Here we will discuss the cause of the ocean tides. A rising tide is when the water level rises, and a falling tide is when the water level goes down. We call this the tide coming in and the tide going out. The tides on Earth are caused by the gravitational pull of the moon. On the side of earth facing the moon, the ocean feels a stronger force of gravity than on the far side of the Earth. Since the gravitational force is an inverse square law, the difference in gravitational pull is very different on opposite sides of the earth. This causes the oceans to bulge, and this is what we notice as the tides on earth. There are two tides per day because the moon's gravity causes two ocean bulges on Earth.

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