How tides are formed?
What causes tides?
Everyone should know the basics about tides. It goes without saying
that if you going to venture on to the water in boat or a kayak that
you need to appreciate what tides are and how they work. Essentially
the moon generates the tides on our planet and it occasionally gets
some help from some other heavenly bodies. However for simplicity
sake let us examine the Moon, Sun and earth system. The
gravitational pull of the moon and earths daily rotation cause the
tides to go in and out and the gravitational pull of the moon and
sun combined with the relative positions of the Earth, moon and sun
cause the tidal range to vary across the month/.
Let us look at a moon earth system. Instead of the moon revolving
around the Earth, imagine them both revolving around each other with
the central pivot point in between. This point is called the common
centre of mass. Imagine the moon as a blob of blu-tack and the earth
as an apple. And them both joined by a pencil. Now imagine spinning
them on your finger on the centre of mass, which is some where on
the pencil near the apple. In reality the common centre of mass is
just inside earth itself. More importantly both are revolving and so
both experience centrifugal force
As both the earth and moon revolve around each other, they are both
trying to fly off away from each other, however gravitational force
holds them together and prevents this parting of objects. The
balance between the two forces is what keeps the Earth moon system
in equilibrium. If they gravity suddenly disappeared then they would
both fly off into space, If they stopped revolving they would both
crash in to each other.
Imagine an earth with just water on the surface. The water has two
bulges, one on either side. The bulge nearest the moon is caused by
the gravitational pull between the earth and the moon. The bulge on
the other side is caused by the centrifugal force from the spinning
of the system itself. Since the two forces must balance each other
both bulges are the same size. These bulges are in fact high tides.
Between these high tides are low tides
Relative to the moon these bulges stay in the same place, however as
the earth is rotating on its own axis underneath the bulges.
Therefore as the earth rotates 24 hrs a day, at a given spot, these
bulges will pass by twice so i.e. a high tide approximately every 12
hours.
High tides do not occur at the same time every day; they actually
occur a little later each day. While the Earth takes 24 hours to
rotate, the moon takes 28 days to rotate around the Earth. Every
time the Earth rotates once, the moon has gone around a bit more. So
for a given spot on the earth to come back to the same position on
the moon, the earth has to rotate a bit more .This extra rotation is
approx 50mins and so explains why at a given high tide on a given
day, the same high tide will be approx 50min later the next day.
Another consideration is also the tilt of the Earths axis of the
systems .i.e. One pole of the Earth is sometimes nearer to the moon
than the other and there is also a natural difference in inclination
during the lunar cycle. The consequence of this is that the tidal
bulges may not line up exactly with the earths axis, i.e. one bulge
may be near the North Pole and the other down near the South Pole.
Therefore as the earth spins underneath the tidal bulges, a spot on
the Earths surface near one of the poles goes right under the middle
of the bulges but when it travels around to the other bulge it
misses completely and so results in only one tide a day. Although
the diurnal tide (1 tide) is experienced in some places more common
to have a semi-diurnal (2 tides) or mixed mostly semi-diurnal is
more likely. Remember this is all just on our total water covered
Earth example, Things get more complicated as always with the
addition of land on this watery surface.
Springs and Neaps
Tides have many facets and have many cycles within each other. One
of these is the fact of the tide going in and out, however its
actually embedded within a longer and larger cycle called the
Springs-Neap Cycle. This Spring –Neap Cycle causes the tidal range
to grow and shrink alternately twice during every 28 days that the
moon goes around the Earth. In simple terms one week the tide goes
in and out a lot and the next week the tide goes in and out a
little. Large or Spring tides usually occur after the Full moon and
then about two weeks later just after the new moon. This period of
time (new moon or full moon is often called Syzygy).After the half
moon tides are small or “neap". To understand about the Spring-Neap
Cycle we need to now look at the interaction of the sun on the
Earth-Moon system. If we look at when the Moon and Sun combine, we
will see that the addition of the Lunar and Solar bulge will cause a
larger bulge than normal.
Due to the relative position s of the Earth, Sun and Moon, the moon
has four “phases” a month. They are:
Opposition: when the Moon is full and this occurs when the
Sun and Moon are on opposite sides of the Earth
Conjunction: when there is a new moon, and when the Sun and
Moon are on the same sides of the Earth.
Quadrature: is when there is a half moon, when the Earth,
moon and Sun forma right angled triangle.
When the Sun and Moon are lined up (opposition or Conjunction)
(either a full moon or a new moon) their gravitational influence
adds together to increase the size of the tidal bulges. The bigger
the bulges the bigger the tides which means Spring Tides. Conversely
when the sun and moon are at right angles to each other (Quadrature
or half moon) the gravitation of the sun acts against that of the
moon and each tries to create bulges in opposite directions. This
evens out the water on the earths surface and makes bulges smaller
and hence “Neap Tides”
