Tides can be diurnal, meaning that they most commonly occur twice a day. Tides can also be semi-diurnal, meaning that they occur as two high waters and two low waters each day. However, these periods do not happen at the same time each day. This is because the Moon takes slightly longer than 24 hours to line up again exactly with the same point on the Earth. This averages to about 50 minutes more. Therefore, the timing of high tides is staggered throughout the course of a month, with each tide commencing approximately 24 hours and 50 minutes later than the one before it. As a result, the high and low tides will appear later and later every day.
There are many factors involved in predicting the tides. In addition to the motion of the Moon and Sun as described above, the timing of the tides is also affected by the Moon's declination. This is the angular height above the equator. The local geography of the coastline, topography of the ocean floor, and depth of the water can also influence the size of tides. For greatest accuracy, tide prediction tables always integrate data from actual observation, often over a period of many years.