What Causes Tides

I gotta keep breathing. Because tomorrow the sun will rise. Who knows what the tide could bring?
- Chuck Noland (From the movie Castaway)

When I think of tides, I can't help but think about Tom Hanks, playing Chuck Noland in 'Castaway' and this dialog that I quote above. He gets marooned on an island, cast away from society and his only hope lies in what the tide brings him! For him the tide brings hope! It brings him a sail one day which makes him build a ship and get back to where he belongs. Tides are so much a part of human culture that we even have a proverb about it - 'Time and tide wait for no man!'. Those of you who have lived near seashores or visited one often, must have noticed the rising and falling of the sea water level every day. This phenomenon is called tidal motion. The advancing and receding volumes of water are called tides. Sitting on the seashore, especially in the evening, we are either too captivated by the splendid view or too involved in our own thoughts to notice the gradual rise and fall of sea water level and the advancing and receding of the tide.

About Tides

To understand the reason behind the occurrence of a phenomenon, one must know the details of the phenomenon in depth. One thing about tides is that they are periodic phenomena. A high tide is the maximum advancement of sea water towards the shore, covering the 'intertidal' zone. This zone is the region of the seashore that has no water, except during a high tide.

Subsequently a low tide is when the water recedes back into the sea, exposing the intertidal zone. Every coast around the world experiences two high tide and low tides every day, separated by a period of approximately twelve and a half hours. Once in a while, a record high tide is recorded, called a spring tide that is the highest of the high tides and they generally occur during a Full Moon and New Moon. Sometimes, the high tides go very low and are called 'Neap Tides'. Neap tides peculiarly occur on a half Moon.

The sea tides have a very deep connection with the rhythm of life in the sea. Life originated in the oceans and perhaps the ancestors of the first land dwelling organisms were washed up on shores by tides. Study of tides is an important part of Earth science.

Let us look at facts of the matter or clues that may help us in understanding how are tides formed and what are the forces that create these phenomena. One thing is that they are periodic. They have something to do with the Moon as their maxima and minima are connected with Moon's phases!

What Causes Tides to Occur?

There is something called gravity that can never be switched off somehow and it's Moon's gravitational pull along with Sun's, that causes tides to occur. Another factor that causes tidal formations in our oceans is Earth's rotation, which creates a centrifugal force. Newton was the one who discovered the law of gravitation and one of the first explanations of gravity that he gave was the formation of tides!

The Sun and the Moon exert gravitational influence on the Earth and a by-product of that gravitational influence, along with Earth's rotation is sea tides. When the Moon is right overhead a region on Earth, the water in the sea below gets pulled up by its gravitational tug. That creates a high tide in that region. When this is happening on one side of the globe, on the exact opposite side too there is a high tide. That is because, though the water on the other side does not rise, Earth itself is slightly pulled by the Moon's gravity, effectively raising the level of water relative to Earth! Since, the total volume of sea water on Earth remains constant, when two points on the Earth's surface have a 'High Tide', regions separated 90 degrees on both sides of a high tide naturally experience a low tide. As the Earth rotates around itself every twenty four hours, these high and low tides, separated from each other by a 90 degree phase, travel around the planet. Rotation of the Earth makes tides change with time, across the oceans of the globe.

On a New Moon or a Full Moon, the Sun and the Moon get aligned in a straight line and their combined gravitational influence causes the highest tides ever, which are spring tides. Whereas, during a Half Moon phase, the Moon and the Sun are at right angles and they nullify each other's gravitational pull, giving rise to the lowest high tide, which is the 'Neap Tide'. Every tide is thus influenced by the gravitational pull of Sun and Moon. In fact the Moon also has an influence on the Earth's crust, which is a floating landmass over a sea of magma. 'Earth tides' are a result of this gravitational influence.

To summarize, tides are caused due to the eternal, periodically waxing and waning force of gravity, exerted by the Sun and the Moon, combined with Earth's rotation.