ADVANCED INSTRUCTOR KNOWLEDGE


Tides are the vertical movement of water caused by the gravitational pull of the Moon and Sun on the Earth. The Moon, being closer to Earth than the Sun, has the greatest influence on tides and controls their timing.


In most tidal areas there are two tides per day, caused by the Earth turning on its axis. In the 6 hours (approximately) between tides, the volume of water does not rise or fall at a constant rate. Slack tide, the hour before and after high tide or low tide, shows very little movement of the water. However, during the third and fourth hours, half of the total volume of water is moving. Tide and current tables are available for most locations.


The range of the rise and fall of the tides is affected by the Moon’s position relative to the Sun. When the Sun and Moon are in a straight line, there is greater gravitational pull on the water along that line. Therefore, there are large ranges of tide movement known as “spring tides” - higher high tides and lower low tides than normal. When the Moon is in quadrature, the Sun’s influence is moderated and the tidal range is less than normal. These are neap tides.


SPRING TIDES Sun's


gravitational pull EARTH


SUN MOON Moon's gravitational pull Moon's gravitational pull MOON


From the figure, it can be seen that the sun and moon are in a straight line twice a month and at right angles twice a month. This produces the two spring and the two neap tides we experience monthly. The vertical movement of tides can be seen as the water rises or falls on a piling or beach. A falling tide will leave a wet beach or piling next to the water’s edge. A dry beach or piling next to the water signifies a rising or high tide


WATER WATER SUN


NEAP TIDES Sun's


gravitational pull EARTH


Table of Contents


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