A portrait, attributed to Murillo, of Galileo staring at the words “E pur si muove” (“And yet it moves;” not legible in this image) scratched on the wall of his prison cell. Image via Wikimedia Commons.
Happy birthday to Italian astronomer, mathematician and physicist Galileo Galilei, born February 15, 1564. Galileo was one of the first to aim a telescope at the night sky, where he saw phases of Venus and four dots of light orbiting Jupiter (now known as Jupiter’s famous Galilean moons). These and other observations began to change the way we saw the universe and our place in it.
In the time of Galileo, people developed the Aristotelian view that the Earth was lying made in the center of a more or less immutable universe. So his discovery of moons orbiting Jupiter (now known as the Galilean satellites in his honor) and the phases of Venus that emerged from the planet orbiting the sun were considered heresy by the Roman Inquisition. In 1633 these tribunals – which had been developed by the Holy See of the Roman Catholic Church – forced Galileo to revoke.
As he left the courtroom, he reportedly muttered:
E pur si muove (and yet it moves).
And so it does. The Earth moves and all objects in space move. The phrase is still used as an answer, implying it doesn’t matter what you believe; these are the facts.
Galileo spent the rest of his life under house arrest, but that didn’t stop him from publishing another work, Two New Sciences, on mechanics and movement.
EarthSky’s lunar calendar shows the moon phase for every day in 2021. Order yours before they are gone!
Galileo grew up in a musical family. In 1574 the family moved to Florence, where 18-year-old Galileo began his education in a monastery. He was very successful in his studies and began to study medicine at the University of Pisa. Financial difficulties prevented him from completing his studies, but his years at university were priceless. They introduced him to mathematics and physics, but most importantly, they introduced him to Aristotle’s philosophy.
At the time, if anyone wanted to know more about the universe, the way to do it was to read Aristotle’s works. As Dante had said several centuries earlier, Aristotle is “the Master of those who know” (Dante, Inferno 4.131). In other words, at that time knowledge was to philosophy what faith was to religion.
And so, despite not being able to complete his degree in medicine and become a college professor, Galileo continued his studies in mathematics anyway. He was able to get a few minor teaching positions for a living. After two years of hard work, he published “La Bilancetta” (The little balance), his first scientific book, which earned him a good reputation. The book commented on the story of how the king of Syracuse asked Archimedes to verify whether his crown was made of pure gold or an inferior mix of metals. Galileo presented an invention of his, the ‘small balance’, now called ‘hydrostatic balance’, which is used to make more accurate measurements of differences in density.
Read here about the king’s crown and Archimedes’ other discoveries.
Galileo’s reputation was bruised after the publication of his “Du Motu” (On movement), a study of falling objects, which showed that he disagreed with the Aristotelian view of the subject.
In 1609 he heard that an instrument had been invented in the Netherlands that showed distant objects as if they were close by. Like many others, Galileo quickly grasped the mechanics of the binoculars, but later improved the original design significantly. He gave the Venetian state a telescope with eight motors, a telescope that magnifies normal vision eight times. His telescope earned him a doubling of his salary and a lifetime employment at the University of Padua.
Over the years, Galileo has improved its telescope to magnify up to 20 times.
One of Galileo’s telescopes. Image via the University of Oregon.
He made many astronomical discoveries with his telescope. For example, he was the first to see the moon magnified 20 times. He drew the surface of the moon and showed that the surface is bumpy and rocky, contrary to the popular belief at the time that the moon was smooth.
In January 1610 he discovered the four most massive moons of Jupiter: Io, Europa, Ganymede and Callisto. Today they are called the Galilean moons. He explained all his findings in his book “Sidereus Nuncius” (The Starry Messenger
Galileo noted that Venus went through different phases, just like the moon.
Composite image overlooking spacecraft from the four largest moons of Jupiter. Known as the Galilean satellites, they were first seen by the Italian astronomer Galileo Galilei in 1610. Shown left to right in order of increasing distance from Jupiter. Io is the closest, followed by Europe, Ganymede and Callisto. Image via NASA.
Galileo was a respected man in 1610, but his increasing public acceptance of the heliocentric system began to cause him problems with the Roman Catholic Church.
In 1618 Galileo got into a controversy about the nature of comets, which did not benefit his social position. Galileo nevertheless published the argument under his own name in “Il Saggitore” (The Assayer) in 1623, which is one of his most famous works to this day.
Read selections from “The Assayer.”
Things did not get much better for Galileo before his death in 1642. His work continued to defy the accepted Aristotelian view and incur the ire of the Roman Catholic Church, which had established a group of institutions within the legal system of the Church – known as the Inquisition – whose purpose was to combat heresy.
In particular, his publication in 1632 of his “Dialogue on the Two Major World Systems, Copernican and Ptolemaic” was contrary to the Aristotelian view. In 1633 the Inquisition summoned Galileo to Rome. He was declared a heresy suspect, sentenced to life imprisonment and formally ordered to abstain. Still, he lived comfortably and was allowed to continue his work.
Galileo’s daughter, Sister Maria Celeste, was a nun in the Catholic Church. They wrote letters to each other regularly, and she kept the letters Galileo wrote to her, which were finally published by Dava Sobel in a book called Galileo’s Daughter in 1999.
Despite Galileo’s struggle with the Church, he was a devoted Catholic. He would probably be happy to know that the Vatican now has its own observatory and that some of his fathers are astronomers. But it wasn’t until 1992 that the Vatican admitted that Galileo was right in his heliocentric beliefs.
Galileo died on January 8, 1642.
A list of all of Galileo’s discoveries is long. While Galileo is hugely praised for his various scientific discoveries, he did much more than just advance science: he also pushed society forward. His life was much more than just a conflict with religion and Aristotelianism. It was a struggle against the suppression of the opinion of an emerging scientific minority.
Galileo was one of the first to liberate science from philosophy. He inspired countless others to pursue the freedom of scientific inquiry.
Portrait of Galileo by Justus Sustermans. Image via Wikimedia Commons.
In short, one of our greatest astronomers, Galileo Galilei, was born on February 15, 1564. His discoveries with the improved telescopes he made have changed the way we view the universe.
