What did Galileo discover?

Galileo pioneered the use of the telescope for observing the night sky. His discoveries undermined traditional ideas about a perfect and unchanging cosmos with the Earth at its centre.

Who was Galileo Galilei?

Galileo was born in Pisa, Italy on 15 February 1564 (Julian calendar; 26 February 1564 by our modern day Gregorian calendar), the first of six children.

Prior to Galileo's conflict with the Church, the majority of educated people in the Christian world subscribed either to the Aristotelian geocentric view that the earth was the center of the universe and that all heavenly bodies revolved around the Earth, or the Tychonic system that blended geocentrism with heliocentrism.

His championship of the Copernican (Sun-centred) planetary system brought him into serious conflict with the Church, which forced him to make a public recantation and put him under restriction in later life.

What is Galileo most famous for?

Galileo, though not the first inventor of the refracting telescope, significantly enhanced its power. In 1609, he learned of the spyglass and began to experiment with telescope-making, grinding and polishing his own lenses. His telescope allowed him to see with a magnification of eight or nine times, making it possible to see that the Moon had mountains and that Jupiter had satellites.

Image of Galilean model telescope
Replica of a handheld Galilean telescope

What did Galileo discover?

Galileo used observation and experimentation to interrogate and challenge received wisdom and traditional ideas. For him it wasn’t enough that people in authority had been saying that something was true for centuries, he wanted to test these ideas and compare them to the evidence. At the time this was quite a shocking idea, and was one of the reasons that he got into trouble. He discovered...

1. Craters and mountains on the Moon

The Moon’s surface was not smooth and perfect as received wisdom had claimed but rough, with mountains and craters whose shadows changed with the position of the Sun. Galileo was able to use the length of the shadows to estimate the height of the lunar mountains, showing that they were similar to mountains on Earth. 

Photograph of Mars Next to the Moon © Eric Toops, Astronomy Photographer of the Year Planets, Comets and Asteroids Commended 2015
Mars Next to the Moon © Eric Toops, Astronomy Photographer of the Year Planets, Comets and Asteroids Commended 2015

2. The phases of Venus

The planet Venus showed changing crescent phases like those of the Moon, but their geometry could only be explained if Venus was moving around the Sun rather than the Earth. This undermined the idea that everything in the heavens revolved around the Earth (although it was consistent with the Tychonic system as well as the Copernican one).

Venus Phase Evolution © Roger Hutchinson Winner
Venus Phase Evolution © Roger Hutchinson Winner

3. Jupiter’s moons

The planet Jupiter was accompanied by four tiny satellites which moved around it. These are now known as the Galilean moons: Io, Ganymede, Europa and Callisto. Again, this showed that not everything in the heavens revolved around the Earth.

4. The stars of the Milky Way 

Galileo saw that the Milky Way was not just a band of misty light, it was made up of thousands of individual stars.

Photograph of The Milky Way View from the Piton de l’Eau, Réunion Island © Luc Perrot, Astronomy Photographer of the Year Earth and Space Commended 2012
The Milky Way View from the Piton de l’Eau, Réunion Island © Luc Perrot, Astronomy Photographer of the Year Earth and Space Commended 2012

5. The first pendulum clock

If that wasn’t enough, as well as Galileo’s contributions to astronomy, he also designed a major component for the first pendulum clock, Galileo’s escapement. This design, however, went unbuilt until after the construction of the first working pendulum clock by Christiaan Huygens.