Many people have wondered what makes the oceans rise and fall on the world's beaches twice a day.Those tides are caused by the force of the Moon's gravity¹ and the rotation² of the Earth.

Gravity acts on the center of any solid object. The Earth is mostly molten³ rock，but compared to the oceans， we can think of it as a solid object.

As you know，the oceans are fluid and lie on the surface of the Earth.When the Moon is overhead，the oceans are four thousand miles closer to the Moon than the Earth's center is. Because the o ceans are closer， the Moon pulls harder on the water than on the Earth. This stronger pull makes the water bunch⁴ up a little under the Moon，creating a high tide.

Atthesame time， the oceans on the opposite side of the Earth are four thousand miles farther from the Moon thantheEarth's center is. The Moon's pull on this water is weak.This water bunches up because it is \"left behind\"(making another high tide) as the Earth is pulled slightly⁵ toward the Moon.

When the waters bunch up on these two sides of the Earth，they draw water away from the other sides of the Earth， which creates the low tides.

As the Earth rotates，different parts of the land and oceans pass through the high-tide and low-tide zones.And that makes the waters on our coasts go up and down.

注釋：

1．gravity n. 地心吸力，(萬有)引力

2．rotation n．旋轉，[天]自轉

3．molten adj．熔化的，熔融的

4．bunchvi. ~up集中，擠在一起

5．slightlyadv. 輕微地，少量地

許多人都想知道為什么海水在海灘上會有一天兩次的漲潮和落潮現(xiàn)象。這些潮汐現(xiàn)象是由月球引力和地球自轉引起的。

萬有引力作用于任何固體的中心。地球主要是由熔化的巖石組成的，但相對于海洋來說，我們可以把它看作是一個固體。

正如你所知道的，海洋是液體并且處于地球的表面。當月亮高懸空中時，海洋距月球的距離比地球中心距月球的距離近4000英里。因為海洋距月球近，所以，月球對大海的引力要比對地球的引力強。這種強烈的吸力使海水在月亮下簇集起來，這就是漲潮。

同時，地球另一面的海洋距月球的距離比地球中心距月球的距離遠4000英里。月亮對它的吸力就弱。這里的海水簇集在一起(形成另一次漲潮)，是由于當?shù)厍蚴芤ψ饔帽焕蛟虑驎r，這一側的海水則被“放任自流”。

當這兩個方向的海水都簇集起來，它們會對地球另外兩個方向的海水產生吸力，這就是落潮。

當?shù)厍蜃赞D時，不同地區(qū)的陸地與海洋都會經過漲潮和落潮區(qū)，這樣就形成了沿海地區(qū)海水漲落的現(xiàn)象。