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### Course: NASA>Unit 2

Lesson 1: Modeling the solar system

# The heliocentric model

## Giving up control

When observing a sunset it is natural to assume the sun is moving around us. Aristarchus of Samos (c. 310-230 BC), a Greek astronomer, was first to maintain that the earth rotates on its axis and revolves around the sun. This was a dangerous assertion at the time and could not win over supporters of the geocentric model, a model that persisted for over a thousand years.
In 1543, Nicolaus Copernicus (1473 – 1543) published “On the Revolutions of the Celestial Spheres”, in which he explained what many had suspected: that the sun is at the centre of the universe and we move around it along with all the other planets. This is called the Heliocentric Model.
This theory resolved the issue of retrograde motion by arguing that these pretzel orbits were an illusion due to our vantage point. It is a perfect example of Occam’s razor: if you have two competing hypotheses, the one with the fewest assumptions should be selected. The simplest explanation tends to be correct. If you look closely you'll see his model correctly assumed the moon is orbiting around us. Notice how this explains the strange wandering motion of planets when observed from telescopes based on Earth.
It's the motion of the Earth relative to Mars which causes this wandering effect. The next simulation will allow you to explore both the geocentric vs. heliocentric models of our solar system. Have fun!

## Want to join the conversation?

• In regards to Occam's Razor, why does the simplest explanation tend to be correct?
• all explanations rely on various assumptions.
the simpler the explanation, the less assumptions.

example:
I see you moving from my train window, although your legs aren't moving.

possible explanations:
1) I am moving, not you.
2) You are standing on a moving pavement (like they have in airports etc), and I haven't noticed it.
3) it is an optical illusion caused by a really complicated set of reflections and refractions.
You can see that the first explanation is most likely to be correct, even though all 3 are possible.
Explanations 2) and 3) require additional factors other than the motion (or not) of my train and your legs.
For 2), why would I assume you were on a moving pavement, when the idea of the motion of my train would adequately explain the phenomena.
For 3) why would I assume that it was an optical illusion, which would require a complicated set of circumstances to occur, when, again, the idea of the motion of my train would adequately explain the phenomena.
incidentally, the whole idea of the relative movement of the planets as observed from earth IS a very similar example to that I gave for the train above.
• In the first paragraph, the text states that the heliocentric model was a "dangerous assertion" to make during the time of Aristarchus. I was wondering, why was it so dangerous? I understand that new knowledge is not always taken to be credible, but the word "dangerous" seems to denote violence or threats of violence of some sort, and I would like to know if that was the case for supporters of the heliocentric model.
• Aristarchus-- and other Greek thinkers at that time, like Anaxagoras-- were often charged with impiety for their proposals. This charge could bring a sentence of death. Anaxagoras was banished from Athens for a while. There is a legend that this was because Pericles pardoned him and let him live. Pericles had been his student for a time before he became an important ruler of Greece.
• are all the planets orbiting the sun in circular orbits?
• In the light of heliocentrism, what should we call sunrise/sunrise?
• Hmm...
An insightful question. But you might as well call it what you wish.
• Do Newtons law happen in space?
• Yes! In fact, Newton did not discover gravity at all. What he came up with was the idea that gravity happens everywhere in the Universe. The word for gravity is a very old one. And, it would have been particularly strange if it had taken humankind that long to discover that things fall to the earth. Newton's apple idea under that famous tree was really about how the moon and an apple are following exactly the same laws. Newton was all about space, you see.
• How did scientists find the planets after mars (e.g. Jupiter, Saturn, Uranus, Neptune, Pluto)