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NASA
Course: NASA > Unit 2
Lesson 1: Modeling the solar systemPlanets & epicycles
Wandering stars
Early astronomers observed that a few of the brighter stars seemed to wander in the sky over the course of many months. Attributing this behaviour to a form of extreme power, these wandering bodies were assumed to be deities by the Babylonians. The Greeks named them “wandering stars” or “astēr planētēs” – the root of the word we use today: planets.
The geocentric model couldn’t yet explain why planets would appear brighter and darker at different times. The most striking issue was how to explain why the planets would occasionally stop and reverse directions. This is known as retrograde motion and would be impossible if the planets were the same distance to us at all times. Below is an image of the geocentric model which assumed that all planets travel around the earth in perfect circles.
The red planet Mars was particularly difficult to explain. If we assume a geocentric universe and map out the wandering motion of the Red Planet we’d see this pattern
It was assumed that something must be wrong with the model. For early astronomers, there was a strong need to keep the Earth at the center of the universe. This led to some very creative ways of justifying the geocentric model of the universe. How could they explain why these pretzel shaped orbits occur if everything travels in circular orbits? Below is a representation of the apparent motion of the sun and planets from the earth. It shows the orbit of Venus over 8 years and Mars over 7 years. Notice the preztel shaped orbits which emerge:
Epicycles
The Greek geometer and astronomer Apollonius of Perga (262 - 190 BCE) came up with a model to explain these changes in position and brightness. He believed that this cyclical variation could be represented visually by mini orbits, or epicycles, revolving around larger circular orbits, or deferents. The following animation shows Mars traveling around a red orbit, while the red orbit is also orbiting around the Earth on a secondary orbit.
Unfortunately, the epicycles made the new geocentric model incredibly complicated, and some planets even needed epi-epicycles for the model to match astronomical observations. There was, however, a competing theory that greatly simplified things. For this theory to gain support, astronomers as well as political and religious leaders needed to give up the dominant view that the Earth was at the center of the universe. This was not going to be an easy task.
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why was Mars's orbit wacky(39 votes)- The true orbit of Mars isn't wacky at all. It's the apparent motion of Mars in the night sky as viewed from Earth that's wacky, with its periodic back-and-forth motion against the background stars. You can see this with the applet in the previous exercise, "the geocentric universe." It's wacky if you assume Mars is moving around the Earth -- which seems reasonable until you study the heavens' motions carefully and realize that the whole entire world is actually just spinning and going around the sun.(79 votes)
If you look closely at the last animation regarding epicycles, you can actually see the heliocentric model! The Earth goes round the Sun on the outer edge of the puke-green circle, while Mars goes around the Sun on the outer edge of the dark red circle. It's just a matter of perspective :D(14 votes)
How come in the first motion, Mars goes in a loop-de-loop, but int he second motion, it does not do that?(3 votes)
It does if you look closely, it's just that the animation starts at a different point in mars's orbit(6 votes)
What is the real definition of a star?(3 votes)
An astronomical body is classified as a star when it achieves the steady fusion of helium in its core allowing it to become a luminous object. It continues as a star until nuclear fusion ceases.(5 votes)
Has NASA discovered any other planets near earth, or a new star, etc..?(3 votes)
Well ur question is good. Well nasa has found 7 exoplanets just like earth but they are light years away. Also they have thought that they have found a new planet just like earth which is outside of our solar system. It is known as planet X. This hasn't been seen by any telescopes or so but it has been proven by mathematics. For more info check https://www.nasa.gov/audience/forstudents/9-12/index.html(2 votes)
1. If they was so many problems with the geocentric theory that scientists could not explain then why didn't they just chuck the theory and adopt the heliocentric theory?
2. Why did they keep on trying to come up with explainations for why Mars' orbit is so weird when it was obvious that there was something wrong with the theory that was being used?
3. How did epicycles help explain anything if they were so complicated?(3 votes)
Very good questions!
1. They desperately wanted to believe that Earth was at the centre of everything. They thought that because they could see what was going on around them and because their planet was so great and important, why would they orbit around anything else.
2. Because again, they wanted to believe that they were at the centre of things because that is how they thought. It just made sense to them
3. They didn't really help explain anything really. They just made the geocentric model more complicated. They needed an explanation for it though and this was the one that they came up with. However, you are right, it did just add a whole lot of confusion(3 votes)
When the diagram showed the sun rotating, is that what the people thought because the sun does not actually rotate right?(3 votes)- Yes the people believed that everything rotated around the earth but it is not true.(3 votes)
i think sun revolves around the earth...(1 vote)
no it's a wrong statement . if it's true pls give a explanation(2 votes)
What prevents the planets in our solar system from orbiting into the sun? Are our planets slowly moving closer to the Sun with each orbit? if so, what is the predicted time it would take for a planet to be absorbed by the sun?(2 votes)
Why doesn't Mars orbit in a circle?(2 votes)- no planets orbit in a circle, it's always an ellipse. Some are more irregular than others, due in part to the presence of other planets, and also due to bombardment.(1 vote)
