Johannes Kepler (1571–1630) worked for eight years on Tycho Brahe’s observational data to discover that planetary orbits are ellipses with the Sun at one focus. But why ellipses? The answer came sixty years later, when Newton showed that a force inversely proportional to the square of the distance () produces exactly conic orbits: ellipses, parabolas or hyperbolas (Newton 1687).
Legend has it that Newton conceived the idea watching an apple fall in the garden of his mother’s house at Woolsthorpe in 1666, during the plague that had forced him to leave Cambridge. True or not, the insight is grand: the same force that makes the apple fall keeps the Moon in orbit (Simonyi 2012, ch. 3).
The precision that paved the way
Kepler’s obsessive precision was decisive: a discrepancy of just 8 arcminutes between Tycho Brahe’s observations and the circular-orbit model drove him to seek a different shape, until he found the ellipse. Kepler wrote his own epitaph: Mensus eram caelos, nunc terrae metior umbras (“I measured the skies, now I measure the shadows of the earth”) (Simonyi 2012, pp. 190–193).
Henry Cavendish (1731–1810) measured the constant for the first time in 1798 with a torsion balance, “weighing the Earth” in his laboratory. The value he obtained differs by less than 1% from the one accepted today (Simonyi 2012, p. 264).
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Topics: Gravitazione Concepts: Leggi di Keplero · Legge di gravitazione universale
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