Transcript Proxigean Spring Tide - hrsbstaff.ednet.ns.ca

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 Eclipses
Total 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 Eclipses
Since 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).
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Information on tides
The Equilibrium Tide
Times of Tide
The earth-sun system
Spring tide and neap tide
Diurnal inequality
The 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