32. Spinning a calculation

To an observer on earth, tidal friction slowing down the rotation of the earth is equivalent to a speeding up (acceleration) of the sun's progress in the course of a day.

The tidal action also affects the motion of the moon. According to Newton's 3rd Law, the moon experiences an equal and opposite reaction to the tidal force it exerts on the earth. The reactive force causes the moon to spiral away from the earth and slow down (negative acceleration). A precise determination of these accelerations is essential for calculating an accurate description of eclipses.