A tide is the periodic movement of seawater caused by the combined effects of the gravitational forces exerted by the moon and the sun. Tides include the periodic rise and fall of sea level and the periodic lateral flow of seawater. The former is called a tidal wave, and the latter is known as a tidal current. In Chinese, the day tide is called ‘Chao’, and the night tide is called ‘Xi’, which are similar. The seawater rises to its highest level at high tide. The high tide mark is the boundary between the land and the ocean at high tide. The seawater falls to its lowest level at low tide, and the low tide mark is the boundary between the land and the ocean at low tide. In most locations, a complete cycle of tidal rising and falling takes 12 hours and 25 min. When there are two equal high and low tides each day, it is called a semi-diurnal tide. When there is only one high and one low tide each day, it is called a diurnal tide.