[I]t is the sidereal day, that is, the duration of the rotation of the earth, which is the constant unit of… — Henri Poincaré

[I]t is the sidereal day, that is, the duration of the rotation of the earth, which is the constant unit of time. ...However ...[many] astronomers ...think that the tides act as a check on our globe, and that the rotation of the earth is becoming slower and slower. Thus would be explained the apparent acceleration of the motion of the moon...

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