- Tides are the movement of water vertically up and down the shore. Currents, on the other hand, are the horizontal movement of water. As a simplified example of this difference, imagine standing ankle deep in the East River. The first thing you would notice is the feeling of water passing around your ankles (this is the current); after an hour or more, you would notice that the water level has either risen or fallen relative to your ankles (this is the tide). Note: because the East River empties into the Atlantic Ocean, and Greenpoint is fairly close to the opening to the Atlantic, the East River at Greenpoint experiences tides.
- Tides are caused by the gravitational pull of the moon on the Earth. As the moon pulls on the Earth, the water of the Earth responds by bulging outward (see fig. 1). The Earth's tides are slightly more complicated, in that there are actually two "bulges" at play (see fig. 2). The second "bulge" is caused by the Earth being pulled toward the moon, and away from the water opposite the moon. If we now imagine the Earth rotating - with the moon staying stationary- we see that after one complete rotation of the Earth, every point on the Earth passes through some part of these two "bulges"* (see fig. 3).
- Every day on the East River there are two high tides, and two low tides. The high and low tides mark the extremes of water hieght fluctuation. One extreme is the deepest part of the "bulge's" (high tides), the other is in the troughs between them (low tides). If we use figure 4 (at right) as an example, the highest points the ball reaches are at 1 and at 3. When speaking of tides, these points correspond to high tides. Simply put: at high tide, the rising water stops going up, and thereafter it begins to fall. Points 2 and 4 in figure 4 represent the low tides, or the height at which the water stops falling and begins to rise again. Remember, tides have to do with height, not movement. In figure 4, the green horizontal lines represent the tides, the yellow tracking lines represent the currents (the speed and direction of water).
- What follows is a only a cursory description of tide tables, meant only to be informative. It is highly recommended that you seek professional instruction prior to entering upon the water.
Tide tables supply you with important information for predicting tides. Most simply, they supply the information concerning the tidal cycle for each day. This information includes the date, day, time, height of water in reference to some point, and the type of tide (high or low) that will occur.
- Referring to figure 5 (below), the date, day and time are straight forward enough. "LST" stands for Local Standard Time, and is defined as such on the original table. The height is given as 3.7, and the type of tide that occured is indicated as H, or high tide.
- The single element that is not intuitive is to what the height (3.7) is in reference to. At the bottom of the tide table we find that heights are relative to something called "MLLW". MLLW stands for Mean Lower Low Water. Mean is just a fancy nautical term for average. To get at the next wording, remember that each day has two low tides, one of which drops lower than the other. The tide which drops lower than the other is called the "Lower Low". The last word, water, simply stands in for "tide". It turns out that MLLW is the average lower low tide to occur on that date over the previous 19 years.