Galileo Galilei: Top 13 Facts for Kids about the Father of Modern Astronomy

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Updated on: Educator Review By: Michelle Connolly

On February 1, 1958, the Russians launched the first-ever artificial satellite to space. 

It was called Sputnik I. Sputnik I orbited the Earth from a height of 577 km at a speed of 8 km/s. It travelled a total distance of 70 million km, completing 1,440 orbits around our planet.

Three years later, Russian Astronaut, Yuri Gagarin became the first man to go to space. In the earliest space capsule ever made by humans, Gagarin orbited the Earth, just like Sputnik I, in 89 minutes. In July 1969, American Astronaut Neil Armstrong became the first human to step on the Moon.

On December 25, 2021, NASA launched the James Webb Space Telescope to an orbit that is 1.5 million km away from the Earth. The telescope, which orbits the Sun like the Earth and all other objects in the Solar System, sent us unbelievably marvellous images of long-dead stars by detecting infrared waves in. That happened in July 2022.

Such great technological advancements that allowed more space exploration were only made possible by the efforts of great scientists, astronomers, and mathematicians. Those are the people who have dedicated their lives to studying our planet, the Solar System, and outer space.

One, therefore, can never mention space without paying tribute to the man who was named the Father of Modern Astronomy: Galileo Galilei. And this is what today’s lesson is about.

So let’s hop into it.

Galileo Galilei LearningMole

(1) Who was Galileo Galilei?

Mostly known by his first name, Galileo was an Italian astronomer. Many people consider him a polymath because he had a wide knowledge in a variety of fields including mathematics, astronomy, physics, and philosophy. He was born in Pisa, Italy in 1564 and he lived until he became 77 years old.

Galileo made great contributions to astronomy which paved the way for everything we now know about the Solar System and the Universe. That is why he is considered the Father of Observational Astronomy.

In the past, the term observational astronomy referred to using a telescope to observe objects in the sky and study them. This was different from theoretical astronomy which refers to assuming things about space then looking for ways to prove them right or wrong.

So Galileo was given that name because he was almost the first person to point a telescope toward the sky. Through his telescopic observations, Galileo discovered a lot of things about Venus, Jupiter, Saturn, Neptune, as well as the Moon, our Moon.

After hundreds of years and by using modern technologies, almost all of Galileo’s observations and discoveries turned out correct.

(2) What is Galileo best known for?

Many people are familiar with Galileo because of his brave confrontation with the Roman Catholic Church back in the 17th century. At the time, the world was settled for a space model in which the Earth was the centre of the Solar System. On the other hand, the Sun and all other planets as well as the Moon revolved around the Earth. This model was called the geocentric model or the Ptolemaic system.

The Roman Catholic Church strongly believed in the geocentric model; the Earth was the centre of the Universe. That is why its reaction was pretty harsh when Galileo suggested otherwise.

Through his mathematical work and telescopic observations, Galileo came to the surprising conclusion that it was the Sun that was the centre of the Solar System. He also said that the Earth was just another regular planet among many others that revolved around the Sun; another speck of dust floating in the void of the infinite Universe.

Everyone was shocked and the Catholic Church rejected Galileo’s new space model. They even considered it against religion. 

(3) Did Galileo always want to become an astronomer? 

In fact, no. At least, not at the beginning.

Interestingly, Galileo was apparently hoping to become a musician. His father was a lutenist. The renaissance lute—not flute—was a string musical instrument that pretty much looked like the Middle Eastern Oud.

Galileo’s brother Michelagnolo Galilei became a famous composer and lutenist. Having grown up in such a music-filled atmosphere, Galileo probably hoped to become a musician too. He was even a good lute player himself.

Years went by and Galileo seemed to have lost interest in music. In his youth, he started preparing himself to become a priest. He took it so seriously that he went to study for it at a monastery in a nearby city.

But Galileo’s father disagreed. He wanted his bright son to take a completely different path in life. So he insisted on Galileo enrolling at the school of medicine at the University of Pisa.

Not a lutenist. Not a priest. Galileo was going to become a doctor.

Galileo Galilei LearningMole

(4) How did Galileo, then, become an astronomer?

As we have just mentioned, Galileo had given in to his father’s urging and agreed to study medicine. But that was not his fate.

Not long after his enrollment, Galileo left medical school and set off a totally unalike route. He turned to study mathematics after he abruptly fell in love with it all because of, you will not believe it, a chandelier.

One day, Galileo looked up and noticed a chandelier hanging down from the ceiling. It was not still but moving. The chandelier was forced to swing by the air currents. Galileo noticed that it took the same amount of time to travel back and forth.

When he got home, Galileo used two pendulums to repeat the process. He swung one of them at a wide angle and the other at a smaller angle and the same thing happened. Both pendulums took the same amount of time to go back and forth. It was not only that. The pendulum that travelled through a wider range was going faster than the one travelling through a smaller range.

That was the spark. And Galileo went straight ahead to his father and begged to let him study mathematics instead of medicine. While it is not recorded whether or not the father approved his son’s request, Galileo went on anyway to study mathematics as well as natural philosophy and fine arts.

Galileo’s passion and talent for maths and inventions sparked that he began applying what he studied to invent real tools. A few years after he started such studies, Galileo began teaching mathematics in terms of geometry and mechanics at the university. He also taught astronomy.

(5) Did Galileo invent the telescope?

Well, not really. This is one common misconception about Galileo. But he did not invent the telescope. Yet, he upgraded it.

It was a Dutch eyeglass maker named Hans Lippershey who invented the first ever telescope; an apparatus which could magnify far objects. After hearing of it, Galileo made a telescope with a three-time magnifying power. Then, he enhanced it to magnify objects up to 30 times.

This advanced telescope was referred to as the Galilean telescope. At the time, people also referred to it as a spyglass since it spied on objects that were barely, if not never, seen by the naked eye.

Interestingly, Galileo also made a good businessman. After he developed the telescope to a reasonably magnifying power, he sold it to merchants who found the new apparatus pretty useful when sailing at sea.

(6) What did Galileo discover about the Moon?

Galileo Galilei LearningMole

In the early 17th century, precisely in 1609, Galileo used his telescope to observe the Moon.

Long before that, people used to think that the Moon was a perfect, smooth, translucent sphere; a pearl hanging in our night sky. This notion was strongly believed by the public as many trusted personalities throughout history supported it.

If the Moon were really so, Its phases would look very smooth throughout the month. But what Galileo saw through his telescope contradicted this perfect sphere theory. He found out that the visible parts of the Moon were uneven. There were small shadows.

Using mathematics and supported by his observations, Galileo discovered that there are mountains and craters on the Moon. He was also able to calculate the height of some of these lunar mountains. This way, Galileo justified the shadows seen on the Moon’s surface and proved it was not as smooth as it was thought to be.

(7) What did Galileo discover about Jupiter?

In early 1610, Galileo made another great astronomical discovery.

Jupiter is easily visible with the naked eye in a clear night sky. So it was not much of a big deal when Galileo observed it. What was revolutionary was viewing Jupiter’s moons.

Using his more advanced telescope, Galileo was able to view three incredibly small objects near the largest planet. At first, he thought they were fixed stars. Fixed stars are so distant that their motion can only be detected when observed over a long period of time. In other words, fixed stars look fixed, motionless, in the sky. So that is what Galileo thought of those three dots near Jupiter.

But on the next night, Galileo found out that those ‘fixed’ stars changed their positions relative to Jupiter. That meant they were not fixed stars but rather something else. After a few observations and the discovery of one more of these objects, Galileo concluded that these four objects were orbiting Jupiter. They were Jupiter‘s moons. 

Galileo named these four moons after the Italian duke Cosimo II de’ Medici and his brothers. The reason for that is most probably gratitude. In 1610, Cosimo lawfully helped Galileo have more freedom in teaching mathematics at universities. Maybe that is how Galileo thought he could pay Cosimo back since it was ‘mathematics’ that contributed to discovering Jupiter’s moons.

Interestingly, Galileo was not the only astronomer observing the sky through a telescope. Many other astronomers were also pointing their telescopes at the sky and studying it. On the same night when Galileo discovered the Jovian satellites were moving, another German astronomer also observed the four moons of Jupiter!

(8) What reaction did the discovery of Jupiter’s moons spark?

The discovery of Jupiter’s moons was pretty much revolutionary. It can also be fairly described as controversial. Here is why.

Aristotle was one of the greatest and most influential philosophers ever lived on Earth. He was Greek and he lived almost a millennium before Galileo observed Jupiter’s moons. 

Despite his great contributions to science and philosophy, Aristotle was a human being after all. He was prone to making mistakes. For instance, he strongly believed in the geocentric model in which the Earth was the centre of the Universe. Thanks to Aristotle’s integrity, the large masses and later the Catholic Church believed in the geocentric model too, for centuries.

So when Galileo discovered there were four natural satellites orbiting Jupiter and not the Earth, many astronomers refused his idea. They thought Galileo must have been mistaken about something. German Astronomer Christopher Clavius was one of the few who believed Galileo because he also observed Jupiter’s moving moons.

This observation would go on and assist other discoveries by Galileo to become the cause for his unfair imprisonment later on.

(9) What did Galileo find out about Venus?

Through his telescopic observations, again, Galileo found out that Venus, the brightest, hottest, and second-closest planet to the Sun, had phases. These phases looked similar to the phases of the Moon.

The mere happening of these phases suggested that Venus must be orbiting the Sun and not the Earth. This was another point that ditched Aristotle’s claim of the Earth being the centre of the Universe.

(10) Did Galileo have any further Solar System observations?

He sure did. 

Like Jupiter, Saturn is always bright and visible in a clear night sky, after 10:00 pm, usually in the southeast. It has been viewed by our ancestors for thousands of years. With the naked eye, Saturn appeared like a bright dot and nobody knew anything about its rings. That was until Galileo aimed his telescope at the ringed planet in 1610.

When Galileo saw Saturn‘s rings, he could only see two small elliptical objects on each side of the planet. So he thought they were parts of it; that Saturn was a three-bodied object.

Sometime later, Galileo observed Saturn again and the attached bodies were invisible. He thought they disappeared. That happened because the Earth was on the same plane as Saturn. So the rings were perfectly horizontal to the observer on the Earth. That is why they looked as if they disappeared. 

This existence/non-existence of what he thought were twin bodies confused Galileo. Because he lacked a more advanced telescope, Galileo was not able to know what those Saturn bodies were. Despite that, he was the first to observe them and therefore receive credit for their discovery. 

It was not until 1655 that a Dutch astronomer named Christiaan Huygens using a much more advanced telescope could see and identify the true shape of Saturn’s rings.

In 1612 and 1613, Galileo also observed Neptune, the outermost planet of the Solar System. But again, just like what happened with Saturn, Galileo could not identify Neptune clearly. So he thought it was just a fixed star.

That said, Galileo recorded the position of Neptune in some drawings. More than two centuries later, a German astronomer called Johann Gottfried Galle could observe Neptune more clearly than Galileo did. According to Galle’s observation, the position of the blue planet was almost the same as that previously identified by Galileo. 

So technically, Galileo was the first to observe Neptune even though he did not know what it was.

In the years to come, Galileo would make many other observations about the Solar System and the Milky Way. Such discoveries were as thorough as they were contentious. They helped reveal the truth about how the Universe was operating. But they also contributed to sentencing the genius astronomer to life imprisonment.

(11) How was Galileo’s trial?

As we have mentioned earlier, many of the astronomical observations that Galileo made contradicted the geocentric model. On top of these discoveries were the phases of Venus and Jupiter’s moons. These findings proposed that planets must be revolving around the Sun and not the Earth. This model was called heliocentrism or the heliocentric model.

Interestingly, Galileo was not the first to propose the heliocentric theory. It was suggested more than 70 years earlier by another Italian astronomer called Nicolaus Copernicus.

The Catholic Church strongly adopted the geocentric theory. That is why it strongly opposed Galileo’s findings. It even went on and declared heliocentrism as unorthodox, completely heretical. That was in 1616.

Up until that time and a decade later, Galileo was free to continue his findings as no official condemnation was made against him. So he went on to propose the theory of tides which attributed them to the Earth’s motion; another thing that contradicted geocentrism.

After that Galileo wrote a book called Dialogue Concerning the Two Chief World Systems comparing geocentrism and heliocentrism. As you might have guessed, he defended heliocentrism in that book.

The book was published in 1632. Soon after its publication, it became widely popular. Such popularity was viewed as a threat by the Catholic Church. So the Roman Inquisition, which was a powerful office set by the Catholic Church to punish those found guilty of heresy, arrested Galileo and sent him to trial.

The judges strongly found Galileo suspect of heresy. As a result, Galileo was banned from teaching anything that would defend heliocentrism. He was also sentenced to life imprisonment at his home. Galileo never left his home after the trial in 1633 except for his grave in 1642. He died at the age of 77.

(12) How did the modern astronomical society pay tribute to Galileo?

With more technological advancements in astronomy, many of Galileo’s observations proved to be correct. Even most of his observations were somewhat accurate despite the not-that-advanced equipment he used back in the 17th century.

Having discovered that Galileo was right, the modern astronomical society wanted to honour him and free him from the unfaithful accusations. One way to do that was to rename Jupiter’s moons after him.

As a group, the four largest Jovian moons Galileo discovered are called the Galilean satellites. Individually, they are called, largest to smallest, Ganymede, Callisto, lo, and Europa. 

In addition, a spacecraft developed by NASA bore the name Galileo. It was intended to study Jupiter and its moons. Galileo was launched in October 1989 on Space Shuttle Atlantis.

The spacecraft was estimated to take 8 years and 1.5 months to reach Jupiter’s orbit. But it could successfully do so in only 7 years and 9.5 months, precisely entering Jupiter’s orbit on December 7th 1995.

Galileo orbited Jupiter and closely studied it as well as its four largest moons, the Galilean moons. Surprisingly, Galileo found evidence of liquid salt water underneath the thick sheets of ice on three of the Galilean moons. Such a discovery suggested the possibility of the existence of any lifeform on Jupiter’s moons.

Galileo also provided information about thunderstorms on Jupiter which were much, much larger than those happening on the Earth.

After spending nearly 14 years in space, Galileo’s mission ended on September 1, 2003. So NASA deliberately crashed the spacecraft into Jupiter. The scientists chose such a tragic ending because they feared if Galileo had been left in orbit, it may have collided with one of the Galilean moons. Such a collision was surely to contaminate any lifeforms scientists believed might have existed on these moons.

In February 1910, German Astronomer J. Helffrich discovered an asteroid orbiting the Sun. The asteroid is as big as a minor planet. It was named 697 Galileo in honour of Galileo.

In addition, American Astronomers Geoffrey Marcy and R. Paul Butler discovered a giant gas planet In April 1996. The planet is blue and is around 80% the size of Jupiter. It orbits another star that is 41 lightyears away from our Sun—one light year is 9.5 trillion km! This planet was called Galileo, again in honour of the great Italian astronomer.

Another thing that honoured Galileo was naming the year 2009 the International Year of Astronomy. This year was the 400th anniversary of the first time Galileo aimed his telescope at the sky, marking the birth of modern science and astronomy.

In the early 20th century, a museum was dedicated to Galileo Galilei, Museo Galileo, in Florence, Italy. Hosted in an 11th-century building, Galileo’s museum contains a large collection of Galileo’s scientific instruments which he invented himself. After some renovations, the museum has been welcoming visitors since 2010.

(13) Did the Vatican ever change their mind about Galileo?

Sadly, not until 1992!

The Roman Catholic Church, which was later given the Vatican City and accordingly referred to as the Vatican, had apparently clung to their accusation of Galileo for a tremendous period of time, around 359 years!

After innumerable great advancements in astronomy that proved Galileo’s findings were absolutely right, Pope John Paul II of the Vatican started an investigation to look into Galileo’s old accusation. That was in 1979. After 13 years, in 1992, the investigation concluded that the court had made a mistake by sending Galileo to trial. So a formal apology was made to him.

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