What is a tide?
In marine navigation, the word "tide" is a common term used to mean the rise and fall in sea level. But do you really understand how to tides work? Tides are the the result of the gravitational pull that the moon and the sun have on our planet. Other factors that influence tides include the depth of water at a particular place, the ocean floor topography and the configuration of the coastline there. Anybody living near the sea observes, daily, that there is ‘high water’ (the highest sea level for the day) and a corresponding ‘low water’. This usually occurs twice a day, (called semi diurnal tides) but not always. Some places have only one high water and low water every day. (diurnal tide). Sometimes they are even mixed.
What causes tides?
The earth’s gravity holds the ocean water close to the surface. However, the gravitational forces of the moon and sun also act on the earth’s water, drawing it towards them. The force of gravity of the Earth and these external forces caused by the sun and moon cause the water to rise and fall on the earth’s surface depending on the strength of these forces at a particular time. (some are variable, as we will see later) High tides are produced in the ocean waters by the "heaping" action resulting from the horizontal flow of water toward two regions of the earth representing positions of maximum attraction of combined lunar and solar gravitational forces.
As water levels rise in one place, water from other places on Earth falls to compensate, causing low tides in those areas. As has been said earlier, other factors also play a part. Also, the earth and moon’s revolution around the sun is elliptical, although the sun is not at the centre of the ellipse. At times, the earth is closer to the sun and so the sun’s effect is greater. Similarly for the moon.
The difference in the height between consecutive high and low tides at a particular place is known as the range of tide.
Spring and Neap tides (see diagram)
The earth and the moon are revolving around the sun. We can see, from the diagram, that the attraction caused by the sun and moon is maximum when they are in line , or on opposite sides of the earth- either at New Moon or Full Moon. This is because they complement each other at those times. This greater attraction obviously causes a higher range than normal and the tides are then called spring tides
When the moon is not complementing the sun in the First and Third Quarter, the range is minimum. The tides are then called neap tides
Prediction of tides
The times of High water and low water are calculated using a mix of astronomical and historical data for any location near the sea. These are tabulated and published yearly around the world in publications such as the British Admiralty Tide Tables, which all ships carry. Knowing the times and heights of low and high water, (LW and HW) is of great benefit to marine navigation, specially when sailing through rivers and entering ports where water levels are shallow. Knowledge of currents related to tides is also beneficial for safety.