Astronomical News
Galileo’s Achievements: What Was Galileo Galilei Famous For?
Feb 6, 2025
History
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Vito Technology, Inc.
February 15, 2025, marks Galileo Day and the 461st birthday of Galileo Galilei — one of the most influential scientists in history. His observations of Jupiter’s moons, Saturn’s rings, and the phases of Venus revolutionized our understanding of the cosmos. Want to follow in his footsteps? Use the Sky Tonight and Star Walk 2 astronomy apps to explore the same celestial wonders Galileo once observed and celebrate his legacy with us!
Contents
Interesting Facts About Galileo Galilei
Galileo dropped out of university
Galileo’s middle finger is on display in a museum
Galileo spent his final years under house arrest
The Vatican didn’t admit Galileo was right until just recently
Four years before death, Galileo became totally blind
Galileo's Breakthrough Observations in Astronomy
Craters and mountains on the Moon
The phases of Venus
Four largest moons of Jupiter
The stars of the Milky Way
Sunspots
Lunar librations
Galileo’s achievements outside astronomy
The law of falling bodies
The basic principles of pendulum
The theory of parabolic trajectories
What didn't Galileo Galilei do?
Galileo didn't invent the telescope
Galileo didn't discover the laws of gravity
Galileo didn't prove that the Earth rotates
Galileo didn't say, "And Yet It Moves"
Examining Galileo's theories: are there any flaws?
The theory of tides
The nature of comets
The idea of a perfect vacuum
Galileo's stargazing traditions continue today
Galileo’s discoveries:
Interesting Facts About Galileo Galilei
Galileo dropped out of university.
Initially, his father sent him to the University of Pisa to study medicine, but Galileo became interested in mathematics and left the university without a degree.
Galileo’s middle finger is on display in a museum.
Around a century after the scientist’s burial, the middle finger of his right hand was removed from his corpse and has been housed at various museums in Italy since then.
Galileo spent his final years under house arrest.
Galileo’s support for heliocentrism put him in direct conflict with the Catholic Church. As punishment, he was first sentenced to life imprisonment and then to house arrest.
The Vatican didn’t admit Galileo was right until just recently.
Only in 1992, Pope John Paul II officially declared that Galileo Galilei was right in his support of heliocentrism and that the Inquisition had made a mistake in imprisoning him.
Four years before death, Galileo became totally blind
Contrary to popular belief, Galileo’s blindness wasn’t caused by his observations of the Sun. Most likely, the cause was a cataract or glaucoma.
Galileo's Breakthrough Observations in Astronomy
Galileo Galilei revolutionized astronomy with his groundbreaking observations of the celestial bodies using a modified telescope. Most of them were published in his book "Sidereus Nuncius" in March 1610.
Craters and mountains on the Moon
Moon Atlas in Star Walk 2
Study the Moon’s surface with the Lunar Atlas in the Star Walk 2 app.
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Galileo Galilei was the first to observe the Moon's rugged and mountainous surface, which was a significant departure from the prevailing view that the Moon was a smooth and featureless sphere. He also observed the Moon's craters, valleys, and other features and made accurate drawings and descriptions. Want to explore the lunar terrain yourself? Open the Moon Atlas in the Star Walk 2 app and discover the names of the Moon’s “seas” and mountains.
The phases of Venus
Venus phases in Sky Tonight
Easily learn the current phase of Venus using the Sky Tonight app.
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Galileo Galilei observed the different phases of Venus, including crescent, full, and gibbous shapes, which were consistent with the idea that Venus orbits around the Sun and not the Earth. These observations were a significant piece of evidence for the Copernican system. Curious about Venus’s current phase? Simply zoom in on the planet in the Sky Tonight app and see its phase in real time!
Four largest moons of Jupiter
Galilean moons in Sky Tonight
Find out how Jupiter’s Galilean moons are positioned right now using the Sky Tonight app.
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Galileo Galilei discovered four of Jupiter's moons in 1610. He made observations of Jupiter and its moons over several nights and found that Jupiter was accompanied by four small celestial bodies that orbited around it. This discovery was a breakthrough in the scientific understanding of the Solar System, showing that planets can have their moons. The moons, now known as the Galilean moons, are among the largest in the Solar System. If you have the Sky Tonight app, you can see the current position of the Galilean moons by simply zooming in on Jupiter.
The stars of the Milky Way
Galileo Galilei made a seminal contribution to the understanding of the Milky Way. Before his observations, the Milky Way was generally considered a cloudy band across the sky. Still, Galileo was the first to recognize it as a collection of countless individual stars.
Sunspots
Sunspots are dark areas on the surface of the Sun. Before Galileo's discovery, they were not widely known or studied. Galileo used a telescope to observe the Sun and found it had dark spots on its surface, which he recorded in a series of drawings. This was a major discovery at the time, as the prevailing view was that the Sun was an unchanging object. Galileo's observation of sunspots proved that the Sun was not a perfect celestial body but was subject to change and imperfections.
Lunar librations
Galileo Galilei studied lunar librations, small oscillations in the Moon's apparent position, and was the first to understand the cause of these movements. He observed the Moon with a telescope and made detailed drawings, including its librations, which he recognized were due to the changing perspective from Earth and the orientation of the Moon's surface. Galileo's discoveries advanced our understanding of the Moon and its movement, supporting the heliocentric model of the solar system.
Discover the incredible contributions of astronomers beyond Galileo by taking our quiz on great astronomers of the past.
Great astronomers: collage of three
Who first proposed the heliocentric system? Which astronomer was burned at the stake for his beliefs? Take this quiz to test your knowledge about famous astronomers!
Take the quiz!
Galileo’s achievements outside astronomy
Galileo Galilei was a renowned astronomer and also a prolific inventor and scientist. Throughout his life, he made numerous contributions to various fields, including physics, mathematics, engineering, and philosophy.
The law of falling bodies
The law of falling bodies, also known as Galileo's law of free fall, states that all objects fall at the same rate, regardless of their mass, as long as air resistance is negligible. He dropped objects of different masses from the Leaning Tower of Pisa and showed that they hit the ground at the same time, disproving the prevailing Aristotelian belief that heavier objects fall faster than lighter ones. The Law of Falling Bodies was a significant step forward in the development of the scientific understanding of motion. It helped lay the foundation for the laws of motion developed later by Sir Isaac Newton.
The basic principles of pendulum
The basic principle of pendulum motion, also known as Galileo's law of pendulum, states that a pendulum will swing back and forth in a regular, repeating pattern, with the time it takes to complete one cycle, or period, being dependent only on the length of the pendulum and the acceleration due to gravity. This discovery laid the foundation for the development of accurate timekeeping devices such as clocks. In addition, it led to a deeper understanding of the laws of motion and the behavior of oscillating systems.
The theory of parabolic trajectories
Galileo Galilei's theory of parabolic trajectories, also known as his law of projectiles, states that a projectile's path under gravity is a parabolic curve. He showed that a projectile's horizontal and vertical motion can be described separately and that its trajectory is a parabolic curve. This was a crucial step in understanding motion and paved the way for Sir Isaac Newton's laws of motion. Galileo's theory remains essential in explaining the motion of projectiles, including objects in satellite orbits, sports projectiles, and missiles.
What didn't Galileo Galilei do?
Galileo Galilei was a man of many talents and achievements, but there were some things he did not do.
Galileo didn't invent the telescope
Galileo made all his famous observations with the help of a telescope he created. He used spyglasses invented in 1608 — low-powered telescopes capable of magnifying objects three times as a prototype. In 1609, after figuring out how spyglasses worked, the scientist built his own improved version that could magnify a normal vision 8 times. As Galileo kept working on his invention, he made a telescope capable of 20x magnification a few years later. Nowadays, amateur astronomers use telescopes with 20x or 30x magnification for planet viewing. Galileo's telescopes were not flawless, as they had a very narrow field of view, making finding objects more difficult. Nevertheless, it didn't stop Galileo from observing.
Galileo didn't discover the laws of gravity
Galileo Galilei is often credited with discovering the laws of gravity, but this is only partially accurate. While Galileo contributed significantly to understanding gravity and its effects, he did not realize the underlying laws governing the force. The concept of gravity can be traced back to the ancient Greek philosopher Aristotle, who believed that objects fall to the Earth because they are naturally attracted to it. However, the work of Sir Isaac Newton in the 17th century led to the laws of gravitation, which describe the universal force of attraction between all masses.
Galileo didn't prove that the Earth rotates
Galileo Galilei is frequently associated with proving that the Earth rotates, but this is also a misnomer. The idea that the Earth rotates on its axis was first proposed by the ancient Greek philosopher Pythagoras and later developed by Aristarchus in the 3rd century BCE. However, this idea was largely dismissed during the Middle Ages and was not widely accepted until the work of Galileo and other scientists during the Scientific Revolution. Galileo made important observations and experiments that helped support the idea of a rotating Earth, but he did not prove it conclusively. Later, the evidence for a rotating Earth became more solid and widely accepted through the work of other scientists such as Johannes Kepler and Isaac Newton.
Galileo didn't say, "And Yet It Moves"
The phrase "And yet it moves" is often attributed to Galileo Galilei as a symbol of his defiance of the Catholic Church's views on the Earth's place in the universe. However, the exact origin of the phrase is unclear, and there is no direct evidence that Galileo ever said it. It is possible that the quote was first attributed to Galileo by later writers who wanted to highlight his bravery in the face of opposition from the Church and other authorities. Regardless of its origin, the phrase has become a popular representation of Galileo's groundbreaking ideas and discoveries and his impact on the history of science.
Examining Galileo's theories: are there any flaws?
While Galileo's ideas were groundbreaking for his time, they were also limited by the scientific understanding and technology available to him. He made many important contributions to the field of science, but some of his ideas were later found to be incorrect.
The theory of tides
Galileo believed that the tides were caused by the sloshing of ocean water back and forth due to the movement of the Earth. However, it was later shown that the tides are caused by the Moon's and Sun's gravitational pull.
The nature of comets
Galileo believed that comets were atmospheric phenomena rather than celestial objects. However, it was later shown that comets are, in fact, small, icy bodies that originate from the outer reaches of the Solar System.
The idea of a perfect vacuum
Galileo believed creating a perfect vacuum, a space devoid of all matter, was possible. However, it was later shown that a perfect vacuum is impossible due to particles, such as electrons and photons, that exist even in empty space.
Galileo's stargazing traditions continue today
Galileo was passionate about stargazing and encouraged others to observe the night sky and explore the universe. He wrote extensively about his observations and discoveries, sharing his knowledge and enthusiasm for astronomy with others.
It is still possible for amateur stargazers to make astronomical discoveries. With the advancements in technology and accessibility to equipment, hobbyist astronomers can contribute to the field of astronomy in meaningful ways.
One notable example is amateur astronomer, Terry Lovejoy, who has discovered several comets using commercially available equipment.
Another example of a contemporary discoverer is Gennadiy Borisov, who has made significant contributions to astronomy. He discovered the first interstellar comet, 2I/Borisov, originally designated C/2019 Q4 (Borisov), in 2019, demonstrating that comets from other star systems can visit our Solar System. His discovery highlights the role of amateur astronomers in advancing the field despite limited resources.
These examples show that enthusiast stargazers can make meaningful contributions to astronomy, and their passion for stargazing can lead to discoveries.
Galileo’s discoveries: Bottom line:
Galileo Galilei (1564-1642) was the first to observe the mountains and craters on the Moon, the phases of Venus, and the four largest moons of Jupiter. He also studied sunspots and proved that the Milky Way was a collection of stars rather than a huge cosmic cloud. For these and many other achievements, he is often called the “father of observational astronomy”. He made many scientific discoveries outside the field of astronomy as well. Want to be like Galileo? Use the Sky Tonight or Star Walk 2 app to locate Jupiter, Venus, and other celestial objects in real time and embark on your own astronomical journey!
Text Credit:Vito Technology, Inc.
