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Fig 23.26.

Precession caused by the Sun. (a) We treat the Earth as a sphere with an extra bulge around the equator, and look at the torque exerted by the Sun on the bulge.We see that F| is greater in magnitude than F2, so there is a net torque on the Earth, whose direction, given by the right-hand rule, is out of the page.The torque would rotate the Earth, but the Earth already has some angular momentum, so the change in angular momentum must be added to the existing angular momentum, as shown on the right.The result is a new angular momentum whose magnitude is the same as before, but whose direction is different. (b) We show the torques at different points in the Earth's orbit.The torque can sometimes be zero, but when it is not zero it always points in the same direction.

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