Originally posted by: Calin
The gravitational influence of the Sun (which contains like 90% or more of the mass of the solar system) is inferior to the gravitational effect of the moon (less than half). This is the reason why tides differ between full/new moon and quarter moons. Why high tides and low tides differ in a smaller degree? Simply because the Sun's influence (gravitational influence) is smaller than the moon's
Actually, the sun's gravitational influence is incredibly greater then that of the moon. If what you said is true, then we would orbit around the moon instead of the sun. That is obviously not the case. However, there is a germ of truth in what you said about tides.
The reason that the moon is the primary influence on tides is not because it has a greater gravitational effect then the sun on the earth but because since it is closer to the earth the change in relative position between different sides of the earth. I'm not very good at clearly explaining things, but here I go. On the side of the earth that the moon is closest to there is a signicantly greater attraction the on the opposite side of the earth (gravitational force is inversely proportional to the distance between the two objects squared). In terms of numbers, the moon is 384,400 km away. The diamtereof the earth is about 12,600 km. What this means is that there is that on the side furthest from the moon, there is a difference in gravitational attraction (compared to the other side) of about 5%. That makes it so there is a resulting tidal bulge in the oceans.
The sun doest not have that effect because the distance between the sun and earth is so great already that the gravitational effects on each side of the earth are nearly equal. So even though the sun has more of a gravitational effect on the earth then the moon, the moon has more of an effect on the tides because it is closer and there is a greater disparaity of gravitational force on each side of the earth.
Like I said, I'm not so hot at explaining stuff... so if this is a pretty ambiguous explanation, let me know and I can try to find a webpage that does a better job of explaining.
Edit: Oh yeah, in addressing the original post question. I think that something around 5 of the planets aligned a few years ago, back in 1990's and there were a bunch of cults and dooms-dayers freaking out about it. But even if all the planets aligned there would be only like a .01% change in gravitational force on the Earth.