## Info

Moon

and is illustrated in Fig. 23.26. To see how this comes about, we just consider the effect of the Sun on the Earth. The Earth is not a perfect sphere, but has a larger diameter across the equator than between the poles. This oblate, or flattened, appearance is due to the Earth's rotation. We can think of the non-rigid Earth as deforming under the effects of the centrifugal force. Since the Earth is not spherical, the Sun can exert a torque on the Earth.

To see the direction of this torque, we idealize the Earth as a sphere with an extra band around the equator. The side of the band closest to the ai - a2 (b)

Fig 23.25.

Tides on Earth. (a) Different accelerations caused by the gravitational attraction of the Moon. (b) We express these accelerations relative to that of the center of the Earth.The result is that the water is pulled away from the Earth on the side facing the Moon (c), and the Earth is pulled away from the water on the far side.

Fig 23.25.

Tides on Earth. (a) Different accelerations caused by the gravitational attraction of the Moon. (b) We express these accelerations relative to that of the center of the Earth.The result is that the water is pulled away from the Earth on the side facing the Moon (c), and the Earth is pulled away from the water on the far side.

To Sun reshaping of the Earth helps to heat its interior, and also dissipates the Moon's orbital energy. The Earth doesn't respond instantaneously to these tidal effects. Its rotation causes the bulge in the side near the Moon to get ahead of the Earth-Moon line. This means that the force can act to increase the Moon's orbital speed. This makes it move farther from the Earth. The corresponding force on the Earth causes its rotation to slow, keeping the total angular momentum of the Earth-Moon system fixed. Similarly, the Earth has exerted forces that distort the Moon. These forces, acting on the distorted Moon, act to slow its rotation, producing a rotation period equal to the Moon's orbital period.

The tidal forces that the Sun and Moon exert on the distorted Earth produce a torque that results in a continuous change in the direction of the Earth's rotation. This change is called precession,

To Sun

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