# High Tide and Low tide?

What is the reason for high tide and low tide?why during full moon period there will be high tide?what is the duration of high and low tides.

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The high and low tide are caused by the gravitational forces between the earth and the moon. However, the source of the real effect takes some explaining to grasp. The incorrect way of thinking is that the moon attracts all the water to itself, therefore causing a high tide on the side of earth close to the moon, and a low tide on the side far from the moon. However, if this was the case, there would be a high tide once per day. But there are TWO high tides a day. The reason is, the part of the earth both AWAY from the moon, and CLOSE to the moon BOTH get high tides...

Now we need to explain why this is so. The earth and moon rotate around each other, each pulling the other towards itself. The moon attracts every piece of matter on earth. Since gravity is inversely proportional to the square of the distance, this force is greater on the side of the earth closer to the moon, and lesser on the side of the earth further from the moon. Since the earth is quite a rigid object, this difference in forces fails to deform the earth (much). However, it succeeds quite well in deforming the oceans -- which are not as rigid. Since the waters on the moon side are attracted more strongly than average, they tend to bulge TOWARDS the moon, hence causing a high tide. The waters on the opposite side of the moon, since they are attracted less strongly than average, tend to 'lag behind' the rigid earth, and bulge AWAY from the moon, which in this case, is also AWAY from the earth, again, causing a high tide. Low tide occurs at about right angles to the moon, where the force on the waters match the average pull of the moon on the earth closely.

The question that usually follows this is, why doesn't the sun cause any tides (comparable to that of the moon) although its pull on the earth is larger? The answer is, although the gravitational pull of the sun on the earth is larger than that of the moon, due to the much greater distance, the force changes very little from one end of the earth to the other. Since it is the difference in the force than the average magnitude of the force that matters for creating tides, the net effect is much less than that for the moon.

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To explain this use need to use your imagination to visualise what is happening to the earth. you are also mixing up high/low tides (daily) with Spring and Neap tides (monthly)

Consider the earth as a sphere about 3000miles in diameter, the oceans are about 11 miles deep at their deepest point and on average a few miles deep. Most of the earth is covered in water and hence, the oceans are only a thin layer of water around the earth. The earth then rotates inside this envelope of water (think of spinnign a glass of water with an object floating in it, the glass can be rotated but the object does not move...untill friction eventually starts the water rotating). Thus for the moment, consider that the earth is rotaing inside the layer of water but the water does not move.

The two most influential gavitational bodies on earth are the sun and the moon. The graviatation pulls on the ocean and this thin veneer of water is pulled towards the sun and the moon, creating a bulge of water towards the moon and sun, and therefore, the water gets less where the influence of the sun and moon are less.

As the earth rotates every 24 hours inside this layer of water a single point will experience higher water as it passes through the bulge and low water in the area that there is less gravitational influence of the sun and the moon. This gives you high and low water, because of the two bulges (either side of the earth) you get two highs and two lows per day (the actual time between tides is about 12.5hours).

So if the sun and the moon are directly inline on the same side of the earth then the bulge of water is greatest and also when they are on either side of the earth. Think of pulling a balloon with both hands.

But if the moon and the sun are pulling at ninety degrees to each other then then they are pulling against each other (thinking of pulling a balloon on the side and at then end) and the buldge is less.

Thus, depending on the orientation of the moon and sun relative to the earth, the bulge will be lower or smaller and hence, you get Spring (highest high tide) and Neap (lowest high tide) tides. The moon rotates around the earth every 28 days (lunar month) and therefore you get a cycle of spring, neaps within a 28 day period.

A new moon and a full moon are when the moon and the sun are inline and hence these are the Spring tides, so by looking at the moon you can predict whether the tides are neap or spring tides.

However, there are land masses, wind factors and fiction between the ocean floor and the water which also influences how tides are experienced in different parts of the world. These play a secondary role of modify the underlying Moon/Sun effect of causing tides.

High and low tides have to do with gravitational pull from the moon. For duration you would have to check the local area you are curious about, because duration, differences in feet from high to low and even how many time the tides change in a day has to do with location.

For the posters above the two high tides a day is not universal Some places only have one. And the times change gradually as the days pass. Noon will not be hight tide next week if it is high tide at noon today.

High and low tide is caused by the gravity of the moon pulling on the earth. The duration of high and low tides vary you can have up to 3 tide changes a day. Weather i.e. high and low pressure systems can have a effect on the amount of tide change. For example a cold front can cause extreme low tides on the gulf coast from the constant north wind blowing the water out.

Source(s): Tug boat capt.
• Anonymous

the tides are controlled by the moon's gravitational pull. the tides are the highest when the sun, the earth, and the moon are all perfectly aligned. the moon orbits the earth every 28 days, so ffor any intensive tide purposes, you can consider it stationary. that means that on average, the tides alternate from high to low every 6 hours. think of it like this:

morning and evening --> low tide

noon and midnight -->high tide

• Anonymous
5 years ago

In my experience here in the tidal marshes and intercoastal canals of southern Louisiana, the OUTGOING tide is best. Incoming tides can be ok if its a strong tide. A high tide all in or a full low tide all out, is never good. No water moving in a tidal system = No bites. Predatory fish feel the slightest water movement and position themselves facing the current. By doing this, their food is almost washed right into their mouths. So we cast up current and let the lure drift and roll along with the current until a fish seeking his lunch grabs it. So basically, when the water starts moving, the fish hear a dinner bell!

Gravitational pull of the sun and the moon do this. As their position varies and the earth spins, the duration and high and low of the tides and the timing vary every time.

But as we know what the relative positions will be there are very accurate predictions about them.

The tide is the cyclic rising and falling of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the Earth. Tides cause changes in the depth of the sea, and also produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation (see Tides and navigation, below). The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is an important ecological product of ocean tides.

The changing tide produced at a given location on the Earth is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of the rotation of the Earth and the local bathymetry (the underwater equivalent to topography). Though the gravitational force exerted by the Sun on the Earth is almost 200 times stronger than that exerted by the Moon, the tidal force produced by the Moon is about twice as strong as that produced by the Sun. The reason for this is that the tidal force is related not to the strength of a gravitational field, but to its gradient. The field gradient decreases with distance from the source more rapidly than does the field strength; as the Sun is about 400 times further from the Earth than is the Moon, the gradient of the Sun's field, and thus the tidal force produced by the Sun, is weaker.

In most places there is a delay between the phases of the Moon and its effect on the tide. Springs and neaps in the North Sea, for example, are two days behind the new/full Moon and first/third quarter, respectively. The reason for this is that the tide originates in the southern oceans, the only place on the globe where a circumventing wave (as caused by the tidal force of the Moon) can travel unimpeded by land.

Many people also believe the time difference between high tides and low tides to be the same. Times between high and low tides will always differ (many people believe them to be 6 hours and 12 minutes, from which they may deviate).

The resulting effect on the amplitude, or height, of the tide travels across the oceans. It is known that it travels as a single broad wave pulse northwards over the Atlantic. This causes relatively low tidal ranges in some locations (nodes) and high ones in other places. This is not to be confused with tidal ranges caused by local geography, as can be found in Nova Scotia, the Bristol Channel, the Channel Islands, and the English Channel. In these places tidal ranges can be over 10 metres.

At the point right "under" the Moon (the sub-lunar point), the water is closer than the solid Earth; so it is pulled more and rises. On the opposite side of the Earth, facing away from the Moon (the antipodal point), the water is farther from the moon than the solid earth, so it is pulled less and effectively moves away from Earth (i.e. the Earth moves more toward the Moon than the water does), rising as well. On the lateral sides, the water is pulled in a slightly different direction than at the centre. The vectorial difference with the force at the centre points almost straight inwards to Earth. It can be shown that the forces at the sub-lunar and antipodal points are approximately equal and that the inward forces at the sides are about half that size. Somewhere in between (at 55° from the orbital plane) there is a point where the tidal force is parallel to the Earth's surface. Those parallel components actually contribute most to the formation of tides, since the water particles are free to follow. The actual force on a particle is only about a ten millionth of the force caused by the Earth's gravity.

These minute forces all work together:

pull up under and away from the Moon

pull down at the sides

pull towards the sub-lunar and antipodal points at intermediate points

Because the Moon's tidal forces drive the oceans with a period of about 12.42 hours (half of the Moon's synodic period of rotation), which is considerably less than the natural period of the oceans, complex resonance phenomena take place. The global average tidal lag is 12 minutes, which corresponds to an angle of 3 degrees between the position of the moon and the location of global average high tide. Tidal lag and the transfer of momentum between sea and land causes the Earth's rotation to slow down and the Moon to be moved further away in a process known as tidal acceleration.