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The moon rises above the sea as the sun sets. Tides are the response of ocean water to the pull of gravity of the moon, and to a lesser extent the sun, on the earth as it spins around on its axis once every 24 hours.
Photo: Chris Johnson

September 24, 2002
  Real Audio

Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey in the Seychelles.

Today was the first clear, calm day we have experienced in weeks. By late afternoon we reached the northern end of the Mahe Bank where we are hoping to find sperm whales over the next couple of weeks. As we sailed north, the sky slowly changed from blue to orange, the entire crew aloft in the hopes of seeing a green flash as the sun disappeared over the horizon. At the same time, the moon was just beginning to rise in the east. It was a full moon in fact. We wondered what effect the moon was having on the ocean beneath us, and the shorelines of nearby islands.

The moon is so far away yet it can move the waters of the oceans in a predictable way by gravitational and centrifugal force. Over the centuries, mariners and those who dwelled along the shoreline had noticed a relationship among the phases of the moon, the heights of the tides and the times of the day at which the water reaches its height. However, it was not until Isaac Newton formulated the laws of gravitation in the seventeenth century that we began to understand the relationship. According to Newton's law, the closer a body is, the more gravitational force it will have. Therefore, even though the sun is 107 times larger than the moon, the moon is 390 times closer to the earth, and so the moon's gravitational force is the main regulator of the tides.

Tides are the response of ocean water to the pull of gravity of the moon, and to a lesser extent the sun, on the earth as it spins around on its axis once every 24 hours. Tides are a predictable, repetitive rising and falling of the water along the ocean's shore. Every day the ocean washes up onto the shore until it reaches a certain point (the high tide mark), then it begins to recede, reaching it's lowest point (the low tide mark). Tides repeat themselves over regular intervals of 12 hours, or, there are approximately 6 hours between each rise and fall of the waterline. That means that in a 24-hour day, there will be two high tides and two low tides.

Now, imagine the moon exerting its gravitational force on the earth - pulling the waters of the oceans toward it. At the same time, another bulge of water is occurring at the opposite side of the earth caused by complex centrifugal forces between the moon and the sun. Centrifugal force is a force that repels a body away from an axis around which it rotates. So at any one time there are two bulges of water, of approximately equal size, on opposite sides of the earth. These 'water bulges' moving over the earth as it rotates are the high tides.

Remember a time when you spent the day at the beach and watched the tide come in or out, these are the bulges of water moving towards the moon, running into or leaving land while on its way.

Although farther away, the sun also plays a significant role in the fluctuations of tides. The sun produces tidal and centrifugal bulges much the same way as the moon except to a smaller degree. Indeed, the tidal bulges caused by the sun are less than half those caused by the moon. However, every two weeks, the moon, the earth and the sun line up. The gravitational pull of the sun and moon reinforce each other and combine to produce extra large tides called 'spring tides'. At the other extreme, when the sun and moon are at right angles to each other in respect to the earth, their forces are separated, therefore producing lower or 'neap tides'.

When the tide goes out, the part of the shoreline that spends half of the time below water at high tide is now left exposed at low tide, this is called the intertidal zone. This is a very special area supporting plants and animals that are highly specialized to live in such a drastically changing environment.

A view of the earth looking down at the North Pole.
The moon's gravitational atraction causes two bulges of water on the oceans, one on the side nearest the moon, the other on the opposite side from the moon. As the earth rotates, the bulges remain aligned aligned and pass over the earth's surface forming high tides.
Drawing: Chris Johnson

So why are tidal ranges so dramatic in some places and so minor in others? This has to do with the size of the landmass, the gradient of the shoreline and local currents. The tidal range, or the difference between high and low tide can vary from only a few feet to several meters. The Bay of Fundy in Nova Scotia can have a tidal range of more than 15 meters.

The tides also produce currents that move toward and away from the shoreline, these are called 'tidal currents'. 'Flood currents' move toward the shore during high tide and 'ebb currents' move away from the shore during low tide.

So, with main and mizzen set, the Odyssey crew sailed across a tranquil ocean under a starry sky and a full moon with a gentle 8-knot wind at our beam. Then the dolphins arrived, first two, then four, then eight, their clicks and squeals clearly audible from the bow, their sleek bodies drenched in moonlight. The crew spent the next half hour in the company of dolphins, beneath the illuminating light of a full moon, sailing through the western Indian Ocean. Some nights at sea are truly magical.

Log by Genevieve Johnson

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