![]() ![]() This could be seen if you took a series of photos every night for a month with a chosen star highest in the sky and laid them over the top of each other. But if you note the location of a planet relative to the background stars, and note its location again several nights later, you will see that it has moved. ![]() The term “planet” originates from the Greek word for “wanderer.” This phenomenon can’t be really be seen on any given night. They change their position in the sky from night to night. However, planets move in the sky relative to the pattern of background stars. However, the pattern of stars that is seen in the sky, how far apart a pair of stars are seen from each other, stays the same over time scales of thousands of years. In the night sky, stars rise and set due to the rotation of Earth. This is illustrated in the video Retrograde Motion (6 minutes, 25 seconds). His model was very successful, however, in solving the problem of retrograde motion in a very elegant manner. Although this solved many longstanding problems in the Ptolemaic model, Copernicus still believed that the orbits of planets must be circular, and so his model was not much more successful than Ptolemy’s in predicting the position of the planets. Copernicus proposed that the Sun was the center of the Solar System, with all of the planets known at that time orbiting the Sun, not the Earth. The astronomer given the credit for presenting the first version of our modern view of the Solar System is Nicolaus Copernicus, who was an advocate for the heliocentric, or Sun-centered model of the solar system. However, because even in its most complex form it still produced errors in its predictions of the positions of the planets in the sky, some astronomers continued to search for a better model. The geocentric model of the Solar System remained dominant for centuries. ![]()
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