G = 6.6742 x 10^-11
M(sun) = 1.9891 x 10^30
D(sun = 149.6 x 10^9
M(moon) = 7.3477 x 10^22
D(moon) = 363,104,000
gravitational effect of sun on 1kg of water
= ((6.6742 x 10^-11) x (1.9891 x 10^30) x 1)
((147.1 x 10^9)^2)
= 0.00603269373 N m^2 kg^-2
of moon
= ((6.6742 x 10^-11) x (7.3477 x 10^22) x 1)
(363104000^2)
=0.00003719534 N m^2 kg^-2
It would appear that despite the Sun's distance from us it has over 100 times the gravitational pull on Earth as the Moon does when both are at their closest. Also circular motion can be defined as acceleration towards the centre of a circle, we are constantly being accelerated directly towards the sun which causes circular motion (no other factors are needed to stop us being pulled into the sun).
However the Moon's orbit around the Earth is about 3 times more eccentric than the Earth's around the sun. meaning that the forces the moon exerts on the Earth change by over 20% in under a month, whereas the forces which the sun exerts on the Earth change by less than 3% in one year. Furthermore the earths rotation means that any one point on the earth will experience a change in gravitational attraction from the Moon by about 6% in a single day. Because the Sun is much further away, the change in gravitational attraction from the Sun on any one point on the earth is equal to about one 1000th of a percent. I haven't studied this particular area much, but it seems like this massive, fast change of the gravitational effect of the Moon on any point of the Earth and the small, gradual change in gravitational attraction by the sun could be the reason that the moon effects tides so much.
In summary, it seems like it is not the power of the gravitational attraction, but the change in gravitational attraction (most likely the daily change) which causes tides. But then again maybe there is another explnation I am unaware of.[/u]