Galileo Galilei (1564-1642)
Galileo was one of the greatest astronomers in the world. He changed the way we think about the universe. Galileo was the first person to use a telescope to look at the universe. He studied the sun, the planets, and the moon.

Galileo was born in Pisa, Italy, on February 15, 1564. Did you know Galileo was born in the same year as William Shakespeare? It is also the same year in which Michelangelo died.

When Galileo was born, Italy was not called Italy. Instead, Italy was made up of independent city-states. The city-states were in regions controlled by powerful families. Pisa was located in the Tuscany region and controlled by the Medici family.

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Young Galileo
Galileo's father was named Vincenzio, and his mother was named Giulia. Vincenzio was a gifted musician and mathematician, and he was also a cloth merchant to earn extra money. Galileo was educated by his father until he was 10 years old.

In 1574, Galileo's family moved to Florence. The next year, Galileo attended the Benedictine Monastery of Santa Maria di Vallombrosa, and he studied Latin, Greek, music, logic, and religion. Galileo's education prepared him to study at a university.

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The Earth Revolves Around the Sun
Before Galileo, people believed the earth was the center of the universe and the sun revolved around the earth. This theory is known as the Ptolemaic theory. It is named after the Roman astronomer Ptolemy. The Greek philosopher Aristotle and the Roman Catholic Church also believed the sun revolved around the earth.

In 1543, Nicolaus Copernicus published a new theory stating the earth revolves around the sun. This is known as the Copernican theory. This theory was considered a serious challenge to the Ptolemaic theory and the Roman Catholic Church. Copernicus died before he could prove his theory.

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The Moons of Jupiter
Galileo studied the Copernican theory and set out to prove or disprove it. He did this by observing the universe through the telescope.

In 1610, Galileo made an important discovery about the planet Jupiter. He noticed there were four "stars" near Jupiter. Galileo then realized these "stars" were revolving around Jupiter. These "stars" were really moons. This observation helped prove the Copernican theory that the earth revolves around the sun.

People knew the moon revolved around the earth. They thought if the moon revolved around the earth, then the sun must also revolve around the earth. However, Galileo's discovery of four moons revolving around Jupiter proved one object in space can revolve around a second object in space while the second object revolves around a third object in space. To put it in a more simple way, it proved the moon can revolve around the earth while the earth revolves around the sun.

After Copernicus and Galileo, we know the earth revolves around the sun.

As you know, Galileo discovered four of the moons of Jupiter. Did you know Jupiter's other moons have been discovered by observations from Earth and by the spacecraft Voyager? In May, 2002, astronomers observed eleven new moons around Jupiter. This made Jupiter the planet with the highest number of known moons in our solar system. Until the May, 2002, discovery, Saturn held the record for the most known moons with 30. In February, 2003, seven more moons were discovered around Jupiter. In March, 2003, four more moons were discovered around Jupiter. As of April, 2003, Jupiter has 60 known moons. Keep checking back because the number of moons could increase.

The four moons discovered by Galileo are Jupiter's largest moons. They are Io, Europa, Ganymede, and Callisto. Today, these moons are named after Galileo and are known as the Galilean moons.

Jupiter is the largest planet in our solar system. Our solar system is in the The Milky Way galaxy.

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Our Moon
In December, 1609, Galileo looked at the earth's moon through the telescope, and he observed many things regarding the moon that could not have been known by looking at the moon with the human eye.

First, the moon does not create light itself. The moon shines because it reflects light from the sun.

Second, the moon's surface is not smooth and perfect as Aristotle had believed. It has mountains, craters, and valleys just like the earth.

In 1637, Galileo also discovered the moon has a regular libration. A libration is a rocking movement. This means a person can always see more than half of the moon's surface.

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Observing the Universe
Galileo made other important observations of the universe with the telescope. He discovered:

 the Milky Way is made up of billions of stars

 Venus has phases similar to the moon

 Saturn has "ears" which are the rings of Saturn

 sunspots move across the surface of the sun

These observations by Galileo led to the development of astronomy as a major branch of science. Astronomy is the study of stars and planets.

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The Father of Modern Science
Galileo is known as the Father of Modern Science. He was one of the first people to use the scientific method to conduct experiments. This means Galileo observed a situation, developed a theory, and then carried out experiments to test his theory.

The idea of testing and experimenting to prove or disprove a theory was revolutionary during Galileo's time. Galileo used the scientific method to disprove some of the beliefs of ancient Greek philosophers, like Aristotle.

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The Laws of the Pendulum and Falling Bodies
Galileo began experimenting with pendulums after he saw a chandelier swinging in the Pisa Cathedral. He noticed each swing of the chandelier took the same amount of time, even though each swing was shorter than the previous swing. This led to Galileo's discovery of the Law of the Pendulum.

Galileo also conducted experiments with falling objects. Aristotle believed heavier objects fall to the ground faster than lighter objects. As legend has it, Galileo dropped two objects from the Leaning Tower of Pisa to prove or disprove Aristotle's theory. These objects were the same size, but different weights. They fell to the ground at the same speed. This disproved Aristotle's theory and led Galileo to discover the Law of Falling Bodies.

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Honoring Galileo
Galileo died on January 8, 1642, in Arcetri, near Florence, Italy. He was 77 years old. He is buried in the Santa Croce cemetery in Florence.

Galileo's observations of the planet Jupiter were very important. To honor his contributions, the four moons of Jupiter which Galileo discovered are called the Galilean Moons. These are Jupiter's largest moons.

There is also a spacecraft named after Galileo. It was launched on October 18, 1989, to orbit Jupiter. It went into orbit around Jupiter in 1995.

Galileo proved the earth revolved around the sun. He was a great astronomer and scientist, and he is a hero in our hearts.

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