Galileo Galilei is one of the most influential figures in modern astronomy. While he did not propose the idea that the sun was at the center of the solar system, he strongly supported it. Galileo proceeded to make a number of observations that strongly supported the claim that the Earth and other planets revolved around the sun.
During Galileo's time, there were two leading beliefs about how the heavenly bodies were organized. The first was the geocentric model, in which the heavenly bodies rotated around the Earth. This was first converted into a scientific theory by the philosopher Ptolemy. Later, Ptolemy's model was adopted by the Roman Catholic Church. The second model was the heliocentric, or sun-centered, model, which proposed that the Earth and other heavenly bodies rotated around the sun. In 1543, The astronomer Nicolaus Copernicus published a work that proposed a scientifically comprehensive heliocentric model of the solar system. Galileo, who was born in 1564, supported this model. Galileo was prosecuted by the Inquisition twice for heresy as a result of publishing his findings in support of a heliocentric model. In 1616, he was warned not to continue publishing his support of a heliocentric model. However, he continued to do so, and in 1633 he was convicted of heresy and lived the rest of his life under house arrest.
Phases of Venus
One of Galileo's contributions in support of a heliocentric model was the observation that Venus had phases similar to those of the moon. Galileo was able to observe this because he made significant use of telescopes, which were a relatively recent invention in his day. He was the first person to develop and use the telescope for astronomical observation. Through his telescope, he saw that Venus went through phases. The only way that this would be possible, he concluded, would be if it were between Earth and the sun. In a geocentric model with Venus between the Earth and the sun, Venus would appear in a crescent or new phase. However, Galileo observed Venus in a complete range of phases. Hence, Galileo's observations of Venus supported a heliocentric model.
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Moons of Jupiter
Galileo also discovered that Jupiter had moons orbiting around it. Through his telescope, Galileo observed Jupiter's four largest moons changing position around the planet. If this were the case, that meant that the heavenly bodies could continuously circle things other than the Earth. The geocentric model proposed that the Earth was the only object around which heavenly bodies rotated, so this observation also supported a heliocentric planetary model.
Sun and Moon
Galileo also observed that the sun and moon were imperfect bodies. One of the philosophies that accompanied Ptolemy's planetary model was that the heavenly bodies were smooth, perfect spheres. With his telescope, Galileo was able to observe the sun's sunspots and the moon's craters. This revealed that these bodies are not perfect, but rather, subject to alterations by physical processes and interactions with other heavenly bodies.
Galileo's contributions were significant because they were revolutionary observations of physical phenomena. Other scientists helped to define the laws that, in combination with Galileo's observations, helped create a new model of the solar system. Johannes Kepler, for example, described laws of motion in which bodies orbited in ellipses, rather than perfect circles. Furthermore, Issac Newton later developed laws of motion and gravity that described all bodies in the universe. All of these developments were fundamental to clarifying a heliocentric model of the solar system.