A view of the tides at Halls Harbour onNova Scotias Bay of Fundy. This is a timelapse of the tidal rise and fall over a periodof six and a half hours. During the next six hours of ebb the fishermen unload theirboats on the dock. Thats a high tide every 12 hours and 25 minutes! There are two high tides every 24 hours and 50 minutes
II. Tides• A. Earth-Moon system• 1. Gravitational attraction
What causes tides? Tide-generating forces (TGF) are a result of the gravitational attraction between the earth, the sun, and the moon and the centrifugal force due to the relativemotions of the moon around the earth, and the earth around the sun. While theseforces exactly balance on average, the local mismatch at the earths surface creates a horizontal force directed towards thesurface points closest and farthest from the moon (the "lunar" TGF) and the sun (the "solar" TGF).
The crust of the earth is slightly elastic, so that it is deformed by theTGFs, 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 earths surface, these tidal budges move fromeast to west around the earth as it rotates each day, thus causing two luner and two solar high earth tides about each 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 moons revolution around the earth every 27 days. These tides are called the"semidiurnal" tides since they have periods of roughly 1/2 day. The inclination of the earths spin axis to the plane of the moons revolution about the earth and the earths revolution about the suncreates in addition weaker "diurnal" tides with periods of roughly 1day. The amplitude of the semidiurnal lunar high earth tide is about1 m at the equator, about twice that of the solar tide. We do not feel these earth tides due to their very large horizontal scales (many 1000s km).
The fluid ocean also experiences the TGFs. Unlike thesimple tidal budges created in the earths 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. The worlds largest semidiurnal tides exist in the Bay of Fundy (maximum high tide ~12-15 m), where the Bay of Fundy/Gulf of Maine acts as a coupled hydrodynamicsystem which is forced near its own resonant frequency bythe semidiurnal tide in the western North Atlantic Ocean. Similar very high tides are found in other coastal areas (e.g., the Amazon and the Patagonia shelves) where the regional topography creates a near-resonant response to the adjacent deep ocean tide.
TIDES TIDES Tides are periodic rises and falls of large bodies of water. Tides are caused by the gravitational interaction between the Earth and theMoon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon(and away from the water on the far side). Since the earth is rotating
The Suns Interaction with the Tides Spring TidesSpring tides are especially strong tides (they do not have anything todo with the season Spring). They occur when the Earth, the Sun, andthe Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.
The Proxigean Spring Tide is a rare, unusually hightide. This very high tide occurs when the moon is both unusually close to the Earth (at its closest perigee, called the proxigee) and in the New Moon phase (when the Moon is between the Sun and the Earth). The proxigean spring tide occurs at most once every 1.5 years.
Neap Tides Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (withrespect to the Earth). Neap tides occur during quarter moons.
A lunar eclipse occurs when the Earths shadow falls on the moon. Lunar eclipses occur, on average, about every 6 months. Types of Lunar EclipsesTotal Eclipse - When the entire moon enters the Earths umbra (the darkest part of its shadow), this is called a total eclipse. Partial Eclipse - When only part of the moon enters the Earths umbra, this is called a partial eclipse. Duration of Lunar Eclipses During an average total lunar eclipse, the moon is within the Earths umbra for about an hour. This is called totality. Frequency of Lunar Eclipses Since the plane of the moons orbit is inclined about 5°: from the plane of theEarths orbit, lunar eclipses are relatively infrequent. There are about two lunar eclipses each year (visible somewhere on Earth).
http://www.waterland.net/rikz/getij/doInformation on tides
Diurnal inequalityThe average angle of the earths rotational axis in relation to the connecting line between earth and moon is 66.5 degrees. The same applies in relation to thesun. The angle of the terrestrial equator is 23.5 degrees (90-66.5) in relation to the connecting line between earth and moon (and thus earth and sun as well).The ellipsoid of the water mass will be oriented towards the moon. During a complete orbit (one day), someone at a particular spot on the earth will thereforeobserve two high and two low tides which are unequal in height. This is called diurnal inequality. At certain spring tides, the diurnal inequality is substantial. The subsequent inequality will, on thecontrary, be insubstantial. This has to do with the point, or the node, where the lunar path intersects the ecliptic surface. The ecliptic surface is the surfaceformed by the orbit of the earth and the sun. The sun, earth and moon are then as far as is possible in one plane. The cycle between two nodes is 27.21days. This gradually catches up with the lunar month cycle of approximately 29.53 days. After about 13 lunar months, that is about 376 days, they are levelagain. Therefore, the inequalities are not systematically larger during either new moon or full moon.In addition to the diurnal inequalities, there are other variations in high and low tide levels.