What are tides and what causes tides?

The waters of an ocean are moving all the time. The surface of the water moves up and down as waves travel across the surface. The level of the ocean rises and falls with the tides. What makes these movements that we call waves and tides? Waves are caused by winds, earthquakes, and the gravitational pull of sun and moon. Waves move forwards, but under each wave, the water moves around in a circle. As each wave reaches the shore, it causes the water near the shore to break into surf.

Water on the surface of an ocean moves up and down while the waves travel along its surface. The water does not really move forward until the wave reaches the shore.
Have you ever gone to a beach along the seacoast to walk or play, but found it completely covered by water? Sometimes, you have to wait as long as six hours before the seashore is uncovered again! These changes in the level of water are called tides. High tide is when the water covers the shore. Low tide is when the beach is uncovered again. High tide and low tide each occur twice at all places on the ocean every day.
Tides happen because of a force called gravity. The gravity of the moon pulls the earth's waters towards the moon. And the gravity of the sun pulls the earth's waters towards the sun.
The moon takes about one month to move around the earth. When the moon is either new or full, the sun, moon and earth lie in a straight line. The tides that happen then are called spring tides, occurring about twice each month. During spring tides, the difference between the level of water at high tide and at low tide is very large. When the sun, moon and earth are at right angles to each other, the tides are called neap tides, also occurring about twice each month. This is when the difference between high tide and low tide is small.
The difference between the level of water at high tide and low tide also varies with the shape of the coast. On the coasts of the Mediterranean Sea, the difference may be as little as 1 foot (30 centimeters). In the Bay of Fundy in eastern Canada, the difference may be as much as 40 feet (12 meters).
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