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Most of the Earth’s seas experience two high and two low tides each day.  There is also a tide in the solid surface of the Earth with a height of about half a metre.  Tides are caused by the gravitation pull of the Moon and the Sun.  To understand why there are two tides each day, we must consider the force of gravity acting on the sea and the fact that the whole system is in rotation.

waves.jpgEach month, the Earth and the Moon both orbit their common centre of gravity, G, located about 46,000 km from the centre of the Earth, O, in the direction of the Moon.  Consider the point B on the Earth’s surface nearest the Moon.  Here the pull of the Moon’s gravity is greatest.  On the opposite side of the Earth at A, the pull of the Moon’s gravity is weakest. 

We must also include the effect of the centripetal force associated with points B and A rotating once a month about point G, which is below the Earth’s surface.  The overall result is that there are equal-sized oceanic bulges on opposite sides of ht Earth.  The oceanic tidal bulges are fixed with respect to the direction of the Moon, while the solid Earth rotates about point O, beneath the oceans, giving rise to two high tides each day.

Exactly the same types of tidal bulge are created by the interaction of the Sun and the Earth.  The sizes of the tidal bulges due to the Sun are slightly less than half the size of those due to the Moon, giving rise toe the phenomena of spring and neap tides.  Roughly twice a month, the Sun and Moon line up and then the effects of the Sun and Moon reinforce each other to produce the largest tides, the spring tides.  When the effects of the Sun and Moon are at right angles to each other, the sizes of the tidal bulges are at a minimum, corresponding to the neap tides.