The tide is the vertical rise and fall of the sea level surface caused primarily by the change in gravitational attraction of the moon, and to a lesser extent the sun.
As the earth spins on its axis the centrifugal force results in slightly deeper water near the equator as opposed to shallower water at the poles. In fact it causes a flow from the poles to the equator.
The earth is also in orbit around the sun (one revolution in one year) creating not only another centrifugal force but also a gravitational interaction. These two yield a bulge on the night site (centrifugal) and a bulge on the day site (gravitational) both of them moving as the world turns. Therefore, a certain place on this world will experience two high and two low tides each day.
With these forces alone, we would not have spring tides and neap tides. Spring tides have higher high tides and lower low tides whereas neap tides have lower high tides and higher low tides. Hence, the range (difference in water level between high and low tide) is much larger in a spring tide than in a low tide.
This animation shows how the tide changes during the lunar cycle. When the sun, moon and earth are aligned : spring tide.
When at right angles the forces are not aligned:
The time between spring and neap is approximately 7 days.
These differences in range can be explained if we include the moon into our earth-sun system. The moon and the earth orbit each other around a point (called the barycenter or baricenter) 2000 odd kilometres inside the earth, creating a centrifugal and a gravitational bulge. Moreover, despite the sun's immensely larger mass, the moon exerts a 2.25 times larger gravitational attraction, since the moon is much closer to our earth.
It is the combined effect of the sun and moon that creates spring and neap tides. In the animation the gravitational forces of both the sun and the moon are taken into account. When aligned with the earth they combine their attraction and otherwise they counteract their attraction. The sun is located in the corner right below, far outside this picture (note the eclipse) while the moon is revolving round the earth. One full circle corresponds to one lunar cycle (about 28 days).
The fact that gravity is an inverse square law leads to the idea of tides and tidal forces. Essentially the Moon exerts its gravity on the Earth as the Earth does on the Moon. The force of gravity from the Moon on the near side of the Earth to the Moon is larger than on the far side. This causes a stretching force. For a liquid like water that can flow freely it leads to the piling up of water on both the near side of the Earth towards the Moon and on the far side. If you wanted to understand why there are two tides a day rather than one consider the stretching of a spring with the distance between any two coils and the spring increases. Thus, tidal forces cause pilling up of the oceans in the direction both away and afar from the Moon, and as the Earth rotates one in 24 hours two tides a day are seen. Tides also apply to the land masses. The gravity force in absolute terms of the Sun on the Earth is stronger than that from the Moon, but the stretching force from the Moon is stronger than the stretching force caused by the Sun. However they do act together. So near a new moon or a full moon the tidal forces of both sun and moon combine to produce extra large tides and extra large probability of earthquakes.
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