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Illustration 12.6: Heliocentric vs. Geocentric

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Does Earth orbit around the Sun or does the Sun orbit Earth? For a very long time people thought that Earth was stationary (as the argument goes, otherwise the birds would be ripped from their perches!) and the Sun orbited Earth. This belief is where we get the terms sunrise and sunset. But the Sun does not orbit Earth; it is the other way around. In addition, the motion of the planets, as seen from the reference frame of the Sun (the heliocentric reference frame) is rather simple. But from the perspective of each individual planet (the geocentric reference frames of the Inner Planet and the Outer Planet) the motions of the other planets are rather complicated. The geocentric view is exactly what we see on Earth when we look at the sun and the other planets of the Solar System.

In this Illustration two planets (the red circle is the inner planet and green circle is the outer planet) orbit a central star (the black circle) as shown in the animation. Along with the animation from the star's reference frame, the heliocentric point of view, two other animations show the motion as seen from each of the planets' reference frames, the geocentric points of view.

As you view the animation, keep in mind that in the Inner Planet animation, if Earth is the red planet, the green planet behaves like Mars as seen from Earth, while in the Outer Planet animation, if Earth is the green planet, the red planet behaves like Venus as seen from Earth.

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