When I was 12-years-old, I spent part of the summer with my father in Oregon. One of the most memorable moments was during a weekend at the shore. Early one Saturday morning I went out to play with several other children on a broad, dry mud flat. By mid-morning my father and I were out fishing, rowing in several feet of water where just a few hours earlier I had been running on dry land. That was this mid-western kid's introduction to the concept of tides.
People who live in coastal communities are familiar with the clock-like rise and fall of the waters. It's regular and predictable. The times of high and low water advance about 50 minutes each day, rising and falling in a rhythmic pattern. The basic mechanics are simple, but variations in the shape of different coasts make the results very different.
THE SUN AND MOON
The vertical movement of water we call the tide is created by the gravitational pull of the moon and the sun. While the moon is much smaller than the sun, it is a great deal closer to the Earth and thus becomes a controlling force. Typically, a given area will experience high tide when the moon is on a certain bearing.
Both the sun and the moon affect the rising of the waters. When the sun and moon are lined up (during a new moon and full moon), their effect is cumulative, causing water to rise higher and fall lower than average. These are called spring tides and they have nothing to do with the season, as evidenced by the fact they occur every two weeks. When the sun and moon are at right angles (sometimes called "at the quarters") their total effect is less and the tidal range (the difference between high and low) is less. High tide is not as high and low tide is not as low. These are called neap tides.
The distance between the moon and the earth also varies. When the moon is closest to the earth it is said to be in perigee and tidal ranges are greater. When the moon is in its farthest position it is in apogee, and tidal ranges are less.
If the perigee coincides with a new or full moon, high tides will be very high. Add an onshore storm to that and you have a recipe for coastal flooding.
HIGH AND LOW TIDES
Most, but not all, places experience two high tides, and two low tides, every day. It is reasonable that the tide is high when the moon is in sight. But what about the other tide? Gravitational attraction produces a "lump" of water that goes around the world under the moon. On the opposite side of the earth is another lump created by the centrifugal force of the earth and moon rotating around their common center of mass (about 1,100 miles below the earth's surface).
Usually the two daily tides are not exactly the same height. In some cases the difference is quite noticeable. Chart depths will be noted as either "Mean Low Water" or "Mean Lower Low Water." "Mean," in this case, is a mathematical term that means "average." During an extreme spring tide, the water may be lower than charted, so you need to pay attention to the tide tables.
Tide tables are commonly available just about everywhere. Simple ones are often given away in bait shops. Most chandleries sell tide tables in book form. And many navigational software packages have tide information. These tables show a more complete set of tidal predictions. They give detailed predictions for a limited number of reference stations and offsets (both for time and height) for a great number of subordinate stations. The depths indicated are relative to the chart datum (usually mean lower low water), thus a negative depth (such as -0.3 feet) tells you that at low tide there will be less water than what is shown on the chart.
