The gravitational pull of the moon
The lunar gravity has a fantastic affect and pulls the sea level higher on the side of the earth nearest the moon (Fig 5.2). If we look down on the earth towards the north pole we can visualise two bulges representing high water and two lower areas representing two areas of low water.
These tidal bulges follow the moon around the earth. The bulge nearest the moon is slightly greater than the one furthest from the moon. The moon has created a massive ‘standing wave’ with two peak and troughs that follows itself around the earth every 29.5 days.
So in a hypothetical universe if the earth did no rotate on its axis we would get two high water and two low water every 29.5 days or lunar month. However we know this is not the case and to make a better model of what is going on with the tides we have to factor in the revolving earth and the effects of the sun.