The basic idea of Newton's approach to the problem of precession was correct. He had shown that one of the perturbing effects of the Sun on the orbit of the Moon was to cause the nodes - the intersections of its orbit with the ecliptic plane - to regress. From his theory, Newton could calculate the rate of regression for a body orbiting the Earth once per day, i.e. at the same rate as the Earth rotates. Much as in the tidal theory, this body was then imagined to be a solid ring encircling the Earth and then finally attached to the Earth around the equator. Newton believed that the ring would be subject to the same regression of the points of intersection between it and the plane of the orbit around the perturbing body, which in this new situation corresponds to precession. Various technical results83 led him to believe that the effect would amount to just over 9" per year. But there was also the effect of the Moon on the equatorial bulge, and, based on measurements of the heights of tides at Bristol, Newton concluded that the attraction by the Moon of a particle on the surface of the Earth was about 4 2

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