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Oceans in Motion

Introduction

The moon's gravitational pull is the primary force responsible for the tides on the Earth. Photo taken by the Galileo spacecraft from a distance of about 6.2 million kilometers from Earth, on December 16, 1992. (Source: NASA).

The Earth/Moon system has a centre of rotation some three quarters of the Earth’s radius from the centre of the Earth towards the Moon. As the system rotates a centrifugal force is produced. The further from the centre of rotation, the greater the force. On the Earth’s surface nearest to the moon, a ‘small’ force is exerted. At the centre of the Earth, a ‘moderate’ force exists. On the far side of the Earth is a ‘large’ force.

The Moon also exerts a gravitational force on the Earth and its seas. The closer to the Moon, the greater the force. On the surface of the Earth nearest to the Moon a ‘large’ force is exerte. At the centre of the Earth, a ‘moderate’ force exists. On the far side of the Earth is a ‘small’ force.

The combination of these centrifugal and gravitational forces explains the production of the tides. At the centre of the Earth, these forces are exactly balanced, so no physical pull is exerted. Parts of the ocean surface which are the same distance from the Moon as the Earth’s centre similarly experience no tide as the forces are also in balance.

On the Earth’s surface nearest to the Moon, the net force is towards the Moon, so there is a high tide (the Moon’s gravitational force exceeds the centrifugal force of the Earth/Moon system). On the side of the Earth furthest from the Moon, the net force is away from the Moon, thus producing a high tide ( the centrifugal force of the Earth/Moon system exceeds the gravitational force of the Moon).

Timing of Tides

The timing of tidal events is related to the Earth's rotation and the revolution of the moon around the Earth. If the moon was stationary in space, the tidal cycle would be 24 hours long. However, the moon is in motion revolving around the Earth. One revolution takes about 27 days and adds about 50 minutes to the tidal cycle. As a result, the tidal period is 24 hours and 50 minutes in length.

The second factor controlling tides on the Earth's surface is the sun's gravity. The height of the average solar tide is about 45% the average lunar tide. At certain times during the moon's revolution around the Earth, the direction of its gravitational attraction is aligned with the sun's.

During these times the two tide producing bodies act together to create the highest and lowest tides of the year. Known as Spring tides, these tides occur every 14-15 days during full and new moons.

Spring Tide

The sun's mass is 27 million times that of the moon, but it is also 390 times farther from the earth. So although the sun affects our tides, the moon exerts the greater gravitational attraction because of its proximity to our planet. The sun's influence is about 45% that of the average lunar tide.

During the periods of new and full Moon, when the Sun, Moon and Earth are directly in line, the solar and lunar pulls compliment each other and produce what are known as the ‘spring tides’, when high water is higher and low water is lower than usual.

Thus the tides experienced on Earth are a result of the interplay between Earth, Moon and Sun, where a sequence of relationships and events are continually repeated.

Neap Tide

It was not until Sir Isaac Newton (who lived from 1642-1727) discovered the law of gravity that the effect of the sun and the moon on the tides was fully understood. All surfaces of the Earth are pulled toward the moon and sun. This force has little effect on land masses, but it does have a very great and obvious effect on the water of the Earth's oceans.

Twice each month, when the Moon is in first or third quarter, the tidal range reaches a minimum and these smaller, weaker tides are called neap tides. The alignment of the sun and moon are at right angles from the Earth, the gravitational pull on the oceans is less, producing a smaller difference between high and low tide.

Some locations have much bigger tides than others. Tidal ranges are usually small in the middle of the ocean but can be very large where tidal waters are funneled into a bay or river estuary.

The crust of the earth is slightly elastic, so that it is deformed by the tide generating forces, creating lunar and solar tidal budges (high land) at the points closest and furthest from the moon and sun respectively. To an observer fixed on the earth's surface, these tidal budges move from east to west around the earth as it rotates each day, thus causing two luner and two solar high earth tides about each day.

Diurnal Tide

The inclination of the earth's spin axis to the plane of the moon's revolution about the earth and the earth's revolution about the sun creates in addition weaker diurnal tides with periods of roughly 1 day.

Cyclical tidal cycles associated with a diurnal tide.

Semi-diurnal Tide

Semi-diurnal tides have two high and two low waters per tidal day. The period of the solar tide is exactly 12.00 hours, while the period of the lunar tide is slightly longer, 12.42 hours, due to the moon's revolution around the earth every 27 days. These tides have periods of roughly 1/2 day.

Cyclical tidal cycles associated with a semi-diurnal tide.

Mixed Tide

Many parts of the world experience Mixed tides where successive high-water and low-water stands differ appreciably. In these tides, we have a higher high water and lower high water as well as higher low water and lower low water. The tides around west coast of Canada and the United Sates are of this type.

Cyclical tidal cycles associated with a mixed tide.

The fluid ocean also experiences the tide generating forces. Unlike the simple tidal budges created in the earth's crust, ocean tides have complex spatial patterns due to the complicated shapes and topographies of the different ocean basins. In general, however, ocean tides at any spot consist of a mixture of semidiurnal and diurnal tides.

Dodge Tide

The tides in the South Australian region are of the mixed type along the coasts of southern Kangaroo Island, south-eastern South Australia, and southern Spencer Gulf. In Gulf St Vincent and the Great Australian Bight, the tides are principally semi-diurnal.

A distinctive feature of these tides is the prominence of the solar constituent which has about the same amplitude as the lunar constituent. This leads to a periodic occurrence known as the 'dodge tide', when the semi-diurnal components cancel and the water level shows little variation. Similar tides are found in Torres Strait and the Gulf of California.

In the Great Australian Bight, three complete tidal cycles may be observed in one day. The amplitude of the ter-diurnal components are small and are more pronounced at neap tides when the major semi-diurnal tides cancel.


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