In an article I located in Astronomy magazine online entitled, What are the Accepted Proofs that Earth Revolves Around the Sun? When Did This Realization Take Place?, the author quickly explores how the model of our solar system has changed over time, who was responsible for the ideas, and where we are today in our thoughts. The author starts the reader out by pointing to Greek cosmology and its role in how we view the universe. He brings up Aristotle’s geocentric model and how it remained the consistent thought of the universe until it was questioned by Aristarchus.
The reader’s attentions are then turned to Galileo and his views through a very early version of a telescope. According to the author, Galileo’s views of Venus lead him to believe that Venus orbited closer to the Sun than Earth. As our telescopes became more advance, our ability to see the motion of the Earth became clearer. The author continues to lead the reader by showing them the Earth’s motion proved that it wasn’t stationary and lead other astronomers to begin to question the geocentric model of the universe. But, the time frame for this detection took a while.
The next astronomer mentioned by the author was that of Friedrich Wilhelm Bessel who measured the parallax of a specific celestial body. The author ends the article with a mention of English astronomer James Bradley and how Bradley was able to learn more about the Earth and its rotation around the Sun at a distinct tilt.
The geocentric view of the universe took a while to unravel because of the lack of ability to truly test the theories that may have been different. The idea of the Earth being the center of the universe was established by the Greeks. The Greeks were the first to not only try to look at the universe and understand how it works but to also write down their findings and reason among themselves regarding what they had found. Aristarchus, although Greek, questioned the idea of Earth being the center of the universe and thought that the model of the solar system should have been Sun-centered. The idea wasn’t widely accepted and was hard to prove at the time. The Greek philosophers encountered a problem when they began to observe retrograde motion among the planets.
The Greeks utilized the Ptolemaic model to explain why some planets appeared to move in retrograde motion. The Ptolemaic model, named after Claudius Ptolemy, attributed the retrograde motion to the planets moving around in their own circle while traveling around Earth which supposedly caused the planets to appear to travel backwards at certain points in their orbit. The circles the planets traveled in were called epicycles and the larger orbit around Earth that they traveled was called a deferent.
As time went on, philosophers and early astronomers (although not called astronomers) began to notice that the planets didn’t travel in the same direction as Earth. Hipparchus determined that the Earth moved in a way that allowed the celestial bodies in the sky to shift. In other words, certain celestial bodies did not always reside in the same position. For example, Polaris is considered to be our North Star. However, it was not always the North Star and will not always be our North Star because of the motion of Earth.
Copernicus really proposed the heliocentric model of the universe. He questioned the geocentric model and offered an explanation for the retrograde motion of the planets. Galileo helped to contribute to the idea that the Sun was the center of the universe through his observation of different planets and the celestial bodies that orbited them. Brahe measured the night sky with a giant protractor but struggled to prove that the Sun was the center of the Earth because of his lack of ability to accurately compute his findings. Kepler utilized his gifts in math and science to make models of Brahe’s work. He ended up discovering that the planet’s don’t travel in circles but in ellipses and the fact that they do orbit Earth.
In our lecture tutorials, we were able to confirm through practice inputting data that celestial bodies or planets that appear to move backward are actually moving in ellipses. During a simulation in the overhead dome, we watched the planets as they traveled in their orbits in retrospect to the Sun and Earth. When Mars traveled, it appeared to be moving backwards temporarily. We learned that as Earth moves in its orbit around the Sun, it moves faster than certain planets like Mars (side note: which we later found out also has to do with the Earth’s closer distance to the central star, the Sun). As it moves, it is as if it laps the other planet and the planet appears to be moving backwards temporarily.
It does amaze me how with the limited equipment and knowledge the early philosophers and astronomers had, they were still able to figure out some of the complexity that is our universe. Kepler’s ideas still hold true today.