Galileo Galilei (1564-1642) made many advances in mathematics and physics, but is best remembered for his contributions to astronomy. He was central to challenging the idea that the Earth sat at the center of the universe with everything revolving around it (the ‘Ptolemaic system’). He also made important discoveries about the sun and moon. After Galileo, people began to recognize the solar system as it is known today, although there were still planets and moons left to discover.
Nicolaus Copernicus (1473–1543) had postulated that the earth orbited the sun, rather than vice versa. This idea was deemed unacceptable by the Catholic Church, but Copernicus’ theory had supporters among the scientific fraternity. Galileo was among them. He did not invent the telescope, but upon learning of its invention, he was able to build one for himself without seeing the original. It was this telescope that allowed him to observe the heavens.
Galileo’s most important discovery was that the planet Venus has phases, just like the moon. These are caused by its orbit around the sun – seen from Earth, the planet has different proportions of its surface illuminated at any time, depending on its position in its orbit. This is only possible because its source of light comes from the object it is orbiting, i.e. the sun. If, as in the Ptolemaic system, the Sun and Venus both orbited Earth, the source of illumination would come from behind Venus, and its phases would look completely different.
This was the first empirical evidence to support Copernicus’ theory, and its importance cannot be overstated. The move from the Ptolemaic to the heliocentric model of the solar system represents perhaps the greatest ever shift in the way mankind understands the universe. It is not surprising that the church persecuted Galileo for publishing this theory, as it challenged everything that was believed about the physical universe.
Another important discovery was a number of smaller bodies surrounding Jupiter. By observing the way that they appeared and disappeared, Galileo deduced that they were moons circling Jupiter. This was another important revelation, as it showed that it was possible for a planet to have bodies orbiting it even though it was itself orbiting the sun. An argument against Copernicus was that, if Earth orbited the sun and the moon orbited Earth, the moon would be left behind as Earth moved through space. Jupiter’s moons showed that this was not true.
Galileo was the first to see that the other planets in the solar system were disks, rather than stars. He saw Saturn’s rings, although his telescope was not powerful enough for him to realize what they were. He also saw Neptune, although he may not have realized it was a planet.
He identified the features visible on the moon as craters and mountains, which he was able to identify because of the shadows they cast, which changed depending on where the moon was in relation to the sun. Galileo was also the first to discover sunspots. These discoveries helped to disprove the idea of the heavenly bodies as perfect, unchanging spheres.
Galileo went blind in later life, and it has often been suggested that this was caused by his looking at the sun through a telescope too often. That provides another valuable lesson for astronomers, although presumably not one that Galileo intended.
By being able to better observe the heavens than anyone before him, Galileo was able to make discoveries which shattered the way the universe was believed to work and laid the foundations of modern astronomy. Even the church forgave him, eventually.