From the Moons of Jupiter to the Laws of Motion: The Contributions of Galileo Galilei to Astronomy and Science

~7 min

February 15, 2024, marks Galileo Day and the 460th birthday of Galileo Galilei — one of the most influential scientists in history. Galileo's contributions to astronomy and physics were numerous and far-reaching. We invite you to join us as we celebrate Galileo Day and honor the man who changed the course of science forever.


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

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.

The phases of Venus

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.

Four largest moons of Jupiter

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.

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 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
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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.

By introducing people to the night sky and helping them explore the universe, Sky Tonight continues Galileo's legacy. It encourages people to engage with science and technology in new and exciting ways. So if you're looking to explore the night sky, download the stargazing app Sky Tonight today and start your journey into the universe!

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.