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Retrograde motion

This animation shows the retrograde motion of Mars. When the animation starts Mars seems to move forwards when compared with distant stars when viewed from the Earth as both the Earth and Mars orbit the Sun . It then moves backwards because the Earth is 'overtaking' it in its orbit. It was this apparent strange movement of the outer planets that led Galileo to propose the sun-centred view of the Solar System.

© Keith Gibbs/John Bourne 2010