Monday, May 24, 2010

Swimming with the Tide in Hawaii

For those of you who do not remember their 9th grade General Science Class, like me, the ocean’s tides have always been sort of a mystery – e.g., why are there two of them each day?

A search of the literature reveals a complicated, and even somewhat controversial, explanation of why and how tides occur. And some parts of the world do not even have two tides per day! So I will keep it general about why tides occur, but be more specific about what happens.

First, a key definition: Mean Lower Low Water (MLLW): the average of the lowest tide recorded at a tide station each day during the recording period, usually nineteen years. It is the “0” in tide charts, but more about them later.

Simplifying, and ignoring inertia, tides are created because the Earth and the moon attract each other, like magnets. The moon tries to pull at anything on the Earth to bring it closer. However, the Earth is able to hold onto everything except the

The gravitational attraction is strongest on the side of Earth that happens to be facing the Moon, simply because it is closer. This attraction causes the water on this “near side” of Earth to be pulled toward the moon (see below).

On the opposite of Earth (the “far side”), the gravitational attraction of the Moon is less because it is farther away. Thus, the moon’s gravity creates two bulges of water. One forms where Earth and Moon are closest, and the other forms where they are farthest apart. That then means in most of the country, each day there are two high tides and two low tides. The ocean is constantly moving from high tide to low tide, and then back to high tide.

A high tide is as high as the water will reach before it starts to fall again. It is highest when the Earth and Moon are closest, and the other daily high tide is somewhat less than the highest tide (shouldn’t these tides have different names (?)). A low tide is as low as the water goes before it starts to rise again. And the same with the two daily low tides; one is lower than the other.

A common misconception is the thought that since there are four tides daily they must be on a six hour schedule. It takes the Earth about 24 hours to rotate once, relative to the Sun. But, because the Moon is moving with respect to Earth and the Earth is spinning, it takes the Earth a little longer to complete a rotation relative to the Moon—24 hours and 50 minutes. Thus, two daily tides occur separated by 12 hours and 25 minutes.

The amount of rise of fall in the tide is directly related to the relative location of the earth, moon and sun, but we’ll address that next time.

No comments:

Post a Comment

Mahalo for leaving a comment!~WesIsland