Planets

The planets are spherical bodies which revolve above the Earth. The planets follow a similar daily route across the sky as the Sun. Five planets — Mercury, Venus, Mars, Jupiter, and Saturn are visible to the naked eye and were known to the ancients as "wandering stars;" entities which appear to move differently from the fixed path of the stars. The word "planet" comes from the Greek word planetes, meaning "wanderer."

On the topic of the history of the planets and the invention of the heliocentric system, by the time of Copernicus and Galileo it was already widely believed that the Earth was a globe, largely based on Greek knowledge. The debate was whether the Earth was in motion and moving in the Solar System or whether the Earth was the center of it all. There is some agreement that they got some things right, such as the Sun being a seemingly central body for the planets, since that is the most natural inclination when one sees Mercury and Venus following the Sun in the sky and moving to either side of it. In this sense, some may accept that the Solar System is Heliocentric or semi-Heliocentric under the Flat Earth model. However, from viewing the motions of the planets above us, it does not necessarily follow that the Earth is a planet in the Solar System. The Solar System sits above the Earth's plane.

Topics

 * Discovery of Neptune
 * Precession of Mercury's Orbit
 * Kings Dethroned
 * Astronomical Prediction Based on Patterns
 * Three Body Problem

Dance of the Planets
The Sun moves in Northward and Southwards paths across the sky over the course of the year when changing seasons and the planets follow along that path, with an apparent relationship to the Sun. Mercury and Venus appear to be rotating around the Sun, while the other planets have a less direct effect. The below animation shows the Sun's progress across the sky over the course of a year as a panorama, its Northward and Southward motions, and the 'dance of the planets' as they follow the sun.



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The Copernican Revolution of the 16th century held that this relationship with the Sun was evidence that all of the planets of the Solar System moved around the Sun. At the time the idea of a Round Earth was already widely prevalent, based on the teachings of the Ancient Greeks. It was deduced that since the Earth is a round body in a Sun-centered celestial system, that the earth must also be a body in motion similar to the planets seen in the sky.

Ptolemaic System
Prior to the heliocentric system of Copernicus the prevailing astronomical model featured an earth-centered system, with main attribution to mathematician and astronomer Claudius Ptolemy. The Earth was at the center of this system while the planets moved around it on epicycles. However, the movements of the planets was a mathematical construct rather than a geometric one, the epicycles being a mathematical construct to predict patterns of movement. Its authors felt that prediction was more important than a geometric depiction of the system. In Beyond Reason: Essays on the Philosophy of Paul Feyerabend its author describes Ptolemy's construct:

Copernican System
The Copernican System, introduced in 1543, attempted to move the Ptolemaic System forward with by proposing a Sun-centered geometric solar system. However, this model still featured the complex mathematical constructs of epicycles, which were arranged in a way so that the Sun was at the center.

What Is This Thing Called Science? By Prof. Alan Chalmers

Copernicus and Epicyles
Some assert that Copernicus ended up with even more epicycles than Ptolmy. From p.42 of Galileo Was Wrong its authors provide the following:

Footnotes:

86 Copernicus writes in the Commentariolus: “Behold! Only 34 circles are required to explain the entire structure of the universe and the dance of the planets!” (Gingerich, The Book that Nobody Read, p. 56). But Koestler remarks: “incidentally, as Zinner has pointed out, even the famous count at the end of the Commentariolus is wrong as Copernicus forgot to account for the precession, the motions of the aphelia and the lunar nodes. Taking these into account, the Commentariolus uses thirty-eight not thirty- four circles," adding that Copernicus makes no mention of the total number of epicycles in De revolutionibus: “Apart from the erroneous reference to 34 epicycles, I have nowhere see a count made of the number of circles in De revolutionibus” (The Sleepwalkers, p. 580), perhaps hiding the fact from his reader that it contained more epicycles than the De revolutionibus. Gingerich adds: “Copernicus must have realized that with his small epicyclets he actually had more circles than the Ptolemaic computational scheme used in the Alfonsine Tables or for the Stoeff‌ler ephemerides" (op. cit., p. 58). Regarding the discrepancies among the orbits of Mars, Jupiter and Saturn in 1504, Gingerich writes: “...the evidence is f‌irm that he had observed the cosmic dance at this time [1504] and was fully aware of the discrepancies in the tables. But what is most astonishing is that Copernicus never mentioned his observation, and his own tables made no improvement in tracking these conjunctions“ (ibid., p. 59).

87 The Sleepwalkers, p. 194-195. One reason Copernicus had so many epicycles is, rather than placing the sun in the center of the universe, he placed the Earth’s entire orbit in the center (although, according to Gingerich: “this was an unresolved mystery in the book, for Copernicus hedged on the issue,” The Book that Nobody Read, p. 163). Koestler remarks that discrepancies in the biographical literature on the number of epicycles in Copernicus’ system is due to the fact that most historians have not read Copernicus’ book but have depended on other biographers for their information. Koestler’s notes show that he did a painstaking analysis of De revolutionibus that allows him to conclude Copernicus used forty-eight epicycles (pp. 579-580). Gingerieh accounts for these extra epicycles as follows: “While he [Copernicus] had eliminated all of Ptolemy’s major epicycles, merging them all into the Earth's orbit, he then introduced a series of little epicyclets to replace the equant, one per planet” (The Book that Nobody Read, pp. 54-55). For mistaken scholarly accounts that settled on Copernicus having only 34 epicycles, Koestler cites the Chamber's Encyclopedia as stating the Copernican system reduced the epicycles “from eighty to thirty-four,” as is the case with Herbert Dingle’s address to the Royal Astronomical Society in 1943. In my research I found the same discrepancies. Ivars Peterson writes: “Copernicus needed more circles in his sun—centered model than Ptolemy did in his Earth-centered scheme [a] total of 34 circles for all the planets and the moon" (Newton's Clock: Chaos in the Solar System, New York: William H. Freeman and Co. 1993, p. 54). Some add even more epicycles to Copernicus, as is the case with James Burke: “To account for the apparent alterations in speed and movement of the planets, Copernicus was obliged to use as many as ninety Ptolemaic epicycles” (The Day the Universe Changed, p. 134).

88 Joshua Gilder and Anne-Lee Gilder, Heavenly Intrigue: Johannes Kepler, Tycho Brahe, and the Murder Behind One of History's Greatest Scientific Discoveries, New York: Doubleday, 2004, p. 38.

89 Owen Gingerich adds that the myth of having to put up with an inordinate amount of Ptolemaic epicycles perpetuated itself like an out-of-control gossip chain. He writes: “The legend reached its apotheosis when the 1969 Encyclopedia Britannica announced that, by the time of King Alfonso, each planet required 40 to 60 epicycles! The article concluded, ‘After surviving more than a millennium, the Ptolemaic system failed; its geometrical clockwork had become unbelievably cumbersome and without satisfactory improvements in its effectiveness.‘ When I challenged them, the Britannica editors replied lamely that the author of the article was no longer living, and they hadn’t the faintest idea if or where any evidence for the epicycles on epicycles could be found” (The Book that Nobody Read, pp. 56-57).

Prediction in Modern Astronomy
Prediction in modern astronomy is still performed through patterns. By analysis of historic tables it is possible to construct functions which can predict where a planet will be in the future. This is how prediction in astronomy has been performed since times of antiquity, and how it is performed today. Indeed, modern astronomers still use epicycles—now called perturbations—to predict the location of planets.

See: Astronomical Prediction Based on Patterns