernican system. However, Galileo thought he had proof based on a radically new technology, the astronomical telescope.
Galileo didn’t invent the telescope. Lens makers in Holland did that in 1608. Operating only on second-hand information, Galileo designed and built his own telescope by the fall of 1609.
Galileo began a systematic tour of the sky. He was so stunned by what he saw that he hurriedly published a book on his findings.
The hastily completed work, “Sidereus Nuncius” or the “Sidereal Messenger,” was a game-changer.
Galileo describes three significant discoveries supporting the Copernican model in the work.
The Ptolemaic model held that everything in the starry realm, the moon included, was perfect. Galileo discovered that the moon had craters, mountains, and valleys on its surface. Galileo even used the shadows of the mountains to measure their height.
Galileo’s lunar observations refuted Ptolemaic perfection. More significantly, it showed that the moon was a world like Earth, complete with mountains and valleys.
Galileo’s second discovery was that the Milky Way was made up of uncountable stars invisible to the unaided eye. That discovery would eventually have a profound impact on our understanding of the cosmos and our place in it. However, it did not seem to factor into the battle between the two solar-system models, which was the overriding astronomical issue of the time. It was small potatoes compared to his third discovery.
Galileo detected four points of light orbiting Jupiter. They seemed to accompany the planet as it moved across the sky.
Jupiter had four “planets,” as Galileo called them, orbiting it. Hence, Jupiter was like Earth in another way. It even beat Earth in that respect. Earth had only one moon. Jupiter had four.
Like most scientists, Galileo needed funding. In 1605, Galileo had been a mathematics tutor for Cosimo de’ Medici, son to the Grand Duke of Tuscany.
Coincidentally, Cosimo became Grand Duke around the same time. To gain financial support from his former pupil, Galileo proposed that the moons be named Cosimo’s Stars.
Cosimo demurred, however. He instead insisted that the moons be named the Medician Stars in honor of his three brothers and, of course, himself.
Although the logic is somewhat esoteric to our modern ears, Jupiter’s satellites provide Galileo’s most telling arguments against the Ptolemaic system. Opponents of Copernicus argued that Earth could not move because the moon would be left behind. Jupiter moved, yet it kept its moons. If Jupiter could do it, so could Earth.
Furthermore, contained within the Ptolemaic model was Aristotle’s notion that Earth was at the center of the cosmos. All the motion in the universe was thus centered on Earth. Galileo’s observations of the orbital paths of Jupiter’s satellites proved that at least one center of motion could and did exist.
Galileo continued his work after “Sidereal Messenger” was published. Two discoveries seemed to refute the Ptolemaic model. His discovery of sunspots suggested that the sun, like the moon, was imperfect.
By systematically studying the motion of the spots across the sun’s disk, Galileo determined that the sun was a sphere that rotated on its axis, just like the Earth and other planets.
Until then, Galileo had close and friendly ties with highly placed officials in the church, including Cardinal Barberini, who would later become Pope Urban VIII.
The church had provided both financial and emotional support to Galileo’s efforts. However, they were also politically and spiritually bound to the Earth-centered model.
Next week: Trouble ahead.
Tom Burns is the former director of the Perkins Observatory in Delaware.
