Understanding Tides

Did you ever wonder what causes the Tides? Many people refer to the tide as a “Lunar Tide.” Due to the “lunar” reference, most mariners assume that the tide is a result of lunar forces only. However, the term “Lunar Tide” refers only to the part of the tide that is produced by lunar forces.

Tides are defined as the “periodic motion of the waters of the sea due to changes in the attractive forces of the moon and the sun on the rotating earth.” That’s right: the tide is influenced by the Sun too! In fact, the sun’s effect on tides is 46% of that of the moon.

Newton’s universal law of gravitation considers the orbits of the sun, moon and earth, as well as the distances between them. The gravitational effects of the moon and the sun are not the same at any two positions within their respective orbits. Based on the ever-changing proximity of the planets involved, we end up with tides of various levels and times throughout the year.

Even year-by-year, on the same day of any given month, the high and low tides are at different times. As far back as the fifth century B.C., an astronomer named Meton concluded that every nineteen years, the full and new moons would recur on the same day of the year. The recurrence is not precise, however, due to a 0.06-day (6/100 of a day) difference in the timing. But, it is as close as it gets to the cycle repeating itself.

This study, resulting in what is known as the “Metonic Cycle,” was done nearly 2,500 years ago. Talk about vision! Today, we use a well accepted body of learning on the subject to calculate and print annual tide tables, which we all use to determine the tides.

Well, how exactly do the moon and the sun cause the movement of the sea on planet earth? There are two primary forces at work simultaneously. First, there are gravitational attractions between the planets involved. Second, there are centrifugal forces relating to the earth’s orbit around the sun, and the moon’s orbit around the earth.

You might ask, if our days are 12 hours long, and our oceans rotate with the earth every 24 hours, why aren’t the tides the same every day? Well, for one thing, if you observe the moon from the equator, it appears to pass by every 24 hours and 50 minutes. That’s because while the earth is rotating on its axis, the moon is orbiting around it. If the moon stood still relative to the earth, we would see it every 24 hours.

Does this sound complicated? It gets worse! The moon moves around the earth in an approximately oval-shaped orbit. At times it is closer and at times it is farther away from the earth. The gravitational forces vary with the moon’s distance from the earth. Due to the number of variables in the total tide equation, you can see why the times and heights of the tide vary so much.

The good news is that it all happens without a government agency being involved and nobody is in charge of it. Maybe that’s why it’s so reliable!

Until next time, we wish you clear skies, fair winds and calm seas!


World Wide Marine Training, LLC, is a U.S. Coast Guard Approved facility authorized to give examinations for captain’s licenses up to Master 200 Tons, Able Seaman up to Unlimited, STCW Basic Training, Radar, ARPA and other Endorsements. Please visit www.worldwidemarinetraining.com or call toll-free 866-249-2135.