什么是潮汐？
Tides are one of the most reliable things in the natural world. Just as we know the Sun will rise in the east and the stars will come out at night, we can be sure that the ocean will rise and fall along our shores.
Tides are very long waves that move through the ocean in response to the pull of the Moon and the Sun. They start in the open ocean and travel toward the coast, where we see them as the regular rise and fall of the sea.
When the highest part of the wave, called the crest, reaches a particular place, it is high tide. Low tide happens when the lowest part of the wave, called the trough, arrives. The difference in height between high tide and low tide is called the tidal range.
Tides play an important part in the natural world, and they also matter a great deal to people who work at sea, from fishers to ship captains.
潮汐是自然界中最靠得住的现象之一。就像我们知道太阳会从东方升起、夜晚会有星星出现一样，我们也可以确定，海水会沿着海岸一涨一落。
潮汐其实是一种波长非常长的波，它在月球和太阳的引力作用下在海洋中移动。潮汐从大洋中形成，向海岸推进，我们在岸边看到的，就是海水有规律的涨落。
当波的最高处，也就是波峰，到达某个地方时，那里就是涨潮；当波的最低处，也就是波谷到达时，就是落潮。涨潮和落潮之间的高度差，叫作潮差。
潮汐对大自然影响很大，对在海上工作的人，从渔民到船长，也都非常重要。