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Galileo’s discovery of the moons of Jupiter disrupted conventional wisdom

It was a warm night in August, and I had gathered with other members of the Popular Astronomy Club at the recently renovated Paul Castle Observatory in Milan for an observing session.

Soon after sunset, club president Alan Sheidler had the observatory’s telescope trained on Jupiter. Minutes later, he announced that Jupiter’s moon Io had just emerged, orbiting into view out of the shadow of largest planet in the solar system.

I looked through the telescope, and could distinctly see four points of light lined up on either side of Jupiter. These tiny dots were the four Galilean moons – Callisto, Europa, Ganymede and Io, the latter now seen just to the right of Jupiter from our terrestrial perspective.

They’re called the Galilean moons because their discovery is credited to the great Galileo Galilea, an Italian born in 1564 who could fairly be described as the ultimate Renaissance man and who’s proclaimed by some as both the “Father of Modern Astronomy” and the “Father of Modern Science.”

The first telescope was patented in 1608, and within a year Galileo began designing and building better telescopes with increased magnification. He turned one of his telescopes to the night sky and recorded his observations.

In January 1610, Galileo observed what he first thought were three fixed stars near Jupiter. He later saw a fourth – coming into view just as Io did on that night in August - and his observations over the next several weeks showed that these four “stars” seemed to move around Jupiter.

Galileo realized that the objects weren’t stars at all, but in fact were natural satellites orbiting Jupiter. This directly contradicted the geocentric theory which dated back to ancient astronomy, and which held that the sun, moon, planets, stars and all other celestial objects rotated around Earth, fixed at the very center of the universe.

Geocentrism is alluded to several times in the Old Testament and so was deeply rooted in the theology of the Roman Catholic Church. Galileo, a practicing Catholic who once considered joining the priesthood and who was acquainted with priests and bishops, knew that his work would bring him into conflict with the then all-powerful church, but still went ahead and published his observations.

Nearly a century before Galileo, the Polish mathematician and astronomer Copernicus postulated that the sun, not Earth, was at the center of the universe, basing this theory on his own pre-telescopic observations and calculations. Galileo’s observations of the moons of Jupiter provided physical proof of Copernicus’ heliocentric theory, which had previously been seen by the church and others as merely a mathematical hypothesis.

In 1616, a commission formed by the Catholic Inquisition proclaimed heliocentrism to be “foolish and absurd… since it contradicts… the sense of Holy Scripture.” Galileo was ordered to “abandon completely… the opinion that the sun stands still at the center of the (universe).”

Though he didn’t really change his mind, Galileo laid low for the next several years. Then, a bishop who was a friend and admirer of Galileo became Pope Urban VIII. This emboldened Galileo, who in 1632 published a book titled “Dialogue Concerning the Two Chief World Systems.”

The book was formatted as a debate between two individuals advocating the geocentric and heliocentric theories. The geocentric argument was made by Simplicio, a name that could be translated as “Simpleton.” This fictional character came across as a fool and was seen by some as a stand-in for a church official, perhaps even the pope himself.

This was simply more than the church hierarchy could bear, so Galileo was charged with heresy and called to Rome to be tried by the Inquisition. In June 1633, the inquisitors sentenced Galileo to house arrest, banned his publications, and directed him to recant the theory of the sun-centered universe.

Legend has it that, as he departed his trial, Galileo muttered, “And yet it moves.” Though he probably never said those words, he continued his groundbreaking scientific work under house arrest until his death in 1642.

In October of 2018, I visited the Museo Galileo in Florence, Italy. The items on display at the museum include originals and replicas of some of the telescopes used by Galileo, as well as the middle finger of Galileo’s right hand, encased in glass.

I don’t know if this display is intended as a symbol of defiance, but I do know that Galileo boldly defied conventional wisdom, and that his theory of the universe is no longer considered heretical. In the centuries that have passed since Galileo turned his telescope toward Jupiter, we’ve learned that our solar system is nowhere near the center of the universe, and that our sun is merely one of countless stars that far outnumber the grains of sand on all the world’s beaches.

As an atonement for trying Galileo, the Vatican now maintains its own observatory near Rome and also operates a telescope at an international observatory in Arizona. The astronomers working at these places are keeping the spirit of Galileo alive, pursuing knowledge that could someday disrupt present-day theories of our place in the universe.

Paul Levesque is a member of the Popular Astronomy Club. The club has suspended its monthly public observing sessions at Niabi Zoo and other public events due to the coronavirus pandemic but will resume them as soon as it is safe to do so. Visit the PAC website at for more information.


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