Tides: Meaning, Types, Duration, Frequency and Facts

Tides are a result of gravitational pull by both Sun and Moon, but the pull exerted by Sun is apparently weak. This is because of the larger distance as the gravitational force is inversely proportional to the square of the distance. The alignment of Sun and moon affects the size of the tides.

Tides are known to Indians since ancient times. In Rig-Veda there was no record about Tides, but in Samveda Chapter 10, Part II, 20, mentions Soma….samudravardhanam, links moon to tides.  In the Indus Valley Civilization, a dockyard was found in Lothal in Gujarat. The 4 walls of this dock towards the estuary are made up of kiln burnt bricks, which prove that these people not only observed but also understood and created structures as per the phenomena & impact of tides.

Earliest geographer to state that the tides are caused by moon was Pytheas, an early Greek geographer around 300 BC.  But he could not understand what the reason was. Newton’s analysis of gravitation explained the phenomenon.

Stages of Tides

There are 4 distinct stages of tides:

• Stage I: Sea level rises over several hours, covering the intertidal zone and this is called flood tide.
• Stage II: The water rises to its highest level, known as high tide.
• Stage III: Sea level falls over several hours, revealing the intertidal zone. This is called ebb tide.
• Stage IV: The water stops falling, this is called low tide.

In general the rising tides are called flood tides and falling tides are called ebb tides and they are known as Jwar & Bhata in Hindi respectively. In astronomy, the alignment of three or more celestial bodies in the same gravitational system along a line is called Syzygy and eclipses occur at the time of Syzygy. Syzygy also affects tides in the form of variations between the High tides and Low Tides.

Spring Tides and Neap Tides

When there is  greatest variation between the high tides and low tides, it is called Spring Tides. Gulf of Fundy is known for highest tides in the world (approximately 50 meters).  At spring tide, Sun, Moon and Earth are in a line. When there is smallest difference between high and low a tide, it is called Neap tide. It occurs when Sun, moon and Earth are at right angles.

• Neap Tides occur when Moon is a quarter phase
• Spring tides occur on Full Moon as well as New Moon.
• Spring tide has higher than normal high tides and lower than normal low tides.
• During the Neap Tides, the high tide is lower and the low tide is higher than usual.

Daily Frequency of Tides

Earth rotates around its own axis and it takes 24 hours to finish its rotation. The direction of Earth rotation is the same in which Moon revolves around earth. The gravitational attraction moon raises the water on two opposite sides of Earth; these points are called Sub-lunar and antipodal points.

The tidal bulge shown in the above graphics follows the revolution of the moon, and the earth rotates eastward through the bulge once every 24 hours and 50 minutes. (This is because, of the relative distance covered by moon in its orbit). This tidal bulge shows that the water of the entire world ocean is pulled by the moon’s gravity.

On both the opposite side of the earth simultaneously there is a high tide. This high tide is on both sides due to inertia of the ocean water and because the earth is being pulled toward the moon by its gravitational field and the ocean water remains left behind. This means that Earth is pulled little away from the water. This creates a high tide on the side of the earth opposite the high tide caused by the direct pull of the moon.

Duration and Frequencies of Daily Tides

Most coastal areas experience two high and two low tides per day. One of these high tides is at the point on the earth which is closest to the moon (sub lunar) and other high tide is at the opposite point on the earth (antipodal).

One tidal cycle comprises two high tides and two low tides. One tidal cycle completes in 24 hours and 50.4 minutes. This is because of the revolution of Moon around the earth and both earth’s rotation and moon revolution are in same direction. (Moon is not stationary, so there is a difference, if moon were stationary the high tides would have occurred exactly in 12 hours). The high tides occur at an interval of 12 hours and 25.2 minutes. This means that if there is a high tide is at 7.00 am, next high tide would be at 7.25 pm and next would be at 7.50 am, and so on. The time difference between two high tides is called “Tidal Interval”. The tidal cycle in this pattern is called semidiurnal. However, most of the enclosed water bodies or away from the open ocean such as Caribbean sea or Caspian Sea, there are only one high tide and one low tide. This pattern is called Diurnal tides.  At the coast of the oceans, there may be two high tides, of unequal length. This is called Mixed Tides.

Apogean Tides and perigean tides

When moon is at closest point to Earth during its revolution around earth it is called Perigee. The high tides are higher than usual and low tides are lower than usual at this point. When moon is farthest, it is called apogee and the high tides are lower than usual and low tides are higher than usual at this point of time. The highest Spring tides occur when the Moon is at its closest to the Earth…the so-called Perigee Tide.

Impact on tides if there was no moon?

If there is no moon, lunar water tides on the Earth go away, but the solar water tides still occur. At the same time, there would be no ‘Spring’ or ‘Neap’ tides.

The Tides Which Are Not Tides

The tide suffix has been used for various phenomena. Some of them are not at all related to the tides. The following are a few:

Storm Tides

Storm tides is the name given to the offshore rise of water associated with a low pressure weather system. Wind causes the water to pile up higher than the ordinary sea level and they are nowhere related to the tides.

Rip Tides

Rip tides are strong channel of water flowing seaward from near the shore, typically through the surf line. They are again caused by the winds and not related to Tides.

Tsunami

Tsunamis are called ‘harbour wave’, and they are result of displacement of a large volume of a body of water, usually an ocean due to Earthquakes, volcanic eruptions and other underwater explosions.

Other Terms Related to Tides

Tidal datum

A chart obtained from the long period of a tidal record is called tidal datum.

Tidal Flat

The sands uncovered by the low tides is called tidal flat

Tidal Range

Tidal range is the difference between the height of water at low and high tides.

Tidal Bore

Tidal bore is a Tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travel up a river or narrow bay against the direction of the river or bay’s current.

Earth’s Tides

Earth’s tides, also known as terrestrial tides affect the entire Earth’s mass. This involves the movement of Earth’s crust in all directions, due to solar and lunar gravitation.

Intertidal zone

Intertidal zone is sometimes known as littoral zone and it is that area of the sea shore or shore of water body such as Open Ocean, which is exposed to air at low tides and water at high tides. The examples are rocky cliffs, sandy beaches, or wetlands. Bay of Fundy is an excellent Intertidal Zone Ecosystem. 

Do tides affect Earth’s Magnetic Field?

Yes. The tidal forces generate currents in conducting fluids in the Earth’s interior and they affect the Earth’s magnetic field.

Tidal Ports

During high tides, water rushes into harbours (Tidal Bore). This helps ships enter and exit harbours safely. High tides make ocean/sea water rush into the mouths of rivers. This helps ships to enter port towns like New York, London, Rotterdam, and Hamburg. This is perfectly advantageous for some ports in India such as Kandla, Mangalore and Kolkata’s diamond harbour.

Tides and prevention of siltation

Tides take away the terrigenous material brought by the rivers at the rivine harbours and thus help in the prevention of siltation.

Tides and prevention of Rivers Freezing

The temperature at which sea water freezes is much lower than that of river water. In cities like London, due to the high tide, the sea water enters the river and prevents it from freezing.

Tidal power

Power can be generated exploiting the huge energy of the tides. This can be done by making dams and the tidal zones where best tides occur and allowing the water to enter and exit through a turbine. However, this is a complicated work and not much success has been achieved as of now. The water accumulation during high tides can also be stored behind specially made dams, which can be then used for hydel power. World first tidal power station is Rance Tidal Power Station, located on the estuary of the Rance River, in Brittany, France. It is also world’s largest tidal power station which started working in 1966. Its annual output is 600 GWh.