Four hundred years ago today Johannes Kepler rediscovered his most important contribution to the evolution of astronomy, his third law of planetary motion.
Portrait of Johannes Kepler 1610 by unknown artist. Source: Wikimedia Commons
He had originally discovered it two months earlier on 8 March but due to a calculation error rejected it. On 15 May he found it again and this time recognised that it was correct. He immediately added it to his Harmonices Mundi:
For when the true distances between the spheres were found, through the observations of Brahe, by continuous toil for a very long time, at last, at last, the genuine proportion of the periodic times to the proportion of the spheres –
Only at long last did she look back at him as she lay motionless,
But she look back and after a long time she came [Vergil, Eclogue I, 27 and 29.]
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