Geography & History Notes | KCSE, NECTA, ZIMSEC & GCSE

  The tide is a periodic rise and fall in the level of the ocean and other large water bodies. Tides occur when the moon and to some extent the sun exert a gravitational pull on the water bodies on the earth. A lunar day is time taken by the earth to complete one rotation with respect to the moon (24 hrs 52 min) Lunar month is time taken by the moon to complete one revolution around the earth (27.3 days) The moon is always ahead of the earth by 52 minutes due to its revolution e.g. if Nairobi is opposite the moon at 6 pm the following day the high tide will be at 6.52 pm. The tidal range is the difference between the highest level reached by high tide and the lowest level reached by low tide. Types of tides Spring Tides In which the highest and lowest tides occur. Occurs when the sun, moon, and the earth are in a line (syzygy position) and pulling in the same plane causing pulling force to be greatest. Neap Tides In which high tide is lower than normal and low tide is higher than ...

Most people grow up thinking that the tides are caused by the moon, and indeed that gravitational 'pull' of the moon is a major factor, as is the gravitational effect of the sun but there is another major factor, which is less often mentioned, and that is the force created by the rotation of the earth itself. So the diagram on the left shows how the water on the earth gets pulled into a bulge one way by the moon's gravity and into a bulge on the opposite side by the rotational force of the earth. As the earth rotates on its axis once every 24 hours, if the moon stood still then the moon will be overhead and 'underfoot' once in every 24 hours giving (in most places) 2 high and 2 low tides a day, but as the moon is also orbiting in the same direction as the earth revolves it actually takes about 24 hours and 50 minutes for the moon to be in the same place overhead. So the period from one high tide to the next is about 12 hours 25 minutes. Spring Tides When the sun, mo...