In addition, Ptolemy's model was the first astronomical system that offered a complete and detailed account of how the universe worked. Not only did his model resolves issues arising out of the Ptolemaic system, it offered a simplified view of the universe that did away with complicated mathematical devices that were needed for the geocentric model to work.
And with time, the model gained influential proponents who contributed to it becoming the accepted convention of astronomy. The geocentric model, in which planet Earth is the center of the universe and is circled by the sun and all the planets, had been the accepted cosmological model since ancient times.
By late antiquity, this model had come to be formalized by ancient Greek and Roman astronomers, such as Aristotle — BCE — who's theories on physics became the basis for the motion of the planets — and Ptolemy ca.
The geocentric model essentially came down to two common observations. First of all, to ancient astronomers, the stars, the sun, and the planets appeared to revolve around the Earth on daily basis. Second, from the perspective of the Earth-bound observer, the Earth did not appear to move, making it a fixed point in space.
The belief that the Earth was spherical, which became an accepted fact by the 3rd century BCE, was incorporated into this system. As such, by the time of Aristotle, the geocentric model of the universe became one where the Earth, sun and all the planets were spheres, and where the sun, planets and stars all moved in perfect circular motions.
However, it was not until Egyptian-Greek astronomer Claudius Ptolemaeus aka. Ptolemy released his treatise Amalgest in the 2nd century BCE that the details became standardized. Drawing on centuries of astronomical traditions, ranging from Babylonian to modern times, Ptolemy argued that the Earth was in the center of the universe and the stars were all at a modest distance from the center of the universe. Each planet in this system is also moved by a system of two spheres — a deferent and an epicycle.
The deferent is a circle whose center point is removed from the Earth, which was used to account for the differences in the lengths of the seasons. The epicycle is embedded in the deferent sphere, acting as a sort of "wheel within a wheel". The purpose of he epicycle was to account for retrograde motion, where planets in the sky appear to be slowing down, moving backwards, and then moving forward again. Unfortunately, these explanations did not account for all the observed behaviors of the planets.
Most noticeably, the size of a planet's retrograde loop especially Mars were sometimes smaller, and larger, than expected. To alleviate the problem, Ptolemy developed the equant — a point near the center of a planet's orbit.
To an observer standing at this point, a planet's epicycle would always appear to move at uniform speed, whereas it would appear to be moving at non-uniform speed from all other locations. While this system remained the accepted cosmological model within the Roman, Medieval European and Islamic worlds for over a thousand years, it was unwieldy by modern standards. In the Ptolemaic model, every planet required an epicycle revolving on a deferent which was offset by an equant, which were also different for each planet.
However, it did manage to predict planetary motions with a fair degree of accuracy, and was used to prepare astrological and astronomical charts for the next years.
By the 16th century, this model was gradually superseded by the heliocentric model of the universe, as espoused by Copernicus, and then Galileo and Kepler. In the 16th century, Nicolaus Copernicus began devising his version of the heliocentric model.
Like others before him, Copernicus built on the work of Greek astronomer Atistarchus, as well as paying homage to the Maragha school and several notable philosophers from the Islamic world see below.
By the early 16th century, Copernicus summarized his ideas in a short treatise titled Commentariolus "Little Commentary". By , Copernicus began circulating copies amongst his friends, many of whom were fellow astronomers and scholars. This forty-page manuscript described his ideas about the heliocentric hypothesis, which was based on seven general principles.
These principles stated that:. Thereafter he continued gathering data for a more detailed work, and by , he had come close to completing the manuscript of his magnum opus — De revolutionibus orbium coelestium On the Revolutions of the Heavenly Spheres. In it, he advanced his seven major arguments, but in more detailed form and with detailed computations to back them up. By placing the orbits of Mercury and Venus between the Earth and the sun, Copernicus was able to account for changes in their appearances.
In short, when they are on the far side of the sun, relative to Earth, they appear smaller but full. When they are on the same side of the sun as the Earth, they appear larger and "horned" crescent-shaped.
It also explained the retrograde motion of planets like Mars and Jupiter by showing that Earth astronomers do not have a fixed frame of reference but a moving one. This further explained how Mars and Jupiter could appear significantly larger at certain times than at others. In essence, they are significantly closer to Earth when at opposition than when they are at conjunction. In the early 11th century, Egyptian-Arab astronomer Alhazen wrote a critique entitled Doubts on Ptolemy ca.
Around the same time, Iranian philosopher Abu Rayhan Biruni — discussed the possibility of Earth rotating about its own axis and around the Sun — though he considered this a philosophical issue and not a mathematical one.
In the 11th and 12th centuries several Andalusian astronomers, centered in the Almohad Moorish territory of Spain, challenged the geocentric model of the Universe as well. For instance, 11th century astronomer Abu Ishaq Ibrahim al-Zarqali aka. Arzachel departed from the ancient Greek idea of uniform circular motions by hypothesizing that the planet Mercury moves in an elliptic orbit.
In the 12th century, fellow Andalusian Nur ad-Din al-Bitruji aka. Alpetragius proposed a planetary model that abandoned the equant, epicycle and eccentric mechanisms. Though these were largely philosophical in nature and did not result in the adoption of heliocentrism, many of the arguments and evidence put forward resembled those used later by Copernicus. In the 16th century, Nicolaus Copernicus began devising his version of the heliocentric model, which represented the culmination of years worth of research.
Like others before him, Copernicus built on the work of a number classical astronomers who did not support the geocentric view, as well as paying homage to the Maragha school and several notable philosophers from the Islamic world. This forty-page manuscript described his ideas about the heliocentric hypothesis, which was based on seven general principles. These principles stated that:. Thereafter he continued gathering data for a more detailed work, and by , he had come close to completing the manuscript of his magnum opus — De revolutionibus orbium coelestium On the Revolutions of the Heavenly Spheres.
In it, he advanced his seven major arguments, but in more detailed form and with detailed computations to back them up.
By placing the orbits of Mercury and Venus between the Earth and the Sun, Copernicus was able to account for changes in their appearances. In short, when they are on the far side of the Sun, relative to Earth, they appear smaller but full.
It also explained the retrograde motion of planets like Mars and Jupiter by showing that Earth astronomers do not have a fixed frame of reference but a moving one.
This further explained how Mars and Jupiter could appear significantly larger at certain times than at others. In essence, they are significantly closer to Earth when at opposition than when they are at conjunction. However, due to fears that the publication of his theories would lead to condemnation from the church as well as, perhaps, worries that his theory presented some scientific flaws he withheld his research until a year before he died.
It was only in , when he was near death, that his treatise was sent to Nuremberg to be published. In many cases, the model is simply an idea—that is, there is no physical representation of it. So, if, when I use the word "model," you picture in your head a scale copy of a battleship that you put together as a kid, that is not what is meant here. However, that doesn't preclude us from making a physical representation of the model.
So, for example, if you are studying tornadoes, you can build a simulated tornado tube using 2 liter soda bottles filled with water. However, for it to be useful as a scientific model, you would want to use the physical model to try and study aspects of real tornadoes.
In modern science, many models are computational in nature—that is, you can write a program that simulates the behavior of a real object or phenomenon, and if the predictions of your computer model match your observations of the real thing, it is a good computer model.
This is a simple statement that paraphrased says: If there are two competing models to explain a phenomenon, the simplest is the one most likely to be correct. For more history, see a discussion of Occam's Razor on Wikipedia. I realize that Wikipedia is not always to be considered a trusted resource, but this is a good overview. What I hope will be made clear in the rest of the course is that in practice science is very non-linear.
In fact, as a fairly frequent judge for the "Pennsylvania Junior Academy of Science" which may be similar to science fairs where you teach , I often complain about their rubric for judging, because they force students to try to approach science in a linear, step-by-step model. Scientists all do the standard steps of the scientific method at some point, however, not necessarily in the order presented in textbooks or in a way that they identify as "Now I am on step 5 of the process", for example.
This process is really completed by a community of scientists working on scientific problems separately. It was, in fact, superior to the Copernican system propsed in the 16th century. Although Copernicus idea that the earth rotates around the sun was correct he still assumed that the planets move in perfect circles, instead of ellipses.
Therefore the Copernican system predicted the positions of the planets less precisely than the - incorrect - geocentric model of Ptolemy.
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