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Originally Posted by Celestial Mechanic
It isn't. 99 percent of it is due to the Moon and the Sun, roughly 60 percent of that due to the Moon, 40 percent to the Sun.
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OK, so there's at least 1% you can't account for; 1% might be enough if the balance of forces happens to be about right.
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Since Venus orbits the Sun, its average inverse distance cubed is comparable to the Sun's, but its mass is about 3 millionths of the Sun's, so the effect is too small to detect.
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Venus is only relevant during inferior conjunctions, when it is only 0.3 AU from the Earth. Also there's the rotational resonance between Earth and Venus; but I take it you are probably skeptical in that regard; if so, check out the paper by
Zhang & Shen (1987, 328) who say that the retrograde trade winds on Venus exert enough torque on Venus to allow libration around the ideal resonance. Therefore, since Earth can cause effects on Venus, it's not unreasonable to suppose that Venus might cause some effects on Earth.
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I can. It is the J3 moment, which has a value of 0.254x10-6, compared with 1.1x10-3 for J2.
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Could we persuade you to explain to us in laymen's terms terms the significance of J
3 versus J
2 and how they relate to the north versus the south hemispheres?