There are many reasons for tidal action thus, tides are very complicated phenomenon involving many factors, all of which must be considered to predict tidal action in any one place at any given time. The alternate rise and fall of the surface of the seas and the oceans is mainly caused by the attractive force of the moon. When the attraction is great, the tide is high conversely, when the attraction is minimal, the tide is low. Between the high tide and the low tide is a period of a little over six hours, being called during its rise a flood tide, and during its fall an ebb(退潮) tide. The sun also exerts its effect on the tide, being a little less than half as much as the moon, although its mass is so much greater. When the moon and the sun are horizontal, the highest tide occurs and when they are vertical the lowest tide takes place. The wind also plays an important part in the modification of the normal rise and fall of the tides. Thus these factors affect each other, either detracting(转移) or adding to the attractive forces to add or subtract from the magnitude of the tide. One thing is certain: the tides move inexorably(不可阻挡) on, rising twice a day, shifting a little in time each day, and coutinually posing the problem of how to correctly predict their highs and lows. The principal idea of this article is ______.