Solution:The periodic rise & fall of sea level, once or twice a day, is called a Tide. When the Sun, the Moon and the Earth are in a straight line, the height of tides will be higher than normal. These are called spring tides, which occur on new Moon or full Moon day. At these times, high tides are very high & low tides are very low. Thus, spring tides refer to the greatest difference in the sea level at high and low tides.