Question 13.9: Theancient Chinese court astronomers were able to predict lu...
Theancient Chinese court astronomers were able to predict lunar and solar eclipses with great reliability. The fact that such eclipses arise only occasionally—while otherwise we have a full moon or a new moon—is caused by the inclination of the orbital plane of the earth–moon system from the ecliptic, i.e., the orbital plane of the motion of the common center of gravity about the sun. This inclination is about 5.15°. It is not fixed in space but precesses because of the tidal forces exerted by the sun. This leads to the so-called Saros cycle, which is of great importance for the prediction of eclipses.
Consider the earth–moon system as a dumbbell-shaped top which rotates about its center of gravity S_{p}; the center of gravity orbits about the sun on a circular path. The gravitation force between the earth and moon just balances the centrifugal force resulting from the eigenrotation of the system and thus fixes the almost rigid dumbbell length r_{0}. The gravitation of the sun and the centrifugal force due to orbiting about the sun don’t compensate for each body independently but lead to resulting tidal forces. These forces create a torque M_{0} on the top. Calculate M_{0} for the sketched position where it just takes its maximum value. Realize that M_{0} on the (monthly and annual) average has a quarter of this value. Calculate from this the precession period T_{p}. Can you find arguments for why the actual Saros cycle of 18.3 years is notably longer?
Hint: The only data you need for the calculation are the distances r_{0}, the length of year, and the length of the sidereal month.
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