|Why does the ocean get bigger gravitational pull
of the moon? Why low and high tides occur
throughout a day? |
The pull is NOT greater on oceans. The pull
of the moon on near side of the earth is a
little greater than the pull of the moon on the
center of the earth. The land and water move
towards the moon when the moon is over head.
Meanwhile 12 hours ON THE OTHER SIDE OF THE
EARTH, the land (and ocean) gets a slightly
SMALLER tug from the moon, RELATIVE to the
center of the earth. Hence, relative to the
center of the earth, the ground and the oceans
MOVE AWAY from the center of the earth, to
create a high tide there, AT THE SAME TIME WHEN
THERE IS HIGH TIDE, 12 HOURS or 180 degrees in
Throughout the day, the oceans are constantly
moving, rising and falling. This change in the
level of the water is called the tide. And the
tide is controlled not only by the moon, but
also by the sun, the way the earth rotates, and
more. The most important factors affecting tides
are gravity and rotation - and as the moon
rotates around the earth because of gravity the
moon is said to `control´ the tides.
But to understand how the moon´s gravity and
rotation cause water to rise and fall as tides
we need to understand a bit about water. Water
molecules (every individual molecule that makes
up the ocean and every body of water) are
attracted to each other in a way that makes them
stick together (think about dew droplets on a
plant leaf in the morning; the water forms this
droplet because the different molecules of water
stick together). This property of water is
called `cohesion´ and because of this property
of water all water molecules want to stick
together and then they act like a single body.
So rather than every water molecule in the ocean
acting in different ways, all of the molecules
stick together and act together.
As the earth rotates, the force of the
rotation pulls the ocean water away from the
earth´s surface (you can test this by holding
your arms our and spinning as quickly as you
can - do your arms feel like they want to fly
off of your body?). Now we know that the ocean
water molecules all stick together which means
that every molecule in the ocean pulls away from
the earth´s surface. This creates a mound of
water around the center of the earth (the
earth´s equator) and a thinner layer of water at
the arctic and the Antarctic poles. If the earth
was all alone in its rotation then we would just
have a mound of water at the middle of the earth
and less water at the north and south poles;
however, because the moon is very close to the
earth, its gravity has an effect on this bulge.
So when you watch the water level rise in Santa
Barbara (as the tide rises) it means that the
rotation of the earth is carrying California
away from the moon because the beach that we are
watching is being rotated toward the part of the
earth where the ocean has a water mound. As we
move right in to the bulge of water we have a
high tide. Then as the earth rotates and carries
Santa Barbara away from that water mound, the
tide falls and we end up in a low tide. So why
do we have two high and two low tides every day?
We have two high and two low tides every day,
because there are actually two mound of water on
opposite sides of the earth. It takes the earth
24 hours to complete one full rotation and in
that 24 hours we pass through these two mound of
water (2 high tides) and two low points in the
water (2 low tides). And that mound of water is
caused by the gravitation force of the moon on
the earth as the spot where you are on the earth
rotates past the moon.
The oceans aren't pulled on any harder than
the land, but the oceans, being liquid, are able
to slosh where gravity pulls them which the
land, being solid, is more resistant to.
Tides occur twice a day because the far side
of the Earth from the moon is experiencing less
force than the midway-point, which effectively
causes it to stretch in the other direction.
Now, that said, the sun has a large effect on
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