Because the only physical forces that could affect you are either 1) the gravitational force by the planet or 2) it's tidal gravitational force. As an example, the force upon a single 200 kg human exerted by each of the planets when they are closest to the Earth are:
Distance Mass Force Mercury 92 0.055 0.051 Venus 41 0.815 3.77 Moon 0.356 0.012 737.4 Mars 78 0.105 0.13 Jupiter 629 317 6.23 Saturn 1278 95 0.45 Uranus 2720 14.5 0.015 Neptune 4347 17.2 0.007 Pluto 5751 0.17 0.00004
The distance is in millions of kilometers. The mass is in units of the Earth = 5.9 x 10^27 grams. The force is in dynes using Newtons Law of Gravity where:
G M m M(planet) --------- = 7788 x ------------- 2 2 d D
where M(planet) and D are from the above table. Even under the best of circumstances, the TOTAL gravitational force of all the planets in their closest orbital locations to the Earth equal only 10.65 Newtons which is less than 2 percent of what the Moon does each month. For a 100 kg human (closer to the average!!!) the forces in the above table are 1/2 as large, and the total gravitational force is only 5.3 Newtons!
This is the direct gravitational force which would change your weight slightly. But the TIDAL force is even smaller because it varies as the THIRD POWER of the distance. So, for each of the above entries for the direct force, we divide by factors of 92 for Mercury, up to 5751 for Pluto to get even smaller numbers for any possible tidal changes to your bodies.
The gravitational effects of the planets are utterly inconsequential in the sphere of human affairs.
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