>> 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
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.
