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  3. Tides are created mainly by the pull of the moon on the earth. The moon’s pull causes water in the oceans to be a little deeper

Tides are created mainly by the pull of the moon on the earth. The moon’s pull causes water in the oceans to be a little deeper

admin2010-03-26  79
问题     Tides are created mainly by the pull of the moon on the earth. The moon’s pull causes water in the oceans to be a little deeper at a point closest to the moon and also at a point farthest from the moon, on the opposite side of the earth. These two tidal "waves" follow the apparent movement of the moon around the earth and strike nearly every coast line at intervals of about twelve hours and twenty-five minutes. After reaching a high point, the water level goes down gradually for a little more than six hours and then begins to rise toward a new high point. Hence, most coast lines have two tides a day, and the tides occur fifty minutes later each day. Differences in the coast line and in channels in the ocean bottom may change the times that the tidal wave reaches different points along the same coast line.
    The difference in water level between high and low tide varies from day to day according to the relative positions of the sun and the moon because the sun also exerts a pull on the earth, although it is only about half as strong as the pull of the moon. When the sun and the moon are pulling along the same line, the tides rise higher, and when they pull at right angles to one another, the tide is lower. The formation of the coast line and variations in the weather are additional factors which can affect the height of tides. Some sections of the coast are shaped in such a way as to cause much higher tides than are experienced in other areas. A strong wind blowing toward the shore may also cause tides to be higher.
The time that high tide occurs at a particular place is affected by the factors like the position of the moon, channels in the sea bottom and ______.
选项
答案variations in the coast line
解析 在短文中清楚地知道，发生潮汐高潮的时间受月亮位置、海岸线变化及海底形状等因素的影响。
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