Updated date:

Galileo Galilei Discovers the Moons of Jupiter and the Phases of Venus


My writing interests are general, with expertise in science, history, biographies, and “how-to” topics. I have written over sixty books.

Galileo Galilei

Galileo Galilei

In sixteenth-century Europe, most believed the Sun and all heavenly bodies rotated around the Earth. This explanation of the movement of celestial objects across the sky had held sway since it was proposed by the first century Egyptian mathematician Ptolemy.

Interpretation of the Holy Scriptures by the Catholic Church appeared to support Ptolemy’s view that the Earth was the center of the universe. Verses in the Bible, such as Ecclesiastes 1:5, “The sun rises and the sun sets, and hurries back to where it rises,” lent support to Ptolemy’s model. This was the view of the world that the young Italian man named Galileo Galilei was taught.

Over time, his beliefs would change to come in line with the Polish astronomer Nicolaus Copernicus, who placed the Sun at the center of the universe. Galileo was a bold thinker in a time when religion and science merged, thus making grand new proclamations on the order of the universe a dangerous thing. After receiving an education in mathematics and medicine at the University of Pisa, Galileo began to teach and investigate nature. His work formed the basis of modern physical science and would lead to the Scientific Revolution. As a talented inventor, he turned a mere parlor toy, a spyglass, into a working telescope that was useful to study the heavens and that was of practical use to mariners.

Galileo’s careful observations of the moons of Jupiter and phases of Venus would help put the nail in the coffin of Ptolemy’s theory; however, it would put Galileo squarely in the bullseye of the powerful Catholic Church.

Galileo at his telescope.

Galileo at his telescope.

Galileo Improves the Telescope

When Galileo Galilei was chair of mathematics at Italy's University of Padua, he got word that Dutch glass makers had invented a device that allowed viewers to see very distant objects as if they were nearby. Astonished by the idea, Galileo had to have one. He made improvements to the Dutch design and crafted one by grinding his own lens. On August 25, 1609, he presented the improved, more powerful telescope of his own design to the senate of the city-state of Venice. The government officials were so impressed with the telescope and its potential uses in spotting ships at sea that they rewarded the professor with a higher salary and tenure for life at his university. Now Galileo would turn his new telescope skyward and make observations that would change mankind’s view of the universe.

Galileo’s Drawings of the Moons of Jupiter on 7 to 10 January 1610.

Galileo’s Drawings of the Moons of Jupiter on 7 to 10 January 1610.

Observations of the Moons of Jupiter

One of the bright “wanderers” of the night sky is Jupiter, which is the fifth planet from the Sun. No one knows exactly when the planet Jupiter was discovered as it has been known since ancient times. Before Galileo, the existence of its family of satellites that orbit Jupiter was yet unknown. When Galileo trained his telescope at Jupiter on January 7, 1610, he saw three fixed stars near the planet’s body. The stars were brighter than others of similar magnitude. He found them lying parallel to the ecliptic, on the same plane in a straight line, with one “star” located west of him and two located east of him. Galileo dismissed them to be “fixed stars” and did not pay attention to their distances with respect to Jupiter. The next night, however, when Galileo viewed the “stars” again, the three points of light were closer to each other than they were the night before. Moreover, they were almost equidistant. Galileo’s inquisitive mind started pondering the reason for the change in position of the points of light he had observed for two consecutive nights. Galileo kept observing the “fixed stars” and found they continued to shift in position relative to the planet. On the night of the tenth, he attributed the disappearance of one of the points of light to its change in position from the front of Jupiter to its hind part, from the point of view of an observer from Earth. Galileo only saw two stars to the east of Jupiter. His telescope revealed the same situation on the eleventh nightof observation, and yet the easterly star was twice as large as its neighbor. He ruminated on his observations and wrote, “…There were in the heavens three stars which revolved round Jupiter, in the same manner as Venus and Mercury revolve round the Sun.”

Galileo’s Observation of Venus

Galileo also observed the planet Venus through his small telescope starting in the fall of 1610. The observations of Venus turned out to be very fruitful. Over the period of several months he observed that Venus went through a series of phases, from a small round disk and then various phases of crescents. The behavior was similar to how the Moon appears in different phases as it is seen from the Earth during the month. These observations would turn out to have implications on which model of the universe was correct.

Galileo’s observations of the phases of the planet Venus.

Galileo’s observations of the phases of the planet Venus.

The Sidereal Messenger

Based on his observations, he concluded that the three points of light were not fixed stars as he originally surmised but were in fact the planet’s natural satellites–like the Moon is to Earth. This was a momentous achievement, one of the most important discoveries tied to his name. Galileo had just discovered three of Jupiter’s biggest moons. In later observations, he came upon the fourth Jovian moon on January 12, 1611. He continued observing the four moons revolving around Jupiter until March 22, focusing his efforts on determining their motions. For reference, he used the fixed stars within the field afforded him by the telescope. With continued observations of these heavenly bodies through the middle of 1611, Galileo also came up with estimations of each of the moon’s periods, which came very close to modern measurements. The scientific community was at first in doubt that Galileo could make such a breathtaking discovery. But his observations were confirmed soon enough by other observers.

Galileo named the quartet of moons “Medicean Stars,” after the family of the Grand Duke of Tuscany, his future patron Cosimo II de' Medici. He dedicated the official account of the discovery in The Sidereal Messenger, which he wrote shortly after his observations. It contained the results of Galileo's early observations of the mountainous Moon, the hundreds of stars that were unable to be seen in the Milky Way with the naked eye, and the Medicean Stars that appeared to be orbiting Jupiter. The “Medicean Stars” were renamed the Galilean satellites by astronomers who came after their discoverer. Modern students of science know the Galilean moons by their individual names—Callisto, Europa, Ganymede, and Io. It was a revolutionary finding because it did not conform to geocentrism, which dictated that all heavenly beings must revolve around the Earth.

Sidereus Nuncius (The Starry Messenger) is a short astronomical treatise pamphlet published in New Latin by Galileo Galilei on March 13, 1610. It was the first published scientific work based on observations made through a telescope.

Sidereus Nuncius (The Starry Messenger) is a short astronomical treatise pamphlet published in New Latin by Galileo Galilei on March 13, 1610. It was the first published scientific work based on observations made through a telescope.

The Implications of the Discovery of the Moons of Jupiter and the Phases of Venus

The discovery of the four moons of Jupiter had far reaching implications; namely, that the Earth was not the center of the universe. The response from the geocentric camp was predictable. The loyal followers of Aristotle dismissed Galileo’s published work and despite the author’s repeated requests to look through his telescope, Padua’s principal professors of philosophy refused to look for themselves. The first printing of The Sidereal Messenger sold out quickly. The foremost thinkers chose to adhere to the view that the natural satellites revolving around Jupiter were non-existent. They continued to deny the possibility of centers of motion in the universe other than the Earth.

Galileo’s observations of the different phases of the planet Venus provided further support of the heliocentric model of the solar system developed by Nicolaus Copernicus. In his model, all phases would be visible because the orbit of Venus around the Sun would cause its illuminated hemisphere to face the Earth when it was on the opposite side of the Sun from the Earth. Likewise, the model predicted that when Venus was between the Earth and the Sun it would be larger in apparent size and observers on the Earth would see the dark side of the planet. In disagreement with his observation was Ptolemy’s geocentric model, in which it was impossible for any of the planet’s orbits to intersect the spherical shell carrying the Sun and thus not exhibiting all the phases observed by Galileo.

Galileo before the Inquisition.

Galileo before the Inquisition.

Galileo and the Inquisition

Galileo’s support for Copernicus’s sun-centered model of the universe put him in direct opposition to the beliefs of the Catholic Church, which supported Ptolemy’s earth-centered model of the universe. As Galileo’s sun-centered ideas spread throughout Italy, the opposition began to grow from within the community of scholars and church officials. Galileo attempted to quell the controversy by writing a letter to the Grand Duchess to explain his position. The letter had the opposite effect, and there were calls for Galileo to be investigated as a heretic.

Being called before the church’s Inquisition was a dangerous affair, as they had the power to imprison or even execute a heretic. Though he was not officially charged with heresy, he journeyed to Rome in 1615 to meet the head of the Inquisition. Galileo pleaded his case, but to no avail; the church’s official found that heliocentrism was “foolish and absurd in philosophy, and formally heretical.” Galileo was admonished by the Cardinal “not to hold, teach, or defend” the Copernican theory “in any way whatever, either orally or in writing.” The astronomer was forced to recant the ideas of Copernicus, and the work of Copernicus was placed on the list of books banned by the church.

Unable to contain his belief in the heliocentric theory, Galileo wrote his magnum opus, Dialogue Concerning the Two Chief World Systems. In this work he further defended Copernicus’s theory and managed to offend the Pope. For this his book was banned and Galileo was placed under house arrest for the remainder of his days. Though efforts were made by the church to silence Galileo and banish his work into historical oblivion, he still stands out as the father of modern science. Such contemporary scientific luminaries as Albert Einstein and Steven Hawking have celebrated Galileo as responsible for the birth of modern science.


Crowther, J.G. Six Great Scientists: Copernicus Galileo Newton Darwin Marie Curie Einstein. New York: Barnes & Noble Books. 1995.

Finocchiaro, Maurice A (Editor & Translator) The Essential Galileo. Hackett Publishing Co., Inc. 2008.

Heilborn, J.L. Galileo. Oxford University Press. 2010.

West, Doug. Galileo Galilei: A Short Biography. C&D Publications. 2015.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Doug West


Doug West (author) from Missouri on September 30, 2020:


I think that Galileo's work not being accepted during his lifetime and his trouble with the Inquisition is one of the reasons his story is so interesting. It is a story of discovery and tragedy all in one.

Bhavesh Shah from Mumbai on September 29, 2020:

It was such a tragedy that Galileo's work wasn't accepted during his days. It hurts to know that sometimes your work won't be accepted in this life, but will benefit many others in the future. This is how the world works, rose on the ashes of the once lived, worked, and forgotten...

Doug West (author) from Missouri on September 29, 2020:


Thanks for the comment. Galileo was ahead of his time and it literally about cost him his head at the Inquisition.

fran rooks from Toledo, Ohio on September 29, 2020:

Excellent and very informative article. I had no idea Galileo was investigated as a heretic. He was ahead of time with his knowledge. I learn something new every day. Thanks for enlightening me.