The Heartbeat of the Ocean

IntroductionTides are one of the most obvious features of

the oceans.Natural rhythms can easily be observed.Long term records of amplitudes and

predictions made since 13th century.1687 Newton applies his law of gravitation to

tides

Visual of seasons

II. TIDES Write this

rhythmic rise and fall of water at a fixed location (handout in student binder)

Lunar day 24 hrs. 50 min. Tides are linked to lunar cycle 28 days for moon to circle the

earth (and be over the exact same spot)

Tidal TheoriesThere are two tidal theories.Equilibrium theory by NewtonDynamic theory by LaplaceEach have there own assumptions.

A. EXPLANATION Write this

1. Equilibrium Model-theory that proposes reasons for large scale patterns.

There is a variation in the gravitational effects of ......

EARTH MOON 2/3 EARTH SUN 1/3

Visual on the next slide Write this

The earth moon are a system like a spinning "barbell" through space.

They rotate around each other .

The gravitational midpoint of this unit is the "barycenter"

The Earth-Moon SystemThe Earth and Moon orbit their common center of gravity, the barycenter (3000 miles from Earth’s center, or 900 miles from Earth’s surface)

Inertial Force Balancesthe Lunar Gravitational Forceat the Barycenter (Center of Mass of the Earth-Moon System

Write this

a. Inertia and gravity work against each other but are over all equal. This allows a consistent pattern of tides as the earth orbits the sun

But wait…

there’s more to TIDES!

Neep tide- first and last 1/4 low HIGH

TIDE high LOW TIDESpring tide- new and full very HIGH

TIDE very LOW TIDE

Dynamic Theory by LaplaceEquilibrium theory is not very realistic, so

100 years after Newton, Laplace developed the Dynamic theory.

Assumptions:Still homogeneous ocean (barotropic)No coriolis accelerations. No analytical

solution to Laplace equations, use numerical approximations and simple boundaries

2. Dynamic model- based on observations Write this

a. Types of tides Write this Diurnal Tide – One high and low per day Gulf of

Mexico

Semi-diurnal Tide – Two highs and lows per day, nearly equal in magnitude Bahamas

Mixed Tide – Two highs and lows per day, with conspicuous inequality in successive high and low water elevations West Coast of USA

(Tidal Datum – The average, over a 19-year period, of water height at a particular stage of the tide (i.e., mean high water, mean low water, mean lower low water, etc.))

b. why? Write this

friction of the ocean floor and coast lines distort the "expected" high and low.

Historical records " Empirical data" is collected for 100's of years. Computer programs help project future tidal patterns.

The Inertial force which offsets lunar The Inertial force which offsets lunar gravitation is due to the gravitation is due to the orbit of the Earth orbit of the Earth around the barycenter around the barycenter … it is … it is notnot due to the due to the spin of the Earth on its axis.spin of the Earth on its axis.

Due to much greater mass, the Due to much greater mass, the gravitational gravitational forceforce of the Sun (F=GM of the Sun (F=GM11MM22/r/r22) is greater than ) is greater than that of the Moon. that of the Moon.

So why are lunar tides greater than solar So why are lunar tides greater than solar tides?tides? Because the Because the tractive forcetractive force (proportional to (proportional to 1/r1/r33) exerted by the Sun is ) exerted by the Sun is only about 40-50% as strong as that of the only about 40-50% as strong as that of the Moon.Moon.

• Each pulley represents a harmonic constituent of the tide.

• A wire running through the pulleys was connected to a pen which traced the shape of the tide.

• First conceived in 1872, similar machines were in use until the 1960s.

Built 1910, in use until 196637 Harmonic Constituents11’ x 2’ x 6’2500 lbs