The waters of oceans are never still.
The oceans actually exhibit three major types of movements – waves, tides and currents.
Waves are oscillatory movements that result in the rise and fall of water surface.
In fact, the movement of each water particle in a wave is circular.
A wave has two major parts.
The raised part is called the crest.
Between the two crests are low areas called troughs.
The vertical distance between trough and crest is called wave height.
The horizontal distance between two crests or two troughs is called wavelength.
The time it takes for two crests to pass a given point is called wave period.
Fast moving waves have short period while slow moving waves have long period.
The size and force of a sea wave depends on three factors
 Velocity of the wind.
 The length of time the wind blows.
 Distance that the wind has travelled across the open sea.
When waves are associated with storms or volcanic eruption, they are very violent and cause damage on coastal areas.
They are also a source of energy.
Along a coast all over the world, we observe the sea water moving both upwards and downwards at rates varying from place to place.
Such a variation in sea level occurs from hour to hour and from day to day.
At the time of a rising sea level, the incoming tide towards the land is spoken of as a flow tide or a flood tide.
At the time of a falling sea level after a few hours, we speak of the tide water going out or withdrawn, is an ebb tide (low tide).
The flood tide is a high tide and the ebb tide is a low tide.
Twice a day regularly at constant intervals, a tide flows in and twice a day it ebbs away.
Twice a month, flow tides are higher and the ebb tides are lower than the average.
Also twice a month flow tides are lower and the ebb tides are higher than the average.
However the regular interval between two high tides or between two low tides is 12 hours and 25 minutes and not exactly 12 hours.
Each day (in 24 hours) the high tide arrives about 51 minutes later than on the previous day.
It is so because each day the rising and setting of the moon also falls behind by 51 minutes.
It takes 24 hours and 50 minutes for the rotating earth to bring the same meridian vertically below the moon every day.
The factors responsible for bringing about such a variation in the regulation and the size of tides are:
 The location of the sun, the moon and the earth in relation to each other which is rarely in a straight line.
 The distances of the sun and the moon from the earth are not constant.
 Our globe is not entirely covered with water.
 The outline or shape of the coast may help or hinder the tides.
The forces that generate the tides—–
The earth attracts and is also attracted by the sun, the moon and by other planetary bodies.
It is called the gravitational force and it operates between the sun, moon and the earth.
It sets the ocean waters in motion producing a tidal current.
Tides are the proof of such a gravitational pull.
The moon and the sun both exert their gravitational force on the earth.
The gravitational attraction of the moon is more effective on the earth than the gravitational attraction of the sun.
Since the water is liquid and mobile, its bulging in the direction facing the moon is easily noticed, yet a lower tidal bulge also develops on the other side of the earth farther from the moon because of moon’s least attraction.