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Astronomy Cast Episode 171 from Monday, January 4, 2010, solar system movements and positions.

Welcome to Astronomy Cast, our weekly facts based journey through the cosmos where we help you understand not only what we know but how we know what we know.

My name is Fraser Kane, I'm the publisher of University and with me is Dr. Pamela Gay, a professor at Southern Illinois University, Everett's Village.

Hey Pamela, how you doing?

I'm doing well Fraser, how are you doing?

I'm doing very well. Alright, let's get at her.

So even in ancient times, astronomers realized there was something different about the planets. They move.

The movement of the planets and their moons are governed by gravity and as we all know, gravity can do some funny things.

So let's kind of go back to ancient history and sort of get an idea of what the ancient people thought the way the universe worked.

Well originally it was all based on philosophy, looking up, imagining how the pieces fit together and using philosophy it was Aristotle who led the idea that all of the planets

orbited on perfect circles and the stars were embedded on a perfect sphere that embraced the planet Earth.

And so it was all nested circles with the Earth at the center moving outwards and outwards.

And standing on the surface of the Earth, that's the natural conclusion that you would come to.

You know, you look up in the sky and the stars seem to be moving and so it seems like the stars are moving around you.

The sun is moving, the moon is moving, the planets are moving.

And from one season to the next, you don't see the stars move relative to one another, which is what you'd kind of expect if we were in a little tiny system where the stars weren't that far away.

So since the stars didn't seem to move, they just seem to rotate around and around and around.

It seemed natural, okay, they're just embedded on a flat, well, they're embedded on the inside of a sphere that's not too big that embraces the planet Earth.

Right. And how well were astronomers able to use this model to still do astronomy?

Well, it made some predictions, but they weren't particularly accurate.

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