galileos observations
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Galileo sparked the birth of modern astronomy with his observations of the Moon, phases of Venus, moons around Jupiter, sunspots, and the news that seemingly countless individual stars make up the Milky Way Galaxy.
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.

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
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.

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