Skip to main content

How did Ptolemy explain the way planets seemed to move at different speeds and even backwards?

The Ptolemic Model of the planets relied on the notion that the planets' movements were determined by two different spheres: the deferent and the epicycle. 

The deferent is just the sphere whose center point, called the eccentric, is an imaginary point offset a little from earth. The planets each have their own deferent, and revolve about it. 


Of course, this alone would not be enough to fully explain the seeming motion of the planets. So the epicycle was posited, which was a second sphere that the planet moved around. Or, more accurately, the epicycle revolved around the deferent, and the planet itself revolved around the epicycle. 


If that's confusing, then it is probably most easily understood by linking each term with its analogous counterpart in the modern model:


  • The eccentric is the sun, and is the point about which all else revolves.

  • The deferent is earth's orbit around the sun. It is not an object in itself, just a path the planet follows.

  • The epicycle is the moon's orbit about the earth. It revolves around the earth, while the earth revolves around the sun, giving it two spheres of revolution. 

In this analogy, the planets' motions most closely mimic that of earth's moon as viewed from the sun; revolving around two spheres. Except that the earth, in the Ptolemic model, is not the same as the eccentric. So, really, it would be like viewing earth's moon from a point a little bit away from the sun.


The idea that planets move around two spheres whose center points are not earth is what accounts for the seeming different speeds and directions in which the planets moved. Even though we now know this to be wrong, this model was impressively accurate at predicting the movements of celestial bodies.

Comments

Post a Comment

Popular posts from this blog

etymology - Origin of the greeting "Sweet dreams".

Does anybody know the etymology of the phrase "sweet dreams"? I tried googling but did not find anything satisfying. Is this a relatively new phrase of the modern world or has this been in use for some time? I think it's the latter one. Answer The OED has the interjection as "a farewell to someone going to bed" from the 20th century: 1908 Sears Roebuck Catal. 198/1 Tenor Solos..Good Bye, Sweet Dreams, Good Bye. But it goes back until at least the 19th and possibly 18th centuries. John Wolcot, writing under the pseudonym of Peter Pindar, used it in his poem "Orson and Ellen; A Legendary Tale" published in 1801: Also from 1801 in The infernal Quixote (Page 287) by Charles Lucas: In the March 1776 of The Universal Magazine was published "The Serenade. A Pastoral Tale. From the German of Gesner" , where the shepherd Daphnis watches over his beloved as she sleeps and sings: The same tale appears in 1776's as Idyl XI, " Daphnis ...
Post a Comment
Read more

Is there a word/phrase for "unperformant"?

As a software engineer, I need to sometimes describe a piece of code as something that lacks performance or was not written with performance in mind. Example: This kind of coding style leads to unmaintainable and unperformant code. Based on my Google searches, this isn't a real word. What is the correct way to describe this? EDIT My usage of "performance" here is in regard to speed and efficiency. For example, the better the performance of code the faster the application runs. My question and example target the negative definition, which is in reference to preventing inefficient coding practices. Answer This kind of coding style leads to unmaintainable and unperformant code. In my opinion, reads more easily as: This coding style leads to unmaintainable and poorly performing code. The key to well-written documentation and reports lies in ease of understanding. Adding poorly understood words such as performant decreases that ease. In addressing the use of such a poorly ...
Post a Comment
Read more

What are the similarities between the relationships of the characters in The Outsiders and Romeo and Juliet?

At first glance, it might appear as if the characters in  The Outsiders  by S. E. Hinton and  Romeo and Juliet  by William Shakespeare do not have much in common; however, when one isolates the core of the conflicts, it becomes clear that the relationships in both stories are rooted in rivalry and secrecy.  In both  The Outsiders  and  Romeo and Juliet , an overarching theme is rivalry. While   socioeconomic status separates the greasers from the Socs in The Outsiders , last names separates the Montagues from the Capulets, who appear to be relatively equal in terms of socioeconomic status. Due to these rivalries, there are instances of deadly interactions in both stories. In  The Outsiders , Johnny kills Bob (whether his actions were justified or not is debatable). In  Romeo and Juliet , Romeo kills Tybalt and Paris, and Tybalt kills Mercutio. These deaths have profound effects on the main characters. In Ponyboy's case, the Johnny's death and other events in the novel inspire h...
Post a Comment
Read more