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Animation showing Kepler's first law of planetary motion. Kepler's three laws were published in 1609 (first and second laws) and 1619 (third law). The first law states the the orbit of each planet is an ellipse, with the Sun at one focus. The animation shows the properties of an ellipse, the blue line: the Sun is at one focus, the pink dot shows the other focus. The foci are equidistant from the centre (orange dot). The semi-major axis (a) is pink. The lines joining the planet to each focus always add to a value twice that of the semi-major axis.
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