HOW TIDES ARE FORMED 7th July 2009 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 forceAs 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”