Galileo Galilei

Galileo Galilei
Credit: J Sustermans

Occupation: Astronomer, Mathematician, Engineer

Year born: 1564

Research Areas: speed, velocity, motion, astronomy, tides

"See now the power of truth; the same experiment which at first glance seemed to show one thing, when more carefully examined, assures us of the contrary."

Source: Discourses and Mathematical Demonstrations Relating to Two New Sciences (1638)


Early Life

Galileo was born in Pisa, in the Duchy of Florence (now a part of Italy). He went to university in Pisa to study medicine. However, after a short while he chose to focus on philosophy and maths instead. 

Career Highlights

When he was 25, Galileo became a professor of maths. He wanted to understand the maths of the world around him and studied the motion of objects for more than 20 years. In one of his most famous experiments, Galileo dropped 2 weights from the tower of Pisa. He did so to prove that the speed at which heavy objects fall, did not depend on their weight. 

When he was in his 40s, Galileo heard about a new invention from the Netherlands, called the telescope. He did not have copies of the plans but was able to design his own using trial and error. Galileo’s first telescope could magnify the night sky by 3 times. His later designs were able to magnify the sky by 30 times!

Galileo used his telescope to study the night sky. On 7th January 1610, he saw “three fixed stars, totally invisible by their smallness”, near to the giant planet Jupiter. Over the next few nights, he saw that these 3 points of light moved with the giant planet and sometimes went behind it. Galileo knew he had discovered 3 moons of Jupiter. He found a fourth moon a few days later, on 13th January.

Galileo made many discoveries in the field of astronomy. He was the first to find out that the Earth's Moon had mountains and craters and was not a perfect, smooth sphere. He also observed the phases of the planet Venus. This confirmed earlier predictions by Nicolaus Copernicus and his model of the Solar System which had the Sun at its centre. 

Copernicus’ ideas did not agree with the Bible, so the Catholic Church had begun to view them as a threat. In 1616, the Church warned Galileo to stop acting as if Copernicus’ theory was fact. In 1633, when Galileo was 69, the Church found him guilty of heresy. They sentenced him to life imprisonment. Galileo spent the rest of his life in a large house in the hills and died at the age of 77.


Galileo saw that he could explain the laws of nature using maths. He would also change his ideas based on what he observed. In 1623, Galileo published 'The Assayer'. This set out a new scientific method and was a huge influence on modern science.  

The 4 large moons of Jupiter that Galileo discovered are now known as the Galilean moons. A NASA space probe which studied Jupiter and its moons was also called Galileo.

Other interests

Galileo also designed and built many scientific tools. These included a thermoscope, now known as a Galileo thermometer.