Although many people believe that Galileo invented the telescope, this is a misconception. However, Galileo was one of the first people to turn his telescope towards the heavens to view celestial bodies. What distinguished Galileo from other early astronomers was that Galileo was the first to publish the results of his observations and he created a sensation.
Galileo was the first born child of Vincenzo Galilei and Giulia degli Ammannati and was born on February 15, 1564 in Pisa, Italy. He became a student at the University of Pisa where his father wanted him to study medicine. Galileo became a mathematician and eventually became a lecturer at the University of Pisa in 1588. Galileo became the chair of mathematics at the University of Padua in 1592. He stayed there until 1610. In 1609 Johannes Kepler published his Astronomia Nova which contained his 1st two laws which were that planets move in an elliptical orbit with the Sun as on of the foci and a planet sweeps out equal areas in equal times. In May of the same year, Galileo heard about an invention in the Netherlands that allows you to see objects that are far away as if they are close by. This invention was to eventually be called a telescope. It was invented by a man named Hans Lipperhey. He discovered that holding two lenses up some distance apart brought objects closer. In June of the same year, Galileo recreated the invention he had heard about and created a 3 power telescope. In the Fall of 1609, Galileo made improvements to the telescope and began to observe celestial bodies. In December of 1609, Galileo made a series of observations of the moon starting on November 30th and ending December 19th. Galileo could only see about a quarter of the Moon's surface at a time. He would have to move his telescope each time to see more of the Moon's face. On January 7th, 1610, Galileo observed three bright little stars by Jupiter. By January 15th, Galileo had figured out that Jupiter had four little satellites. Today they are called the Galilean Satellites. When Galileo published his findings that Jupiter had 4 moons that orbited it, it caused a wave of controversy. At that time everyone believed that everything in the sky orbited the Earth.
In February of 1610, Galileo continued making observations and made maps of star formations. Galileo observed many things with his telescope. He saw craters and mountains on the moon, sunspots on the Sun as well as the rings of Saturn. The rings appeared as "horns" because his telescope could not resolve the gap between the rings.
Galileo began to travel all over Italy sharing his observations. Others around the world began to verify and expand upon his observations. In December of 1610, Galileo confirmed that Venus goes through phases like the Moon. He observed that Venus appeared small in its gibbous phase and appeared larger in its crescent phase. This was proof to him that Venus goes around the Sun. Galileo's published works were widely read and caused such a stir. This eventually led to the Roman Catholic Church getting involved. Galileo's publishing of the Sun having "spots" was on the verge of blasphemy and heresy. The church attacked Galileo's ideas because they could not be reconciled with certain Bible passages or with the writings of Aristotle and Plato. The Sun was a heavenly body and thus had to be perfect. Claiming that the Sun had "spots" was an attack on God's beauty and perfection. Galileo was first censured by the church, then, he was excommunicated. Finally, Galileo was held under house arrest until his death in 1642. It is very important to know that you should never look directly into the Sun with a telescope. It could damage your eyes irreparably. It has been reported that Galileo was to have damaged his eyes from his observations of the Sun. There are safe ways to observe the Sun with a telescope and can easily be found on the internet.
Before the widespread use of the telescope, people could not clearly see what was happening in the nighttime sky. Much of their ideas and beliefs about the universe were based on philosophy and religion. Galileo began the trend to observe and measure objects in the sky. This showed that the worlds above seemed to obey the same physical laws that were known to work on earth. An example of this was Galileo was able to determine the height of mountains on the Moon after observing their shadows move on the Moon's surface.
To think that an ordinary person could actually know something about distant worlds just by writing down what is seen and then applying basic mathematics to the problem was a revolutionary idea. Due to Galileo's extensive observations of the Moon, we will next take a look at Earth's Moon.