<Li> Mean sea level (MSL)--This is the average sea level . The MSL is constant for any location over a long period . </Li> <Li> Mean low water neaps (MLWN)--The average of the two low tides on the days of neap tides . </Li> <Li> Mean low water springs (MLWS)--The average of the two low tides on the days of spring tides . </Li> <Li> Lowest astronomical tide (LAT) and Chart Datum (CD)--The lowest tide which can be predicted to occur . Modern charts use this as the chart datum . Note that under certain meteorological conditions the water may fall lower than this meaning that there is less water than shown on charts . </Li>

Explain why the magnitude of the tides varies from time to time