Stand upon a beach on the ocean shoreline and watch the waters. Dependent upon the time of day the sea will be either coming towards you or receding away from you. Stand and watch long enough and you will see the point where the movement of the tides changes direction, either at the tide’s highest point known as high tide, or at its lowest point known as low tide. The same effect occurs on both the calmest and stormiest of days, so what is causing the ocean tides?
Tides are a periodic rise and fall of the waters of the ocean, and it is the gravitational attraction of the Moon and its orbiting of the Earth which is the primary cause. The Sun also has a gravitational attraction and thus an influence on the tides, but because of the closeness of the Moon its effect is far greater. As the Moon’s distance from the Earth varies throughout the year, so too does its attractive pull. This means the times and heights of high and low tides change from day to day. The involvement of other forces, such as those caused by the rotation of the Earth, and the interactions of the Earth, the Moon and the Sun as they all travel through space, means that predicting the tides is difficult.
The average time between one high tide and the next is approximately 12 hours 25.5 minutes. However, the interval between high tides can vary from less than 11 hours to more than 14, dependent on location. In the North Sea there are two high tides and two low tides a day, these are known as semidiurnal tides. The Caribbean has only one high tide and one low tide a day, and these are diurnal tides. Most coastal regions experience semidiurnal tides. The two high tides and the two low tides of semidiurnal tides, are not necessarily equal in height on any given day. To complicate matters further some coasts, especially of the Pacific and Indian Oceans, may experience a mixture of diurnal and semidiurnal tides.
When the Moon and the Sun are in alignment, either at the time of the full or new moon, both gravitational pulls are combined to produce a maximum tide range. These events are known as spring tides, where high tide are higher than average and low tides are lower than average. The highest spring tides occur when the Sun is directly over the equator, and are thus at the equinoxes. The lowest spring tides occur at the solstices. When the Moon and Sun exert gravitational pulls that are at right angles to each other, during a waxing or a waning half Moon, tide ranges are generally lower. These are known as neap tides.
As well as processes outlined above, the tides at a particular location are also dependent on other factors. Both the topography of the local coastline and seabed are factors, amongst others. Accurate measurements of the tides over many years are needed to be able to reasonably predict the tides in any given location, as all tides and their characteristics can vary on a daily, monthly and yearly basis.