The first telescopes, invented in 1608, consisted of two simple concave and convex lenses. When placed at a specified distance apart from one another, these lenses could magnify an image about three times to make distant objects appear nearer. Although the actual instruments were not readily available, news of the invention spread quickly through Europe. By mid-1609, Galileo had heard about this new device and, despite never having seen one, reproduced a low-magnification telescope using his own materials. In fact, he quickly improved upon the existing design by grinding his own lenses to make a telescope with a magnification power of eight. Within months, he had created a telescope with a magnification power of about 20.

Although Galileo did not invent the telescope, he was the first person to use it to systematically study the sky, and what he found was amazing. Through his careful observations, he recorded evidence that would eventually overthrow long-held beliefs about the universe.

In Galileo's time, the traditional view of the universe was adopted from the ideas of Ptolemy. The majority of people believed that Earth was the center of the universe and that all other celestial bodies were perfect and revolved around it. However, Copernicus had proposed a Sun-centered system, which Galileo instinctively believed was correct. Using his telescope, Galileo was able to gather quantitative evidence in support of the Copernican theory. He found many things that contradicted the geocentric view: viewed through the telescope, the Sun and Moon had visible imperfections such as sunspots and craters, Jupiter had satellites orbiting it, and Venus was orbiting the Sun.

When Galileo published his book debating the Copernican and Ptolemaic systems, the Church found him guilty of heresy and sentenced him to house arrest for the rest of his life. Today, Galileo is recognized for his revolutionary work in both astronomy and physics, and has been referred to as "the father of modern science." In addition to his findings in support of the Copernican system, his theories regarding falling objects and objects in motion laid the groundwork for classical mechanics. He was also a pioneer in the approach we call the scientific method, which utilizes observation and experiment. In 1992, three hundred and fifty years after his death, the Church officially apologized for condemning Galileo and his scientific research.

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