He... explain[ed] how the astronomers... had to grapple with the problem of defining definition, because for 2 1/2 millennia there had been just one … - Julian Barbour

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He... explain[ed] how the astronomers... had to grapple with the problem of defining definition, because for 2 1/2 millennia there had been just one standard of time... the rotation of the earth. ...[T]hat had provided an incredibly accurate clock. It's... lost only a few hours in 2 1/2 thousand years. ...Very easy to use. Astronomers only... had to glance at the night sky, at the... Big Dipper... to tell the time within a minute or two... But in the 1890s a crisis developed when they found that using the earth... and... Newton's laws of motion and gravity... the Moon was speeding up. ...[T]hey thought ...the earth was slowing down because of the tidal effects of the moon... and... this would mean that the earth was not a good timekeeper at the accuracy that they wanted.