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Galileo Galilei lived, 1564-1642 in Italy. Galileo is famous for using the newly invented telescope and making it more advanced so he could observe celestial bodies. He was the first to observe Io, Europa, Callisto, and Ganymede orbiting Jupiter. In these days they are called Galilean satellites. This showed that not only earth can be orbited, which gave more evidence that the our Solar System is heliocentric. He also observed that Venus goes through phases like our moon. Phases are when sunlight strikes different spots. We use his ideas today because they are correct. His model changed the way we thought, and it was very advanced for the 1600s.

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Johannes Kepler lived, 1517-1630 in Germany. He formatted 3 principles which are now known as Kepler’s laws. “His first law states that planetary orbits are ellipses that are with the sun at one focus. The second law is that planets rotate faster in their own orbit when closer to the sun. The third law relates the distance of a planet from the sun to the time it takes to go once around its orbit.” He also invented ellipses which I understand as ovals that the planets rotate on. His expansion on Copernicus' ideas helped support the idea of heliocentric and that if other planets have moons, Earth must be rotating something as well.

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Nicolaus Copernicus is a Polish astronomer 1473-1543. Copernicus basically took Ptolemy's idea, that the planets are spherical and move in a circular motion, and made it a more efficient way to see space without it being so complicated. His model was heliocentric. His model is one we are currently using today because it is the most accurate on what the universe looks like and how it works.

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Ptolemy was born about 100 CE and died in 170 CE in Alexandria, Egypt. He developed a geocentric model that was used by astronomers for quite a few centuries after he built it. He believed that “a celestial body traveled at a constant speed in a perfect circle” this helped astronomers develop an understanding of motion looked like in space way in the future. Model was geocentric. Ptolemy is still being talked about today because of his thought about how the planets are circular and are on a rotating axis.

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Aristarchus was born about 310 BCE and died 230 BCE in what is now known as Greece. He attempted to measure the distances from Earth to the Sun and moon, but he didn’t know how large space was compared to earth and voids which gave him a much too small ratio. But he used distances, observation, and geometry to solve a scientific equation. He made a heliocentric model. Aristarchus is still famous today because his model was the first heliocentric model of space with distances.

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384 BCE-322 BCE, in what is now known as Greece.

Aristotle is famous for thinking that the Earth was the center of the universe. He made a model that he later realized that the Earth rotates the sun because the relative positions of the stars would change, which would later be called the parallax. This is important because in such an early time period we already were learning things about space without him giving us a basic understanding. He thought that the universe was around earth, geocentric. The reason we are still talking about him today is because he was the first person to really even have thoughts about space, which made him famous to this day.