Boaters in coastal areas often learn about tides the hard way. Tying a boat fast to a piling will cause two things to happen in areas that experience high tidal fluctuations. Returning to the boat after a couple hours, the owner will either find it hanging from the piling by its bowline or under the water. Learning how to read a tide table is an important boater skill. So what causes the tides?
The oceans of the world all join each other at least one point. Tides are easier to describe if you picture all the oceans uniformly covering the earth. The earth's gravity pulls water towards its center and thus holds it balanced in place on its surface. The gravitational pull or traction of the moon and sun upset this balance and cause the water surrounding the earth to bulge out towards them. Earth's rotational or centrifugal force causes a matching bulge on the opposite side of the earth. These two humps are the high tides of the earth. If you picture the oceans of the earth as a balloon, forcing two sides outward will cause the side lying 90 degrees from the bulge to contract. These are the low tides.
While the Sun is much larger than the moon, the moon has a greater impact on tides because of its closer proximity to the earth. The Sun exerts only about half the pull generated by the moon on the earth. Most coastal boaters have noticed that greater tides occur in the spring and fall. The moon and Sun rotate around the earth at different rates measuring months and years. You can imagine that having both bodies inline and on the same side of the earth will cause larger tides due to the combined tractional force. Just to stretch your imagination a little more, the same effect occurs when the Sun and moon are inline and on opposite sides of the earth from each other. Mariners know these high periods as spring tides. When the Sun and moon are in positions roughly ninety degrees from each other, minimal tides occur. These are called neap tides. Spring and neap tides vary from normal tides by about twenty percent.
Earth's daily rotation causes two high and two low tides each day, making most tides we experience semidiurnal. Some parts of the earth vary from this norm. Alaska has what are called mixed tides where the highs and lows are not the same height, that is for example the first high tide will be different than the second one occurring in the same day. Tides occur 50 minutes later each day due the complete rotation of the moon taking 24 hours and 50 minutes each day. One other factor influencing tides is ocean topography. Tides in deep water are minimal, but as the tidal bulge approaches shore, the shallow water amplifies the tidal height.
Tides cause the ocean's currents. You can picture water rushing from a low tidal point to the high tide's bulge. Local topography modifies current as well. In many areas, narrow passages accelerate the water flow. Reaching speeds of over eight knots, some of these currents can out perform the top speed of slower vessels. Picture the tide forcing you backwards with your boat! If you would like to know more about tides and currents, visit the online edition of Bowditch for an exhaustive discussion.
