Causes: Two forces are acting on the earth: (a) the moon’s attraction, (b) the centrifugal force. Al­though these two forces are equal and opposite to each other at the centre of the earth yet that is not the case everywhere on the earth.

Tide is caused by the difference in these two forces. Spring Tide: It is that tide when the rise and the fall of water are the highest. It is caused when the sun and the moon are in a straight line and their attractive force acts in unison on New Moon and Full Moon days. The gravity of the moon operates in favour of the gravity of the sun.

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Neap Tide is that tide when the rise and fall of water are the lowest. It occurs when the Moon and the Sun make a right angle with the earth, i.e., on the half Moon (the 1st and the 2nd quarter).

Eclipses: Eclipses are caused by the fact that light travels in a straight line, producing clearly well- defined shadows of objects in its path. Lunar Eclipse: When the earth comes between the moon and the sun, and it shuts off the light of the sun from falling upon the moon. Solar Eclipse: When the moon comes directly between the sun and the earth and it obstructs light of the sun from falling upon some portions of the earth and the people in these regions cannot see the sun. Currents: They are great circulatory movements of ocean waters either warm or cold and are caused by the permanent winds, i.

e., Trade Winds and the Westerly’s and by the difference in density of sea water. The best known example is Gulf Stream in the Atlantic Ocean.