Ptolemy's Model Of Planetary Motion example essay topic

Claudius Ptolemy was a citizen of the Roman Empire, who lived around the year 100 AD. Ptolemy was influential in the fields of mathematics, astronomy, and geography. In astronomy Ptolemy relied somewhat on the theories and observations of previous astronomers, such as the geocentric model designed by Aristotle; to create his own model of our solar system. Ptolemy's model of planetary motion was one of his greatest contributions to the science of astronomy.

The model (seen in 1.1) was based on a geocentric theory, and was so important at the time because it seemed to accurately explain the motions of the planets and the heavens. Ptolemy believed that each planet moved on a small circle, called an epicycle, and that these epicycles moved on a larger circle called a deferent. He explained the movement of the stars by theorizing that they existed on a separate celestial sphere outside the planetary spheres. Ptolemy was very adamant that planetary motion should be explained using only uniform circular motion. As shown in diagram 1.1 Ptolemy also noted that Venus and Mercury's motions were linked by the Sun.

He theorized that there was an imaginary line connecting the center of their epicycles that would in turn, connect the Earth to the Sun. This imaginary line also ran in parallel to the radi uses connecting the other planets to their own epicycle. Ptolemy's model was seen as the dominate explanation for planetary motion for centuries; due mainly to social, political, and religious reasons that favored, not only a Earth centered solar system, but an Earth centered universe. It wasn't until 1543 that Ptolemy's model met its first real competition in the form of the Copernican model. Nicolas Copernicus was born in Poland in 1473. Much like Ptolemy, he studied mathematics and astronomy.

Unlike Ptolemy however, who was known for his Earth-centered model of the universe, Copernicus varied radically from many of the thinkers of his time and theorized a heliocentric, or Sun-centered, model of the universe. Although Ptolemy's model was good at predicting the motion of the planets, Copernicus observed that it was not precise, and that, in fact, it was getting progressively worse at predicting planetary motion as the centuries went by. Copernicus also disagreed on Ptolemy's use of numerous epicycles to explain the retrograde motion of the planets. By having the Earth orbit the Sun along with the other planets he eliminated the need for numerous and large epicycles. However, he did not do away with the use of epicycles all together; in fact, they were still necessary for his model to work accurately. Much like Ptolemy, one of the faults in Copernicus' model was the theory that planetary motion must be uniformly circular.

As we now know, the orbits of the planets are not circular but elliptical. This fault made epicycles necessary, there were however, fewer epicycles needed in Copernicus' model compared to Ptolemy's. The Copernican model, by putting the sun at the center, and the Earth as the third planet of the inner solar system, set up the ordering of planets that we still use today. This new ordering explained the varying degrees of brightness of the planets as seen from Earth, because it was now understood that the planets were at different distances from Earth; unlike the Ptolemaic model which theorized that the planets were all the same distance from the Earth. Copernicus also theorized that planets with smaller orbits moved faster than planets with larger orbits. Unlike Ptolemy, who believed the stars moved on their own separate celestial sphere, Copernicus believed that the stars were fixed objects that were quite distant from Earth.

He theorized that the Earth rotated once every 24 hours, which would give the appearance that the stars were moving across the sky in the opposite direction of the Earth's orbit. The main differences between the Ptolemaic model and the Copernican model are their beliefs in a geocentric and heliocentric system of the universe. Both models expressed the theory that the planets had a uniform circular orbit, and therefore, both had to rely on the use of epicycles to predict the motion of the planets. Both models were highly influential in their time, and in the years to follow; both making a significant contribution to the science of astronomy..