At the end of 1608 Galileo had an idea that forever revolutionized astronomy: using the telescope to look at the sky.
The astronomer, from his home in Padua, began to point his instrument in different parts of the sky and realized that the telescope allowed him to see stars otherwise invisible to the naked eye. This was already an extraordinary discovery in itself, but the true revolution came in the first days of January 1609, when Galileo observed the Moon.
According to Aristotelian cosmology, all heavenly bodies belonged to the domains of perfection and thus had to be perfectly spherical and regular, including the Moon. Our satellite, however, also by eye has some color variations, which for the beliefs of the time were a problem. In the Middle Ages these were explained as differences in density from one part of the surface of the Moon: thus at least its sphericity, and together with it Aristotelian cosmology, was saved.
Galileo's observations, published in March in the Sidereus Nuncius with a further myriad of findings, destroyed the beliefs of the time about the perfection of celestial objects. Galileo discovered that the surface of the Moon had structures similar to Earth’s, such as plateaus, mountains and craters.
In particular, Galileo used the terminator, the line which divides the hemispheres night and day, to reconstruct the morphology of the lunar surface. In this region, in fact, the sun’s rays arrive very inclined and the mountains and other structures project long shadows easily observable from Earth.
Galileo’s observations, some of which are shown here thanks to his magnificent drawings, thus allowed the demolition of all Aristotelian beliefs and laid the foundations for the development of modern astronomy.
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