Tides

http://aa.usno.navy.mil/data/docs/MoonPhase.html#y2001

Phases of the Moon Data

A view of the tides at Halls Harbour on Nova Scotia's Bay of Fundy. This is a time lapse of the tidal rise and fall over a period of six and a half hours. During the next six

hours of ebb the fishermen unload their boats on the dock. That's 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

Fig. 3.26

II. Tides

• A. Earth-Moon system– 1. Gravitational attraction

2. Barycenter

Fig. 3.26

II. Tides

• A. Earth-Moon system– 1. Gravitational attraction– 2. Barycenter

3. Centrifugal force

Fig. 3.26

II. Tides

• A. Earth-Moon system– 1. Gravitational attraction– 2. Barycenter– 3. Centrifugal force

4. Lunar day

Fig. 3.27

II. Tides

• A. Earth-Moon system

• B. Earth-Moon-Sun

system

–1. Tidal differences

Tidal range

II. Tides

• A. Earth-Moon system

• B. Earth-Moon-Sun system– 1. Tidal differences

–2. Sun/moon gravitational interaction

Fig. 3.28

Earth-Moon-sun interaction

II. Tides

• A. Earth-Moon system

• B. Earth-Moon-Sun system– 1. Tidal differences– 2. Sun/moon gravitational interaction

•a. Spring tides

Spring tides

II. Tides

• A. Earth-Moon system

• B. Earth-Moon-Sun system– 1. Tidal differences– 2. Sun/moon gravitational interaction

• a. Spring tides

•b. Neap tides

Neap tides

II. Tides • A. Earth-Moon system• B. Earth-Moon-Sun system

Tide types

Semidiurnal

• Tide types

– 1. Semidiurnal

–2. Diurnal

• C. Tide types

–3. Mixed

Fig. 3.30

Research information on following slides

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 relative

motions of the moon around the earth, and the earth around the sun.  While these

forces exactly balance on average, the local mismatch at the earth's surface creates a

horizontal force directed towards the surface 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 the TGFs, 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. 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 are called the "semidiurnal" tides since they have periods of roughly 1/2 day.   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. The amplitude of the semidiurnal lunar high earth tide is about 1 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 1000's km).

The fluid ocean also experiences the TGFs. 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.  The world's

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 hydrodynamic system which is forced near its own resonant frequency by the 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 the

Moon. 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 while this is happening, two tides occur each day.

The Sun's Interaction with the Tides

Spring Tides Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the 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 high tide. 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 (with

respect to the Earth). Neap tides occur during quarter moons.

A lunar eclipse occurs when the Earth's 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 Earth's umbra (the darkest part

of its shadow), this is called a total eclipse. Partial Eclipse - When only part of the moon enters the Earth's umbra, this is

called a partial eclipse. Duration of Lunar Eclipses

During an average total lunar eclipse, the moon is within the Earth's umbra for about an hour. This is called totality.

Frequency of Lunar EclipsesSince the plane of the moon's orbit is inclined about 5°: from the plane of the

Earth's orbit, lunar eclipses are relatively infrequent. There are about two lunar

eclipses each year (visible somewhere on Earth).

http://www.waterland.net/rikz/getij/doc/ehoofdsch.html

Information on tides

The Equilibrium Tide

Times of Tide

The earth-sun system

Spring tide and neap tide

Diurnal inequalityThe average angle of the earth's rotational axis in relation to the connecting line between earth and moon is 66.5 degrees. The same applies in relation to the sun. 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 therefore

observe 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 the

contrary, 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 surface formed 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 27.21 days. 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 level again. 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.

Variations

The astronomical tide

Age of the tide