why do tides occur on both sides of the earth

Gravity is a major force responsible
for creating tides. Inertia, acts to counterbalance gravity. It is the tendency of moving objects to continue moving in a straight line. Together, gravity and inertia are responsible for the creation of two major tidal bulges on the Earth (Ross, D. A. , 1995). The gravitational attraction between the Earth and the moon is strongest on the side of the Earth that happens to be facing the moon, simply because it is closer. This attraction causes the water on this near side of Earth to be pulled toward the moon. As gravitational force acts to draw the water closer to the moon, inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the moon, causing a bulge of water on the near side toward the moon (Ross, D. A. , 1995).


On the opposite side of the Earth, or the far side, the gravitational attraction of the moon is less because it is farther away. Here, inertia exceeds the gravitational force, and the water tries to keep going in a straight line, moving away from the Earth, also forming a bulge (Ross, D. A. , 1995). In this way the combination of gravity and inertia create two bulges of water. One forms where the Earth and moon are closest, and the other forms where they are furthest apart. Over the rest of the globe gravity and inertia are in relative balance. Because water is fluid, the two bulges stay aligned with the moon as the Earth rotates (Ross, D. A. , 1995). The sun also plays a major role, affecting the size and position of the two tidal bulges. The interaction of the forces generated by the moon and the sun can be quite complex.


As this is an introduction to the subject of tides and water levels we will focus most of our attention on the effects of the stronger celestial influence, the moon. Tides are said to be caused by the gravitational pull of the sun and moon. A particularly high tide (Spring tide) occurs when these two bodies are in line and both pull in the same direction. How is it possible that when it is high tide at one point on the Earth's surface it is simultaneously high tide at a point on the opposite side of the planet? To my knowledge tides are caused by two effects. The first is the gravitational attraction of the moon (and to a lesser extent the sun). This causes the tidal bulge that forms in the direction of the moon. The second effect arises because the earth and the moon are rotating about their common centre of mass.


Due to centrifugal (inertial) forces as the earth rotates a bulge is also formed on the side of the planet that is facing away from the moon. As far as I am aware it is coincidence that these two bulges are approximately the same size. All the molecules of sea water are attracted by the earth moon and sun. Those on the moon-and-sun side of the earth are close to the moon-and-sun and are most influenced by the moon-and-sun and thus move towards them causing the obverse tidal bulge. On the other side of the globe, the molecules are least effected by the moon-and-sun, being most distant from them. So they move away from the moon-and-sun, causing the reverse tidal bulge. Or something. The earth-moon system orbits at a rate based on the distance between the centres of gravity of earth and moon.


The water at the point on the earth's surface nearest the moon is closer to the moon than the centre of the earth is, but is travelling at the same speed as the earth - ie, too slow to have a stable orbit given it's distance. So it falls towards the moon, giving rise to a bulge on the earth. Similarly, the water opposite the moon is also travelling at the speed of the earth, which this time is faster than the appropriate speed for the water's orbital distance, so it attempts to move away from the moon to a further out orbit, giving rise to the bulge opposite the moon. There is a similar effect due to the sun. Spring tides occur when the sun-induced and moon-induced water humps co-incide - at both full and new moon.

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