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We see Sun, Moon and stars rise daily roughly in the East, climb higher through the South and set in about the West. Constellations keep their relative positions, but the planets are wondering stars.
Without a telescope we see five planets (who in many cultures give names to the days of the week, together with Sun and Moon): Mercury, Venus, Mars, Jupiter and Saturn. These five planets have their place in the models shown to the left .
These wandering stars show a retrograde movement relative to the background of the fixed stars: at certain intervals they seem to move backwards (in the animation below this is shown for Mars). This phenomenon can be explained as follows:
The almost unchanging brightness of Venus supported Ptolemy until Galilei saw Venus having phases (like the Moon), changing size at the same time (see illustrations below). which can only be explained assuming Venus orbits the Sun (Tycho and Copernicus: see picture below).
Above is shown how Copernicus explains the retrograde movement of Mars: the Earth orbits the Sun faster than Mars, causing Mars to be seen from changing angles.
This principle also applies to other planets.
In two drawings is shown how Ptolemy explains it:
first you see the system with deferent (the big circle) and the epicycle (A). The centre of the deferent is slightlt out of centre: the blue dot is the Earth, B is the equant. Next the movement of planets around the Earth is shown.
It is simpler in Tycho's model: the planets orbit the Sun, which in turn orbits the Earth: this will result in the retrograde movement.
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