Practice English Speaking&Listening with: Ocean Tides Video 2

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>> Welcome back

to this module on tides.

This second part will help you

finish the pre-lab exercise.

We're looking back again

at that tidal curve

for the month and one thing

that we want to try

to understand now is why the

range of the tides varies

with the phases of the moon.

For example, notice these days

around the full moon.

The high tides are quite high

and the low tides are

quite low.

And then you go over here

to where we have the new moon

and you can see again,

very high high tides,

very low low tides.

But if you go in the

in between times you can see

that the tides are more muted.

There's not very high high

tides, and low tides are not

very low.

In other words,

the tidal range is less here

and the tidal range is greater

here and here.

And the reason for that has

to do with something we

haven't thought about yet,

which is the fact

that the Sun also pulls

on the ocean

and it also makes bulges

of water.

Take a look

at this figure here

because it separates things a

little bit better.

You'll notice

that there's a bulge

of water facing the Sun

and a bulge of water away

from the Sun.

So in other words,

the Sun creates bulges

like the Moon does

but they aren't as big

as the Moon's bulges.

Now the key thing here is

that if we get

the Sun's bulges

and the Moon's bulges to line

up it makes the total ocean

bulge more, and so

as the Earth spins

under these extra large bulges

we see extra high high tides

and extra low low tides.

This lining

up of the bulges happens

during what's called the new

moon and the full moon,

when the Earth is,

when the Moon is either

between the Earth and the Sun

or on the opposite side

of the Earth from the Sun.

And so if we go back

to that previous figure you

can see the high tidal ranges

occurring during the full moon

and the new moon.


but 7 days

after the new moon the Earth

moves to this position,

what we call the first

quarter, and 7 days

after the full moon the moon

moves to this position,

what we call the third

quarter, and in either

of these positions those

bulges, the bulges

of water made by the Moon are

out of line with the ones made

by the Sun, and that results

in less of a tidal range.

There's less of a change,

less of a change

between the high tide

and the low tide during what's

called the first quarter

or what's called the third

quarter, sometimes the

last quarter of the Moon.

Now another factor

that controls the range

of the tides is how close the

Earth is to either the Moon

or the Sun.

The orbit of the Moon

around the Earth is not a

perfect circle,

it's an ellipse.

And the Moon is actually

closer to the Earth during

certain times of the month

and we call it perigee

when the Moon is closer

to the Earth and apogee

when the Moon is farther

from the Earth.

And as the Earth orbits the

Sun it doesn't follow a

perfect circle,

it's also an elliptical orbit.

And during the time

when the Earth is closest

to the sun we call

that perihelion, and it happens

in January,

and we call it aphelion

when the Earth is farthest

from the Sun, and that happens

to be in July.

The key idea here is the

closer the Earth is

to either the Moon

or the Sun, the stronger the

gravity's going to be pulling

on the ocean,

and the bigger the bulge is

going to be.

In other words,

the larger the change

of the tides,

the bigger the change

from high tide to low tide.

Now one final factor

that we need to think

about is how the tides

actually move as they move

around an ocean basin.

This bowl of water here mimics

an ocean basin.

And the movement of the tide

around the ocean basin

actually follows what we call

a rotating wave,

and that's what this video

is illustrating.

The water sloshing

around in the bowl here

actually mimics the way

that the tide rises and falls

as it moves

around an ocean basin.

So the edges

of the ocean basin would

represent shorelines.

And if we were

to put a marker here

into the water,

which I'll do right there,

if you're standing

on the shoreline right there,

well what you're seeing is the

tide rising

and the tide falling,

but the actual movement

of the wave

that the tide makes is a

rotary pattern.

And there's a point

in the middle of the bowl

where the water doesn't go up

or down at all, and we call

that an amphidromic point,

an amphidromic point,

a place where there is

no tide.

And what we find is the tide

can rotate

around the ocean basin,

around that amphidromic point

in a way that we're going to

try to figure out in lab.

So during the last part

of the lab,

we're actually going to go look

at the North Pacific Ocean,

and we're going to try to figure

out what the tide is doing

in this ocean basin.

We're going try to figure

out if it's sweeping

around this direction,

counter clockwise,

or is it sweeping

around clockwise?

And the way we're going to do

that is we're going to zoom

in on the coast

of North America,

and we're going to gather some

data for four stations,

one down near San Diego,

one maybe up in Northern

California, one in, say,

Oregon, and maybe one

up in Washington.

And we're gonna look

at the timing and the height

of the tides

as they move past the

West Coast

of the United States,

and we'll try to figure

out what

the tide is actually

doing, which direction

it's rotating.

So with that information you

can finish your questions

on the pre-lab

and enjoy your day in lab.

The Description of Ocean Tides Video 2